U
NIVERSITA’ DI
P
ISA
Tesi di Dottorato in Scienze e Metodi per la Città e il Territorio Europei
Ciclo di Dottorato 2011
KEY ELEMENTS TO DRAW UP A TRAFFIC PLAN FOR
RURAL ROADS
English Summary
Relatore:
Prof. Ing. Antonio Pratelli
Secondo Relatore:
Prof. Ing. Alessandro Marradi
Relatore esterno:
Ing. Maria Carmela Iaconis
Candidato:
dott. ing.
Matteo Rossi
Considerate la vostra semenza:
fatti non foste a viver come bruti,
ma per seguir virtute e canoscenza
I
Sommario
1. Introduction ... 1
2. Traffic Plan for Rural Roads: a general overview ... 3
2.1. PTVE Standards Framework ... 3
2.1.1 Administrative and functional roads classification ... 4
2.1.2 Road Safety: an overview on road safety devices Italian regulatory and laws ... 5
2.1.3 Road Safety Audit and the instructions contained in DL 35 – 15th april, 2011 ... 7
2.2. PTVE objectives ... 8
2.3. PTVE structure ... 9
2.3.1 Knowledge framework...10
2.3.1.1 Infrastructural supply characteristics ...10
2.3.1.2 Transport demand analysis ...11
2.3.1.3 Maintenance level of the road network...11
2.3.1.4 The road safety ...12
2.3.1.5 Road Network Critical aspects ...13
2.3.2 Methodological activities aimed to the management of infrastructures ...14
2.3.2.1 PTVE main targets ...14
2.3.2.2 Mathematical model support tools for scenarios analysis ...15
2.3.2.3 Functional and administrative road network classification ...15
2.3.2.4 Road Network Maintenance Plan ...16
2.3.2.5 Targets achievements ...17
2.3.2.6 Actions efficacy evaluation ...18
2.4. Necessary data to collect to develop a PTVE ...19
2.4.1 Mobility and transport demand data ...19
2.4.2 Accident data ...20
II
2.5. Support tools ...22
2.6 Performance indexes ...23
3. PTVE developed elements ...26
3.1 Design speed for existing roads ...26
3.2 Operative speed ...31
3.2.1 Straights ...33
3.2.2 Bends ...40
3.2.3 Road operative speed ...41
3.3 Risk Assessment for safety devices ...43
3.3.1 Safety devices census ...44
3.3.1.1 Characteristics ...44
3.3.1.2 Point where the devices is installed ...49
3.3.1.3 Other possible critical issues ...49
3.3.1.4 Safety devices to be installed according the Italian Standards ...50
3.3.2 The Risk concept and its application to safety devices ...50
3.3.3 Risk Level Assessment for existing safety devices ...51
3.4 Risk Assessment for pavement maintenance level ...57
3.5 Dangerous points identification ...60
3.5.1 Criterion n. 1 ...61 3.5.2 Criterion n. 2 ...63 3.5.3 Criterion n. 3 ...63 3.6 Accident analysis ...64 3.6.1 CNR procedure ...64 3.6.2 Swiss Standard SN 640 009 ...66
3.6.3 Our procedure developed ...69
3.7 Functionality indexes ...70
3.8 Geo-Referencing ...73
III
4. Practical application: the Province of Pisa ...80
4.1 Actual state of the road network of the Province of Pisa ...80
4.2 The actual transport demand ...83
4.3 Design Speed model application ...92
4.4 Operative Speed model application ...97
4.5 Risk Level Assessment for safety devices ...103
4.6 Black points identification ...111
4.7 Accident rate analysis ...113
4.8 Functionality indexes ...117
4.9 Taxonomic model applied to Province of Pisa ...120
5. Final considerations ...127
6. Further applications and future development ...130
Bibliography ...131
Acknowloedgements ...138 Annex A ... A.0 Annex B ... B.0
1
1. Introduction
In recent years in Italy there has been a population growth of large urban centers. The causes of this increase, compared to a negative natural balance, are to be found in a positive net migration certainly due to an increase in people from abroad who, presumably, have been established in the provincial capitals to small distances from the place of job.
At the same time there remains a redistribution of population by municipalities in areas outside the capital, in the municipalities of the first and second crown or, such as in the case of Milan, in common with neighboring provinces. At the same time, however, the use of the city is growing, resulting in greater attractiveness for users and, therefore, increased services demand. The result of the combination of these processes is the growing need for rapid and efficient transport services between large areas of territory connected by relationships that intersect at different levels, a requirement which, due to an almost complete absence of a common strategy inspired by the coherence between planning urban and transport policies, has led to the development of a predominantly individual mobility, related to use of cars that now has reached the limit of congestion: for example, in the last 20 years the development of the road network was lower than 11 %, while the circulating fleet showed an increase of over 45% (Table 1).
Table 1 - Comparison between road network extension and the size of the circulating vehicles fleet YEARS
1970 1990 2011
Road network extension (km)
Highways and freeways: National Rural Road: Minor Rural Roads:
TOTAL: 3,913 - - - 6,185 44,742 111,011 161,938 6,668 20,773 151,583 179,024 Circulating vehicles fleet Northern Italy: Central Italy: Southern Italy and islands:
ITALY: - - - ≈11,000,000 17,034,145 7,315,756 9,205,217 33,555,118 22,262,899 10,556,206 16,335,735 49,154,843 Source: Ministero delle Infrastrutture e dei Trasporti
The inevitable consequence of such a lack of development of the infrastructure network is the strong traffic congestion: Italy is now to be the second country in Europe to vehicle density with 1347 veh/km on the main road network (source: Autostrade per l'Italia), 286 veh/km over the entire network comprehensive of motorways, national roads, regional roads and provincial roads (source: Ministero delle Infrastrutture e dei Trasporti). In general, the cause of this lack of development is the lack of financial resources destined not only to the creation of new infrastructure, but also to the maintenance of existing ones: in the years 1990 to 2009 investment in public works have in fact recorded a decrease of
2 over 30%, with a current trend continues downward and with the result that the Italian road network , which in the early '70s was one of the most modern and developed in Europe, is showing signs of 40 years of under-investment and is chasing the nations where infrastructure development has gone hand in hand with the growth of the development of the nation.
Figure 1 – Investments in non-residential construction (millions Euros)
Next to the strengthening of the trans-European corridors and the completion of the TEN-T networks is necessary, now more than ever, to give priority to networks of national importance through the implementation of the principal Italian corridors and improving the efficiency and safety of road traffic on the existing infrastructures. This process can be pursued only through appropriate policies for land use planning, not only made of "great works", but also small and medium-sized operations. Already the New Italian Highway Code, issued back in 1992, contained (Art. 36) information on the adoption of planning tools dedicated to these types of roads, such as the Urban Traffic Plan (PUT) and the Traffic Plan for Rural Road (PTVE).
However, while the PUT were issued "Guidelines for the preparation, adoption, and implementation of the Urban Traffic Plan" (dir. Ministero delle Infrastrutture e dei Trasporti, 1995), nothing has been done or planned for regarding the PTVE.
In accordance with the purposes of such instruments, as indicated by the same NCDS, (the improvement of traffic conditions and road safety, the reduction of noise and air pollution and energy saving), the present work was intended to address some of the elements that each PTVE cannot ignore (the classification of existing roads, the estimation of operating speed and design speed, analysis and evaluation of the risk associated with various aspects of road safety, etc..), and each of them here is treated separately and in detail to arrive at the development of original tools for calculation and application, meaning in this way provide Public Administrations of real application tools that act as a guide for the preparation of PTVE, as well for the planning of the actions to include in them.
3
2. Traffic Plan for Rural Roads: a
general overview
It is widely accepted that there is a need for coordination among Urban Traffic Plans and planning of higher order (including also the PTVE) to avoid that tendency to treat local problems through interventions that can move problems from one area to another without finding a permanent solution, simply moving the criticality of traffic from one point of the territory to another.
To achieve the objectives in a satisfactory manner is therefore necessary to take an overview of the issues, contents and methodologies to be adopted in the preparation of traffic plans at possible different levels.
The PTVE must therefore be composed of a set of interventions, coordinated with those of other planning tools, aimed at improving the road traffic conditions, and must contain general guidelines to encourage the use of collective transport systems and/or alternative. All the actions contained in this plan must also be achievable in the short time period, and should be placed in a framework of infrastructural facilities and transport modes essentially unchanged: this means that it should optimize existing resources.
It should therefore be understood as a plan of immediate feasibility, low financial commitment, and which has the containment of the traffic circulation criticalities as its main objective, which means that the PTVE is a plan in which we must give more importance to maintenance and optimization of functionality of the existing network rather than the planning of new infrastructure.
2.1. PTVE Standards Framework
The processing of PTVE is provided in art. 36, paragraph 3, of the New Italian Highway Code (Legislative Decree no. 285/1992): "Provinces ensure the adoption of traffic plans for the suburban rural road network". Paragraph 4 of the same article also identifies the objectives of PTVE: there is no distinction whatsoever among the objectives of the different traffic plans (for urban and interurban road), which is likely to consider the objectives of PTVE quite similar to those provided for the Urban Traffic Plan (hereinafter PUT). These objectives are to "obtain the improvement of traffic conditions and road safety, the reduction of noise and air pollution and energy consumption, in accordance with the existing planning instruments and the Transport Plans and in compliance with environmental values."
4 The purposes of the above PTVE ranging in fields very different from each other, so in a PTVE there are several areas of investigation and planning, some of which are governed by dedicated rules and laws.
2.1.1 Administrative and functional roads classification
To make a choice on the allocation of financial resources available for the implementation of the interventions included in the plan is necessary to establish the prominent role and use more appropriate that each road of the network must perform within the road network.
The New Italian Highway Code (art. 2) identifies six road categories on the basis of geometric and functional characteristics (from A – Highways to F – local roads).
Again the art. 2 identify four administrative categories for rural roads (national roads, regional roads, provincial roads and municipal roads) and a single category for urban roads.
The DM 6792/2001 (Functional and geometric guidelines for road construction) identifies a functional road classification on the basis of a hierarchical order based on the identification of the role played by the individual road in the territorial belonging context and in the global road network: this classification lead to four functional family (primary network, principal network, secondary network and local network, in decreasing order of importance).
The values of the structural and regulatory standards prescribed shall be considered binding for new roads, and to be considered as a goal to be achieved for existing roads. A classification of existing roads is due to a number of conditions very different between them, which are the result of choices, behaviors and habits of users, surrounding environmental conditions and spatial features in flux, as well as choices and activities carried out by various public administrations, that very often do not reflect conditions directly and objectively observable as, for example, the geometrical characteristics.
In the year 1998 the Italian commission for research CNR developed a document to try to solve the problem for the classification of existing road, described in Figure 2, but it was really difficult to apply and it remained at a pre-standard level.
5 Figure 2 - Block diagram for the classification of existing roads, according to the methodology of the CNR
2.1.2 Road Safety: an overview on road safety devices Italian regulatory and laws
The main PTVE objective is to achieve improvements in traffic circulation conditions and in road safety. To achieve these goals it is necessary to thoroughly investigate different areas, including for example the functionality of road safety devices and the maintenance level of pavements.
The argument of safety devices will be discussed in depth up to reach the development of an application tool useful to estimate the level of risk associated with each device installed or to be installed on a road network. Because of the importance that this argument holds in the development of this work and the level of detail achieved, it is necessary to proceed with a detailed analysis of the relevant Italian regulations.
Until 2004 the issue of the installation of road safety devices was regulated by national laws. The DM 2367/2004 has been transposed European Standard EN1317, which in its various parts identifies a common criterion for the installation and maintenance of road safety devices, and establishes the criteria for the construction and approval of such devices. Subsequently, following the updates of the same standards EN1317, were drawn up some explanatory Italian documents for the correct application of the DM 2367/2004: these are the circular no. 3065/2004 and n. 0062032/2010. Finally, with the
NO SI NO * NO SI SI ** SI NO NO SI NO SI SI confronto con i requisiti ex art.13/c.4 determinanti
ai fini classifica e desunti dai requisiti ex art. 13/c.1 (nuove norme geometriche e funzionali) difformità localizzate la classe ottenuta è quella attesa ? CLASSIFICAZIONE IN
DEROGA NELLA CLASSE OTTENUTA + CLASSIFICAZIONE IN DEROGA NORME DI SALVAGUARDIA Risponde ai B.U. CNR 78/'80 *** e 60/'78 **** SI RITIENE DI MODIFICARE LA CLASSE ATTESA ? CLASSIFICAZIONE SI RITIENE DI MODIFICARE LA CLASSE ATTESA ? verifica sicurezza e inquinamenti la classe ottenuta è quella attesa ? SI vuoi prendere (se possibile) in considerazione una classe inferiore CARATTERISTICHE RILEVATE (dalla Fase 2) NON CLASSIFICABILE
* Le difformità sono generalizzate. ** Conseguente modifica del Piano di Fase 1.
*** Limitatamente a cap.2: paragrafi 2.1, 2.2, 2.3, nonché a cap. 3: paragrafi 3.1, 3.2.2, 3.2.3 e 3.3.2 (soltanto per la parte relativa ai raggi minimi).
**** Limitatamente a paragr. 2.1, a TAB.2 di paragr. 3.1 (colonne A,B,C,D,E,F)ed a TAB.3 di paragr 3.2 (colonne relative a velocità min., pt min., raggio min., raggi minimi raccordi convessi e concavi).
CLASSE O CLASSI ATTESE (X0,Y0) o X0 (dalla Fase 1) CLASSIFICAZIONE NELLA CLASSE OTTENUTA + NORME DI SALVAGUARDIA prendi in considerazione la classe inferiore confronto con i requisiti derivanti dall’ Art.2 comma 3 NO SI NO NO NO SI **
6 Ministerial Decree of 28/06/2011 (published in Official Gazette 233/2011) were enacted additional provisions on the use and installation of road safety devices, receiving the contents of the European Standard EN1317-5: 2007 + A1: 2008 regarding the CE marking.
The current legislation is composed by the following (with original Italian name):
- DM 223, 18/02/1992 “Istruzioni tecniche sulla progettazione, omologazione ed impiego delle barriere di sicurezza stradale”
- DM 03/06/1998 “Istruzioni tecniche sulla progettazione, omologazione ed impiego delle barriere di sicurezza stradale” except parts replaced by DM 2367, 21/06/2004
- DM 2367, 21/06/2004 “Istruzioni tecniche per la progettazione, l’omologazione e l’impiego dei dispositivi di ritenuta nelle costruzioni stradali”
- DM 28/06/2011 “Disposizioni sull’uso e sull’installazione dei dispositivi di ritenuta stradale” - UNI EN 1317 – Road Safety Devices
- UNI CEI EN ISO/IEC 17025 – General requirements for the competence of testing and calibration - DM del 05/11/2001 “Norme funzionali e geometriche per la costruzione delle strade”
- DM del 19/04/2006 “Norme funzionali e geometriche per la costruzione delle intersezioni stradali” - Circ. 3065 del 25/08/2004 “Direttiva sui criteri di progettazione, installazione, verifica e manutenzione dei dispositivi di ritenuta nelle costruzioni stradali”
- Circ. 3533 del 20/09/2005 “Direttive inerenti le procedure ed i documenti necessari per le domande di omologazione dei dispositivi di ritenuta nelle costruzioni stradali ai sensi del DM 2367/2004” (for all still appliable parts)
- Circ. 104862 del 15/11/2007 “Scadenza della validità delle omologazioni delle barriere di sicurezza rilasciate ai sensi delle norme antecedenti il DM 2367/2004” (for all still appliable parts)
- Circ. 62032 del 21/07/2010 “Uniforme applicazione delle norme in materia di progettazione, omologazione ed impiego dei dispositivi di ritenuta nelle costruzioni stradali”
- DM del 28/06/2001 “Disposizioni sull’uso e l’installazione dei dispositivi di ritenuta stradale” - DL 285 del 30/04/1992 e s.m. “Codice della strada”
In the original document there is a depth analysis of the most important parts of DM 2367/2004 regarding installation of road safety devices. Here is important to outline following facts: the actual laws do not impose an adjustment of the existing devices to the new standard requirements except in special cases (extraordinary maintenance); the actual laws is intended only for new road constructions and for that roads projected with a speed of at least 70km/h. All other case must be evaluated case by case.
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2.1.3 Road Safety Audit and the instructions contained in DL 35 – 15th april, 2011
The use of a system of control for road safety (procedure known as Road Safety Audit) is certainly an appropriate tool for both new and existing roads, for which are more evident the need for action to reduce the consequences of accidents.
The Road Safety Audit is a procedure established in the United Kingdom for verification, in terms of safety, of road projects related to new infrastructure, and has only recently been extended to existing roads as part of a broader strategy aimed at raising the level of road safety by detecting the presence of potential risk situations alongside road: it can be very useful especially because it can tackle the problem from the point of view of road users.
Because PTVE must manage existing roads, it proves useful to describe here the main objectives of the RSA procedure applied to existing roads. The objectives can be summarized in two main points:
- identify and assess those potential risky situations or that may exacerbate the risk exposure of road users, which means identifying the technical, design and operational requirements that are not responsive to the actual conditions of use, before the statistics accident denounce the presence of dangerous situations;
- suggest corrective measures to reduce or eliminate the potentially hazardous situations from the point of view of road users.
The key criteria upon which the whole procedure of RSA must therefore outlines all situations of potential danger to road users. These criteria must therefore take into account those properties that are relevant for the road safety, which can be briefly summarized in the following figure.
Figure 3 - Criteria for appreciating the road safety
The Guidelines introduced by art. 8 of DL 35/2011, implementing the European Directive 2008/96/EC, establish criteria and procedures for the conduction of road safety audits on projects and safety
8 inspections on the existing infrastructure, and the implementation the process for classifying the safety of the road network.
To date, the provisions contained in Legislative Decree 35/2011 are mandatory for infrastructure including the Trans European Road Network (TEN - Trans European Network) defined according to the European Decision 1692/96/EC and subsequent amendments, while for all other roads the contents of the decree are rules of principle until they become mandatory on the basis of the temporal evolution of the field of application.
For the remaining streets of competence of the regions, autonomous provinces and local authorities the provisions of DL 35/2011 will provide even rules of principle, to be followed in the preparation of their own discipline on the management of road safety.
This last activity consists in maintaining the efficiency of infrastructure, aimed at achieving an appropriate level of security, through proper routine maintenance and repairs, carried out according to a precise and rigorous programming according to the order of priority of interventions, identified by the outcome of the inspections.
2.2. PTVE objectives
The PTVE, dealing with the management of existing resources in the field of rural roads, pursues the objective of improving the circulation through the improvement of road safety (objective explicitly indicated by NCDS), since the problems of congestion and noise pollution/environment are more marginal compared to the urban situation. It follows then that all the choices that are made within a PTVE, mainly in the field of maintenance and optimization of existing infrastructure, must always have as its primary objective of increasing the level of road safety.
Improve the level of road safety is also involved in the DL 35/2011: it is therefore desirable that a PTVE is deeply integrated with the criteria described here for the analysis of road safety. In particular, the PTVE has as its scope the rural road network falling under the direct management of the provinces. In general, this network do not fall within TEN (Decision 1692/96/EC), nor within the network of national interest (DL 461/99), so the provisions contained in the decree having the character of rules of principle to be followed in the preparation of the discipline inherent road safety that regions, autonomous provinces and local authorities must prepare before year 2020.
The road safety is therefore the goal which should be pursued in dealing with all the issues that should and/or can be part of PTVE. The achievement of these targets will be easier by the implementation of databases containing all the information needed to better manage the entire road network, to organize
9 maintenance work, in order to control the supply and demand of mobility. The creation of the database may follow different methodologies: with respect to the database regarding the security level of the network, it may no doubt be followed, at least in principle, the provisions of Legislative Decree 35/2011 and outlined in the following figure.
Figure 4 - Methodology for business process management of road infrastructure
2.3. PTVE structure
A PTVE can therefore be structurally divided into two parts: a first part that contains the cognitive picture of the current situation, in which is photographed the actual state of infrastructures on which the PTVE must be applied, both in terms of characteristics and of maintenance level; a second part containing all the planned activities resulting from analysis of the status quo, in which to develop simulation models of different future scenarios provided for the provincial road network, information systems to support the management, maintenance and design roads, administrative and functional road classification, and so on.
10
2.3.1 Knowledge framework
The first part of PTVE, developed about the knowledge framework of the supply infrastructure level can be developed on the basis of the following points.
2.3.1.1 Infrastructural supply characteristics
This first point can match with the first part of the RSA procedure.
Figure 5 – Network analysis phase block diagram
Here, the analysis must be developed, at the network level, about the management of the road network in the province. Here can be also deepen the description of the characteristics of the traffic by dividing the whole network into homogeneous sections, as required by the Guidelines contained in DL 35/2011, indicating homogeneous section as a minimal element of reference for the description of the state of the road network and maintenance needs. The division into homogeneous sections of the whole road network must follow the article 4 of NCDS and article 4 and 5 of its Implementing Regulation (DPR 495/92), regarding the demarcation of the urban center. This is a relevant aspect for the management of roads: being the homogeneous section the minimal element of reference, for those that fall within a urban area worth the provisions of Article 7, paragraph 3, of NCDS, that with the aim to avoid overlap gives the municipality the right power to regulate the traffic on the roads also non-proprietary (following an opinion issued by the owner administration) with the exception of the measures for the protection of road assets and technical requirements, as well as signage on the geometric and structural characteristics of the road, which remains to be met by the owner (Article 37, paragraph 1, letter d).
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2.3.1.2 Transport demand analysis
Travelers and goods moving in a certain area are the demand for mobility using the transport service offered by the system: it is clear that the infrastructure and transport services are closely joined to the need to meet the mobility needs. Conversely, the choices of travelers can significantly influence the performance of the elements of the offer bringing them till the congestion.
A simple moving produces no utility in itself, but is rather a complementary activity to other activities to be carried out in different places than the one where the moving originates: people travels to reach their workplace, the places of study, for entertainment, shopping, etc.; goods are transported from the place of production to those of consumption, and so on. The demand for mobility is therefore a derived demand, the result of the combined action of spatial planning and the transport system, as well as user habits.
One of the possible definitions of transport demand may be as follows: it is the number of users with certain characteristics that consumes the service offered by a transport system in a predetermined period of time, or as a flow.
The demand for transport is, therefore, the need for users to move from one place to another with the different possible transport modes. Both in the case of transport by private car or by public transport, the transport demand is always be characterized by an O/D matrix (origin/destination), consisting of a number of rows and columns equal to the centroid identified within the territory and among which trips are made. For simplicity, a first subdivision can be made on the basis of the Italian Statistics Institute (ISTAT) census areas, but then refine it further by combining or subdividing the areas identified above as appropriate. The aij element of the matrix indicates the number of users moving from ith to the jth zone.
The assignment of the application to the transport network by mathematical models and software dedicated, after construction of the entire network model, is used to determine the traffic flow on the whole network, that play a relevant role both in road design and in road safety analysis and management.
2.3.1.3 Maintenance level of the road network
Article 14 of the NCDS says that "Public Administrations owners of the roads, in order to ensure the road safety, ensure the: a) maintaining and cleaning of the roads, their pertinence areas and furniture, and equipment systems; b) monitoring technical efficiency of roads; c) placing and maintaining the required signs."
12 In order to fulfill these tasks is useful and necessary to develop some information tools that enable you to monitor and summarize the situation of the state and functionality of the entire road network in order to best allocate available resources to solve/improve all that situations at the highest risk level in terms of road safety. The aspects related to road safety are manifold: it is therefore necessary to have, in an age of digitized information tools, databases related to maintenance, to the state of repair and to the characteristics of road surfaces, safety devices, signs, etc. Each of these databases can be realized with all the data that will be available only after appropriate inspections to be conducted on-site: this activity is once again in line with the provisions of the Guidelines contained in DL 35/2011. The methods of inspection and control are established independently by each Administration, and can be introduced and described in this part of PTVE.
The analysis of data collected in the database, to be performed with procedures and methodologies developed by each Administration, provides an overview of the state of maintenance of the provincial road network and an estimation of the resources necessary for its maintenance.
2.3.1.4 The road safety
This part can be identified mainly with the second part of the analysis phase described by the RSA procedure (Figure 6).
The road safety planning deals with analyzes, evaluations and project developments in the field of road safety in accordance with the provisions of the National Road Safety Plan. The activities should be aimed at:
- Develop and strengthen the capacity of government to improve road safety;
- Encourage the formation of a new road safety culture not only for users but also for decision makers and technicians that can contribute to the improvement of road safety;
- Promote the development of highly effective interventions that, for an identical level of available resources, would achieve the broadest and most rapid reduction of the number of road accident victims.
Since this is still a fact-finding phase, it is therefore necessary to conduct joint analysis with the accident data, with the aim of identifying those sections of road (at the level of single homogeneous section of that entire path) for which interventions are needed for the mitigation of consequences of possible accidents or the elimination of their possible contributing factors related to the infrastructure.
It is, therefore, through a deep analysis of accident data, conducted by the evaluation of appropriate statistical indicators, to arrive at a classification of the single homogeneous section from a point of view of the accident. The main data needed in order to reach such a classification is according to the chosen methodology and its selected indicators: some examples can be found in the literature, from the one
13 proposed by the Italian Research Commission CNR [63] to the one contained in the Switzerland Statement SN640009 [71], until the methodology developed by R. Lamm [29].
Consequently, it must be possible to reach the elaboration of specific studies to determine the risk classes of the investigated road sections, with the aim of optimizing resources aimed at managing all aspects related to road safety and to plan all those measures deemed necessary on the road network.
2.3.1.5 Road Network Critical aspects
At the end of this learning phase, it is therefore possible to draw up a report on the analysis of the maintenance level of the network, obtaining an overall picture of the same criticality refers to a single homogeneous section especially from the point of view of road safety. The results derived from this analysis can be crossed with the accident data to identify that section with high risk and to arrange procedures for Road Safety Audit on the basis of the provisions of the Guidelines contained in DL 35/2011.
In addition, depending on the classification of road safety resulting from the previous analysis, it is possible to prepare the plan of inspections (the second phase of the Guidelines contained in Decree Law 35/2011, Figure 6), which initially will be focused on those homogeneous sections identified as most critical.
Figure 6 – Check phase block diagram
The final result of the fact-finding phase is therefore description of the entire road system criticism, and is the start point for the second part of PTVE: here it is possible, on the basis of the geometric characteristics of the road, the functional classification of TGM and all other data collected, to define the level of service and then to plan interventions to be included in the second part of the PTVE.
14 The resulting classification inspection is a road network classification performed from a point of view of road safety, which is necessary for the planning and programming of interventions (the central themes of the second part of the PTVE). Only thanks to a joint assessment of the analysis of accidents and the results of the inspection, it is actually possible to identify the causes directly attributable to the infrastructure, thereby eliminating external causes and highlighting all the criticism of the same infrastructure: this means that it will be possible to identify that sections where Administration must concentrate its efforts firstly.
Figure 7 - Detail of the classification stage aimed at planning of interventions
2.3.2 Methodological activities aimed to the management of infrastructures
This second part of a PTVE can be developed on the basis of the following points.
2.3.2.1 PTVE main targets
Main targets to be reached are a direct consequence of the whole analysis contained in the first part of the same PTVE. These targets are generally related to the development of the management of the whole road network, from the maintenance of existing roads to traffic and accident statistical analysis, from the planning of new infrastructures to the improvement of the actual level of road safety, till a development of tools aimed to an administrative and/or functional network classification.
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2.3.2.2 Mathematical model support tools for scenarios analysis
Scenarios analysis must be carried out necessarily using appropriate modeling tools that provide results deriving from all the possible choices, and to analyze the form a point of view of the PTVE targets. This modeling phase requires to properly build the entire network by appropriate dedicated software (eg. TransCAD, Visum, etc.).
Developed the network model, in order to obtain reliable data it is necessary that the transport demand used within the model is constantly updated, and the model properly calibrated and refined by successive steps until correct representation of reality is reached. To have a calibrated and validated mathematical simulation model of traffic flow enables you to analyze the different impacts provided by any new infrastructure or improving the existing ones in the traffic flow conditions, in order to identify those that best meet the application requirements; in addition, results from the model can be supportive for all analyzes of the assessments of the risk levels of the roads (TGM is, in the procedures for risk assessment applied within the road, the parameter of risk exposure).
In this, data collected through a monitoring system are of fundamental importance, and can be also used to develop mathematical models aimed at calculating the operating speed relating to homogeneous sections or entire tracks.
In addition to the mathematical models of the network, can be useful to have more mathematical models relating to particular issues, including models for the estimation of operating speeds or taxonomic patterns for the functional classification of roads, all topics deeply treated in the following chapters.
2.3.2.3 Functional and administrative road network classification
As previously noted, very often the administrative and functional classification of roads does not reflect the role played by them within the territory.
The analysis of the operation of the road network must then be made by determining the characteristics of homogeneous sections from a point of view of both geometrical and functional. The review of the administrative and functional classification of roads, carried out in accordance with NCDS, allows you to review the maintenance needs by the importance of the same within a broader and more complex territorial system, and therefore to efficiently allocate economic resources available in order to obtain the maximum possible benefit.
In this phase it is also possible to operate a first distinction between main and secondary roads, and by the use of certain corrective coefficients identify acceptable levels of maintenance of the second lower than the first, with a consequent reduction of expenditure and further optimization of economic resources.
16 A valid help to achieve a classification of the road network from a purely functional point of view is given by the possibility of using mathematical models (for example, taxonomic models) that provide important information about the role played by each road.
2.3.2.4 Road Network Maintenance Plan
A maintenance plan must find an answer to the 4 basic questions: where to intervene, how to intervene, when to intervene and with which economic resource intervene. It is then necessary to know the state of the road network maintenance level under different aspects.
It is here possible to generalize the concept of PMS (Pavement Management System), a tool aimed to maintenance planning of road pavements that, linked to the road cadaster and the administrative and functional classification of roads, allows effective and efficient use of available resources.
According to the American Association of State Highway and Transportation Officials (AASHTO) a PMS can be defined as "...the effective and efficient orientation of the various activities involved in the maintenance of road pavements in acceptable conditions for the traffic flow, at the lowest possible cost of their life cycle.” Hudson and others (1979) define a PMS instead as a "...series of coordinated activities, all aimed at achieving the best possible use of public funds made available for the smoothest, safest and most economical performance of pavements".
Generalizing the concept just expressed, therefore, it is theoretically possible to prepare a Plan for the Maintenance of the Road Network as an integral part, if not predominant, of a PTVE. This Plan can be organized in such a way that make possible to coordinate all procedures involving planning, programming, design, construction, maintenance and recovery, in order to determine the best maintenance program in a given reference period in relation to available economic resources. In this sense, a PMRS is a great help to decision makers for finding optimal strategies for maintaining an acceptable level of operating conditions of the network in a given period.
A PMRS, like PMS, can be developed on two operating levels: network level and project level. At the network level, the main purpose of PMRS is to develop the maintenance plan and establish priorities among the various programs planned. That comes with a number of steps: the definition of the road network, the importance of the maintenance condition of the different elements (safety devices, pavements, signs, lighting, etc.), the prediction of the evolution of these conditions, the action planning at the network level. The network layer is a cognitive framework that, flanked both the data contained in the road cadaster and the data collected during the first phase of PTVE, allows us to identify optimal strategies for maintenance work.
At the project level, then, such decisions are taken to pursue those targets (costs, service levels, etc.) identified at the network level.
17 In order to define these targets, at the network level we need a large amount of data, accurate processing, and reliable predictive models: even if only one of these aspects is not properly defined, the solutions that are identified at the project level will not be optimal and does not reach the targets identified.
A PMS can be represented by the following scheme.
Figure 8 – PMS schematic diagram
Therefore appear extremely obvious advantages of an accurate and precise scheduling of maintenance, as this makes possible the optimal management of economic resources in relation to the equally optimal management of the pavement. The same principle can be applied to the more general PMRS: the timeliness of intervention is, indeed, a necessary means for the proper and optimized allocation of economic resources available, before the operating conditions reach unacceptable levels, and the costs to restore the functionality dramatically grow.
2.3.2.5 Targets achievements
It is the end point of the PTVE. All previous analysis, elaborated in the previous sections, allows you to identify the general criteria (guidelines) relevant to the choice of possible project alternatives to maximize the benefit obtained in terms of improving traffic safety, while optimizing the economic resources available: i.e. you must identify those minimal management actions, almost immediately feasible, characterized by a high cost/benefit ratio.
18 Figure 9 – Actions carrying out details
2.3.2.6 Actions efficacy evaluation
At this point the choice of measures to be realized has already been carried out on the basis of information provided by mathematical models prepared in the previous steps. However, it is necessary to carry out a field assessment of the real impact that these actions have produced after their implementation. It is possible to use, for example, cost/benefits analysis of before-after type, characterized by relative computational simplicity and good results. Measuring the actual effects of the measures introduced must in any case take into account both the statistical variability of the phenomenon and the influence that the external factors, as well as the observation period (which must be chosen in an appropriate manner ).
19
2.4. Necessary data to collect to develop a PTVE
It is necessary to describe now the data to be collected to realize the basic structure of a generic PTVE just described.
In subsequent chapters will then be detailed mathematical procedures for the analysis of the data collected.
2.4.1 Mobility and transport demand data
The transport demand takes very different characteristics depending on the spatial characterization which it relates. A PTVE, dealing almost exclusively on issues related to rural transport networks, should be reported to a transport demand that is characterized essentially by medium/long-distance trips, mainly with work or study purposes. With the increasing of distance to be traveled, is expected for users to make use of roads of gradually increasing category, generally characterized by lower travel time per kilometer.
There are several methods for detecting the distance and the origins/destinations of trips. It is preferable to work on complete information regarding the whole population living in the area of study: in this case we can pass over all possible issues and approximation deriving from sampling procedures. Work on information of the whole population means to collect demand data in a census way. It can be achieved by dividing the territory according to ISTAT census municipalities, and then assigning at each of them a demand concentrated in a centroid considered barycentric regarding the resident population. In this case, we can use the statistical data provided by the same ISTAT regarding trips between municipalities.
At the end, we will have an O/D matrix characterized by a number of rows and columns identical to the number of municipalities within the provincial territory. This type of modeling is not without flaws: the most obvious is of course that is not taken into account in any way domestic trips within a certain municipality, that could happen along the road network under study.
This type of scheme could be definitive, or can be improved by dividing/merging two or more municipalities to obtain more reliable data to analyze.
With the advancement of technologies, it is now easier than ever able to update the O/D matrix using mathematical models based on measurements of traffic in a certain number of road sections (for example by using the Le Blanc algorithm, 1973): in this case we can maintain the O/D matrix always up-to-date. Traffic counts are a very attractive source of data, because generally available, inexpensive (especially compared to survey methods, such as eg. the method of interviews) and feasible in an automatic way by making use of electronic equipment now widely accepted.
20 Also these procedures must be provided by a PTVE, also providing scheduled plans for traffic counts period. This means that a Public Administration that wants to adopt a PTVE must be provided with adequate instrumentation for traffic counts, both fixed-type and mobile-type.
At the end of the analysis of statistical data, measuring traffic flow and analysis of the gathered information, will be available an O/D matrix that will be reliable, up-to-dated and well suitable to reality.
2.4.2 Accident data
Most of the procedures that may be introduced in a PTVE, also in relation to the basic structure described above, need of statistical data regarding accidents, also useful for the estimation of the efficiency of the actions provided by PTVE through the calculation of appropriate performance indicators (eg. assessment of the reduction in the number of accidents). Accidents analysis falls within the characterization of routes in terms of road safety, which allows you to highlight, in relation to a statistically significant threshold comparison, the level of accidents involving the entire route.
One of the possible procedures is the one provided by Italian Research Institute CNR, or the one proposed by Switzerland Standards SN640009.
Finally, also performance indicators can be related to accident data: following the primary objective of a PTVE (the improvement of road safety), in order to assess the efficiency in the medium term it is possible to refer to the reduction of accidents, or the reduction in the severity of their outcomes, as a possible yardstick.
It is therefore of fundamental importance, in the development of a PTVE, the availability of accident data constantly updated, also using digital databases
2.4.3 Infrastructure data
Infrastructure data represent the corpus of the basic data of a PTVE: in these data is included everything about the cognitive aspect of the infrastructure network where a PTVE must be applied, from pavements to safety devices, from signs to possible obstacles placed along roads, all of which closely relative to the broader aspect of road safety.
All these data can only be collected by a census, to plan and to conduct on the whole road network. Furthermore, the data must be kept up-to-date: having stale data, otherwise, would means to operate with an inadequate instrument that hardly reflect the real needs of the network. We also need all of the geometric data such as road width, radius of curvature, length of the straights, and anything else that might be deemed necessary. These data can be collected by two way: planning and conducting topographic surveys, method certainly more precise, but also extremely costly and therefore realistically applicable only in all those cases where high precision is required; or using a CTR cartography (scale
21 1:10,000 or 1:2,000) and subsequently extracting geometric data: this method is certainly faster and cheaper. With the second method, however, cannot be evaluated the side slopes, which must however be found with in-site surveys or can be estimated according to a mean value representative of the actual conditions.
Pavements: requires census of all types of degradation detectable on the pavement, the relative extension and the location on the roadway. It is considered useful to distinguish the type of degradation according to a well-defined classification method that diversifies between degradation of structural type from those of the functional type, because their recovery has deep differences with regard to ways, cost, time and efficiency for the purpose of safety.
Safety devices: as for pavements, also for safety devices is necessary a census of all types of degradation detectable. Also in this case it is essential to differentiate the problems encountered according to the type, distinguishing between defects of a structural nature, an accessory nature and a functional nature.
You also need to point out that between safety devices are not covered by the parapets of bridges, stone walls and other similar items, as part of more complex structures for which there are special procedures for investigation and census.
Black points and obstacles: simultaneously with the census of the safety devices, is necessary to provide a census of obstacles on the roadside. It means to collect data concerning the type of the obstacle, its distance from the roadway and position with respect to it, in order to evaluate its possible protection or removal, and in general in order to take all the possible countermeasures designed to knock down the risk level.
Signs: in this case must be collected data related to the function of both horizontal and vertical signs, the adequacy to the function they are to perform (including in relation to its clarity and efficiency in communicating to road users the correct information) and the state of maintenance using retroreflection measures both in day and night conditions.
Homogeneous sections: these are the result of a subdivisions that can be made on the basis of different data collected during the census (location of intersections, whether variation in the tracks originating traffic volumes, urban areas, etc.). From this combination it will be possible to obtain homogeneous sections effectively under all points of view, as shown in the following Figure 10.
22 Figure 10 – Homogeneous sections example
In addition, the identified homogeneous sections also need to be characterized by a working length, meaning that sections of a too small extension cannot be considered functional for the implementation of possible maintenance actions: it is always important to keep well clear the operational nature of the PTVE, for which therefore requires procedures that can be employed in directly on the field.
2.5. Support tools
The bulk of the data to collect is huge and would necessitate appropriate tools that allow proper management.
The organization of data in geo-referenced databases is undoubtedly a great advantage, since it allows you to have an immediate and effective view of the data.
There are essentially two possibilities: the organization of data in GIS database, and the organization of data in KML database, used for example by Google Earth software (KML stands for Keyhole Markup Language: KML is an XML grammar and file format for modeling and storing geographic features such as points, lines, images, polygons, and models to be displayed in Google Earth, Google Maps and other applications; such as HTML, KML has a tag-based structure with names and attributes used for specific display purposes). The second one has the advantage of immediate readability of the database referred to a satellite image positioning, but modification of the database is more complex and the creation itself requires knowledge of KML. In addition, a further advantage of KML is its portability: the display of KML databases is now possible on any device (mobile or not), which also allows an operator to use a file build in this language directly on the field.
23 Figure 11 – GIS accidents database example
Figure 12 – KML database example for safety devices
2.6 Performance indexes
A PTVE must always achieve the ultimate aim of improving traffic conditions in terms of road safety. This implies that within the same PTVE must be developed some procedures for evaluating the efficiency of the planned actions. These procedures must necessarily be based on before-after performance indexes, which are able to assess the differences between the previous situation (presumably the current status) and the one after the planned action.
24 For each area of action can be also provided specific performance indicators, (for example, regarding the safety devices may be useful to assess the risk level and the global efficiency index before and after the actions).
However, since the PTVE has as its purpose the general increase in the level of road safety, it is useful to have also an indicator of overall performance, which provides a measure of the efficiency of the whole corpus of actions implemented into the plan. The indicators to prefer are regarding accident rates, that make possible to provide adequate evaluations on the dangerousness of each homogeneous section as a function of its actual use (for example, according to their actual traffic flow): a comparison between the value of these indicators before and after the carrying out the planned measures provides a clear indication of the efficiency of actions themselves. In addition to the accident rate, it is useful to evaluate also the frequency of accidents (expressed in function of the extension of the single homogeneous section) and the number of accidents. All three of these performance indicators should be evaluated on the number of accidents involving fatalities and/or injuries, due to the difficult to trace the deaths resulting from accidents at a later date (i.e., fatalities due to consequences of the accident took place but after some time from the accident itself).
The following table shows the possible performance indicators, their priority (i.e. the importance of the information that can be obtained from each indicator) and the unit of measure. Priority 1 indicators provide further information, as the data of the number of accidents/fatalities/injuries is correlated with both the extension of homogeneous section and their traffic flow. Priority 2 indicators provide information on the number of accident/fatalities/injuries related to sections characterized by a standard extension of 1 kilometer.
These are obviously complementary indicators, as everyone is able to represent and provide information on specific aspects of road safety.
As a further indicator of the level of safety of the road network, it may be useful to evaluate the potential safety (SAPO), defined as the potential in accidents reduction and in lowering the cost associated with this reduction.
Table 2 - Performance indicators
Priority Performance index Measure unit
1 Fatalities accident rate . ℎ
ℎ ∙
1 Injuries accident rate . ℎ
ℎ ∙
1 Accidents rate on traffic flow .
25
Priority Performance index Measure unit
1 Fatalities rate on traffic flow .
ℎ ∙
1 Harmfulness rate on traffico flow
. + . ℎ ∙
1 Injuries rate on traffic flow .
ℎ ∙
2 Fatal accidents frequency .
2 Accidents frequency . 2 Fatalities frequency . 2 Injuries frequency . 3 Fatality rate . . 3 Harmfulness rate . + . . 3 Injury rate . . 3 Number of fatalities . 3 Number of injuries . 3 Number of accidents .
It is necessary in this case to know the average social cost of accidents; however, in literature there are numerous studies on this topic (for example, Studio di valutazione dei costi sociali dell’incidentalità stradale – Anno 2010, prepared by the Italian Ministero delle Infrastrutture e dei Trasporti), so as a first approximation it is possible to refer directly to the results of these studies without providing further investigations and calculations. If the SAPO is assessed in relation to the expected reduction of accidents, this can also be used as a parameter to evaluate a priority for intervention and therefore the choice of actions and measures to be implemented firstly, as it makes it possible to identify those homogeneous sections where the expected efficiency of the planned actions is potentially greater than others.
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3. PTVE developed elements
In this chapter are described the procedures developed to: - Design speed evaluation for existing roads;
- Operational speed estimation for tangent elements;
- Risk level evaluation deriving from pavement maintenance level;
- Risk level evaluation deriving from existing safety devices and from needed safety devices; - Accidents analysis;
- Black points identification;
- Administrative and functional road network classification by taxonomic models.
Is also presented, from a theoretical point of view, an analytical treatment of the main performance indicators, some of which actually used in the calculations.
3.1 Design speed for existing roads
The design speed range is defined by chap.1 of DM 6792/2001 (Norme funzionali e geometriche per la
costruzione delle strade): it is the range of speed according to which must be defined the characteristics
of the various elements of a roadway (straights, variable radius curves and circular curves).
It is clear that this parameter is known for new construction roads but it isn’t for older infrastructures, which derive from older road often simply paved during years.
In these cases, the VP parameter can be evaluated by assimilation with the characteristics of road categories described by actual laws and standards. Into circ. 62032/2010, regarding safety devices installation, we can read this suggestion: “in case of actions to be planned on existing roads, the design speed must be evaluated by assimilation, on the basis of DM 6792/2001 “Norme funzionali e
geometriche per la costruzione delle strade”, for the same functional category and the same radius of curvature of the section of the road”.
Since the problem of design speed estimation exists only for the existing roads, it was developed a procedure that allow evaluating this parameter as a unique value or as a range, only on the basis of geometric data of the road. Based on this consideration, it was used the formula proposed in paragraph 5.2.4 of the DM 6792/2001 for the evaluation of transversal slope for a circular curve in function of its radius
27
∙ 127 (1)
where:
q transversal slope
fT maximum transversal grip coefficient, inversely proportional to the design speed
R radius of curvature
VP design speed
For the purposes of this procedure, we need first the geometrical data described in the previous chapter, derived from topographic surveying or cartography.
In case of use of geometrical data derived from cartography, for q parameter in (1) it is possible to assume a medium value taken on the basis of experience and measurements carried out on a sample of roads in different conditions and representative of all possible conditions detectable on infrastructures under investigation. This value can be assumed equal to 0.035, corresponding to a medium transversal slope of 3.5% (within the range provided by Italian Standards DM 6792/2001).
Regarding fT parameter, the standards DM 6792/2001 provided the following maximum values, as a function of the design speed VP:
100/ % 0.11
40/ % 0.21
In our case, VP is the parameter to evaluate. Because of DM 6792/2001 provides the following table for fT parameter, we can evaluate the fT parameter as a function of VP, obtaining in this case a form of (1) where VP is the only parameter.
Table 6 – Transversal grip coefficient as a function of VP and road category
28 Figure 14 – Correlation between VP and fT
By using multivariate linear regression analysis, we identified regression curve that best fits the data. This curve, of parabolic shape, takes the following form:
0.000012054 ∙ (− 0.003355357 ∙ ( 0.325285714 (2) Then, (1) can be written again in the following form:
∙ 127 (0.000012054 ∙ ( − 0.003355357 ∙ ( 0.325285714)
(3)
where VP is the only unknown parameter. By assuming the following coefficients: , 0.000012054
-, −0.003355357 ′ 0.325285714 the deriving VP equation has a parabolic shape:
( ∙ 127 ∙ ′ − 1) ∙ ( ( ∙ 127 ∙ -′) ∙ ( ( ′) ∙ ∙ 127 0 (4) where its solutions are:
(/, −( ∙ 127 ∙ -′) ± √∆2 ∙ ( ∙ 127 ∙ ′ − 1) (5) and because of the right solution is the one that gives positive values for VP, we can obtain the design speed for the bend i-th as:
(,4 −( ∙ 127 ∙
-,) − √∆ 2 ∙ ( ∙ 127 ∙ ′ − 1)
(6)
It is necessary also to introduce a lower and an upper limit to the value of VP,i just evaluated. These limits should not be applied to 180 degrees turns (or similar), because DM 6792/2001, in chap. 1,
y = 0.000012054x2- 0.003355357x + 0.325285714 R² = 0.996212625 0 0.05 0.1 0.15 0.2 0.25 0 20 40 60 80 100 120 140 160 f T p a ra m e te r Project Speed Vp (km/h) fTparameter
29 expressly says that “for mountain roads it is generally not possible to follow project criterion here contained”.
For purposes of this research, for bends with a center angles higher than 90 degrees, we can evaluate the design speed ,46 as a function also of the center angles according with the following Table 7:
Table 7 – Assumptions for the evaluations of 78,9: ,4 < 30/ℎ
<4 ≤ 90° ?@,AB = CDEF/G <4 > 90° ?@,AB = ?@,A 30 ≤ ,4 ≤ 100/ℎ ∀<4 ?@,AB = ?@,A
,4 > 100/ℎ ∀<4 ?@,AB = JDDEF/G
We assume the lower limit and the upper limit respectively of 30km/h and 100km/h as representative of rural roads in case of high speed bends and high center angle bends.
It is the possible to evaluate the average design speed and the weighted average design speed of an entire road: ,KLMNKOM = ∙ P 1 ,46 Q 4R/ (7) ,SM4OTUMV ∑ X,4 6 ∙ Y4Z Q 4R/ ∑ YQ4R/ 4 (8)
where Li are the extensions of each single bend within the road under investigation. The evaluation of VP,weighted makes possible to pass over the limit deriving by the fact that the simple VP,average is evaluated on the basis of the geometrical characteristics of bends only: the VP,weighted makes possible to take in account the influence of the design speed of each single bend on the value of VP,road.
Another important advantage of the VP,weighted is to make possible the analysis of the homogeneity of the path: by a theoretical point of view, in case of a path with homogeneous geometric characteristics (regarding bends only, in this case), we have VP,weighted and VP,average values almost close. Contrariwise, if we obtain significantly different values for VP,weighted and VP,average, it means that the path can be divided into two or more sections, each one with homogeneous geometric characteristics.
This homogeneity analysis can be performed by the evaluation of the variance of,46:
X,46Z 1
∙ PX,46Z− ,KLMNKOM Q
[R/
(9)
30 ∗(]) 1 ∙ PX,46Z− ] Q [R/ (10)
where C is within the speed range [30; 100]. This curve, in according with variance properties, has its minimum values for C = VP,average.
Figure 15 – Var* curve
This curve has a parabolic shape: in its general equation ^ _ -_ we have: 1
- −2 ∙ ,KLMNKOM ∗(] 0)
We assumed as acceptable a tolerance of 5% around the value of Var*, tolerance for which it is possible to consider a path (or part of it) as homogeneous. The threshold of acceptability for the tolerance of the variance must meet two important requirements: to make possible the identification of the possible situations of inhomogeneity in a path, and to be sufficiently large to make possible the division of a path into homogeneous sections characterized by an operative extension. The threshold of tolerance thus identified is represented on the graph as a horizontal line:
31 We can then identify two different speed values, 4Q` and ab(, for which we have two correspondent different values of the Var* function. It is possible, then, to identify a range c4Q`; ab(e, defined as
homogeneity range: if VP,weighted is within this range, the path can be considered as homogeneous from
the design speed point of view: in this case, VP,weighted and VP,average are quite close and we could assume as VP,road the one or the other. Conventionally, into the developed procedure, we assume VP,weighted as VP,road.
Otherwise, if VP,average and VP,weighted are quite different, the path cannot be considered ad homogeneous: in this case, the path must be divided into j sections, each one characterized by its design speed.
The j homogeneous sections can be easily identified by the use of a chart like the one shown in Figure 17, where it is possible to identify two sections, one between pk 0+000 and pk 12+600, and the other between pk 12+600 and pk 17+700.
Figure 17 – Example of 78,9:
Observing the chart, we can establish also if inhomogeneity are spreaded along the entire path or not: in the first case, we can assume that
- if,SM4OTUMV f ab( ⇒ ,NhKV ab( - if,SM4OTUMV % 4Q` ⇒ ,NhKV 4Q`
while in the second case the path will be divided into j sections and the procedure described above must be applied to each one of them, obtaining j VP,section.
3.2 Operative speed
The evaluation of design speed for entire path (or part of it) is performed according the circ. 62032/2010. It is however necessary observe that the choice of the design speed as the reference parameter could be not appropriate: this is a pure theoretical parameter, which cannot be assumed as representative of the real traffic flow conditions. An immediate consequence is that some actions, planned only on the basis of VP, could be inadequate for the real road operating conditions.
