Mode of realization of a study of road traffic

The definition of the network

The network studied is consisted of the whole of the existing or projected sections of road for which one or the other of the solutions under consideration within the framework of the project is likely to exert an influence in term of traffic.

One proceeds to the inventory of flows of traffic concerned, directly or indirectly, and any route likely to be borrowed by one of them, will belong to the network.

Cutting

One carries out geographical cutting in zones. These zones correspond to electric fluxes. Their determination allows the establishment of the matrix origin/destination. An geographical area corresponds to a generating or receiving whole of homogeneous traffic.

The zones, resulting from cutting, are selected so that the users going from a zone to another, have and will have the choice between the same routes. The more one moves away from the project, the more the zones will be wide. For the zones external with the surface of study, one will be able, in general, to gather them by entries and exits. Cutting will take account of specificities of the specific generators of traffic (schools, industrial parks, shopping malls…).

The zones are thus identified for their main role (habitat, economic activities, commercial, downtown area, hypercentre…)

Characteristics of the network

All the component elements the network and the zone of study are raised: - characteristics of the ways concerned - visibility on each section - characteristics of the profile longitudinally if necessary - speed limits - crossroads with their mode of priority - generating hard points of deceleration

The measurement of the traffics

This measurement is carried out by various complementary processes:

manual countings
automatic countings

These two types make it possible to measure the traffic on a section, with regard to the automatic meters, the devices have now the capacity to discriminate light vehicles and heavy trucks.

investigations of the cord type. It makes it possible to distinguish the through traffics from the local traffics, and the origins and destinations of each flow.
qualitative investigations. They make it possible to know the appreciation of the user compared to the network, the reasons of its displacement….
number statements of plates.

At the conclusion of these countings, the traffic is modelled. The highway network then constitutes a mathematical graph made up of arcs (sections of roadway system) and tops (crossroads and exchangers). After having identified the possible routes, the most delicate phase is to determine the principal route for each exchange between zones. It is estimated with this intention, that the user makes his choice in order to minimize the cost of the way.

Each section is evaluated in term of run time, which they are the internal zones to the perimeter of study, or between entrance and the exit points of this perimeter for the through traffics.

Analysis of the existing traffics

Once the known traffics, one will study for each section the consequences of the increase in traffic on the flows and the durations of course.
The consequences of solution of installation are evaluated then, which it is of new infrastructures or development of urban areas or activities.
The difficulty lies in the projection of the traffics on a 5,10 15 year scale or more. Indeed, the increase envisaged is based on the current development model: prevalence of individual transport for the agglomerations of intermediate size in rural environment, prevalence of the transport of the goods by heavy trucks.

Moreover the local evolution of the urban development is considered on the basis of assumption of prospective reflection, translated in the documents of town planning (SCOT, RAINED). This evolution can be upset by modifications of the economic context related to creations or, on the contrary, closings of economic activities.

It is thus important to appreciate at the stage of the analysis of the existing traffics, the factors of influence and the possible margin of development inside the studied zone. It is also important to take into account external factors which can affect this evolution (construction of a highway, of a railway line at high speed…).

Determination of the circulation terms

Other factors that the traffics are to be taken into account at the time of the study of circulation, in particular what characterizes the circulation terms.

Indeed the distribution of the traffics on various routes depends on the circulation terms. This aspect is translated for the user into term of comfort (fluidity of circulation, safety…) and in economic term (run time, consumption…). It is also translated for the community into economic term (cost of the accidents, consumption, pollution, wasted time…).

The user takes into account, consciously or unconsciously three elements: journey time, freedom of circulation, safety.

The journey time is an essential component for the user, thus it must be measured for each possible circuit. The regularity of these durations must also be measured (it affects also the behavior of the user).

Freedom of circulation is appreciated by the evaluation of the time spent in group (vehicles passed spins one from there behind the other). Several thresholds then are distinguished:

the notable gene threshold (50% of the time spent in group)
the dense threshold of circulation (65% of the time spent in group)
the threshold of risk of congestion (80% of the time spent in group) of the risks of congestion apparaissent

It is then very important to measure for each course the latencies to each crossroads, the lengths of queues and to evaluate for each possible scenario the incidence of the increase in the traffic.

The evaluation of the evolution of the traffics

The traffic on the whole of the French highway network doubled between the years 1965 and 1985. It continued a very strong growth since. The direction of the roads of the ministry for the Equipment launched a study on the households to determine the circulation terms by 2015.

This investigation took into account the car fleet of the households, their average revenue, the vehicle ownership, the foreseeable trend of the price of the fuels, the length of the highway network. This study concluded with the increase continues from the traffic to this horizon, without saturation of the park. A ministerial directive specifies the method evaluation of the effect of construction of road infrastructure in open country (http://www.route.equipement.gouv.fr/RoutesEnFrance/circulaire/ree/instruct.htm)

Assumptions usually retained on principal highway network

linear 2,5% until in 2010
linear 1,25% beyond 2010

Assumptions usually retained on linear highway network

3,5% of 2000 to 2010
1,5% beyond 2010

Thus in the absence of data more precise, it is allowed on the principal network to use the following formula to determine the traffic at year N: traffic at the year N = traffic at the year 0 X 1,025 N

The assignment of the traffics

To affect the traffic between 2 points consists in distributing the currents of circulation on the parallel routes (and competitors between these two points). In the case of existing routes between 2 cities for example, the assignment makes it possible to reconstitute the current location and to fix the model of assignment used. This method is especially effective for a new layout doubling one or more route existing. It will offer the occasion to evaluate the interest of installation according to the alternatives of layout as well as the types and the number of connections with the existing highway network.

As regards assignment, one limits oneself to the only mode of road transport. The distribution enters the various means of transport is the model and other rule object other. The assignment takes into account an economic dimension: each route is evaluated in term of cost for the user. A rule defines the bond between cost and traffic: T1/T2 = (C1/C2) 10

Thus, a weak variation on the costs of circulation results in a strong effect on the traffics. In fact, if the variation of traffic is important, the route the charged risk to become gravitational with the profit of the route charged. It is necessary to follow the evolution of the traffics in an iterative way. The model balances in the long term. This modeling is used for the economic studies related to the construction of an road infrastructure.

Origin of the text

Internal bond

Study of traffic

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