Impact of the type of the cross sectional profile on urban zones road

1.3. Functions of a roadway system in urban environment

Indeed, during the estimates in study of traffic with the macroscopic model^4(*)(COSTESEQUE Guillaume, Avril 2012), the streets (minors) are supposed to be non-existent and neglected because not being useful which the vehicle has to fold back on the main roads (object of the study of traffic). It is to say that during a displacement from one point A to a point B (relatively distant) of a city, a motorist starts at the time of the first minutes of his travel by folding back himself on one of the «main roads» in order to go faster and carries out there most of his travel. Arrived at the end of the course, it is folded back towards the small streets which will thus enable him to arrive at destination. One can consequently speak: «of bursting and folding back»(STRANSKY, 1995).

Thus described, urban space is connected with a whole of block made up of the various urban sectors such the downtown area, the residential zones, etc. which the lanes will have to connect between them.

It is thus clear that the roadway system in urban environment will have to fulfill various functions namely:

· A circulatory function because the streets are judicious to support the displacements carried out by the motorized vehicles or by the alternative modes (bicycle, walk, transport, etc.). This function often induces stereotyped installations being based on the separation of the modes: a roadway for the motorized users, of the bands or cycle tracks and the pavements for the pedestrians.

· A function local life where the streets are also to regard as public spaces, places of animation, user-friendliness and meetings, relaxation, walk and even sometimes of plays for the children. This local life pains to exist vis-a-vis installations of the «road» type which privilege the car.

Practiced and/or authorized speeds are adapted often little to the cohabitation between functions circulatory and local life.

However the development with the profit of this function of local life answers 3 stakes (Direction départementale des Territoires de l'Ain, 2013):

· A stake of social cohesion: The experiment shows that the more speeds are raised, the more the axes of circulation are felt as urban cuts which do not support the social relations of vicinity. The moderation speeds thus makes it possible to weave more easily of the bonds (space, social) to facilitate the meeting with others.

· A public health issue: Excessive speeds induce a deterioration of the quality of the frame of life by accentuating noise pollutions with consequences often underestimated on the human health. Sound environment in night period has an influence on the quality of the sleep. The moderation speeds in medium of social life thus proves to be of a public health issue.

· A stake of public safety: The moderation speeds supports the reduction of the number and the gravity of the traffic accidents. It contributes in particular :

o To reduce the feeling of road insecurity which always does not result in an important accidentology but constitutes an important brake for the use of the walking and bicycle in particular for the children and the elderly;

o To widen the field of view of the motorists and thus to support the taking into account and the interpretation of information over all the width of the roadway and around. That makes it possible in particular to anticipate the random movements more quickly users pedestrians or cyclists in places with strong frequentation pedestrian as in the neighborhoods of the schools or the markets.

o To reduce the stopping distances, the violence of the possible shocks and thus the gravity of the accidents. Indeed, an even light reduction speed to the impact, strongly limits the consequences of an accident. The chances of survival of a pedestrian are much higher at the time of a shock to 30 km/h than to 60 km/h. It is advisable moreover to take account of the reaction time of the drivers before braking vis-a-vis an obstacle. This time is estimated at approximately 1 second whatever the speed. Thus, for example, to 60 km/h (16,67 m/s), a motorist seeing a pedestrian engaging has thus 17 meters to traverse in front of him before it has time to start to slow down; he will thus strike it (certainly) at full speed; on the other hand, while rolling to 30 km/h (8,33 m/s), it largely has time to stop and avoid the collision or at least the violence of the shock will be largely attenuated.

From these various functions, it will thus result from this the different characteristics from the roadway according to whether one is in a zone with strong pedestrian frequentation or machines with two wheels as in zones residential or on the other hand that one is in zones has strong traffic where the use of the car remains dominating such on the loop lines, ways of penetration and those leading towards the urban center where the practice of an high speed remains necessary and/or authorized.

From this concept of difference in characteristics of the roadway according to the traversedmiddle, we can say that it results from this the notion of hierarchisation of the roadway system which is particular with urban environment. Indeed, the road which hardly starts to adapt on arrival of the car will have to be constrained now in turn to adapt to the frequentation pedestrian as Bruno d'ARCIER underlines it(d'ARCIER, 1992)when it quotes A. GUILLERME at the time of a communication to the seminar of the Urban plan «City and transport» of March 18th, 1992 which, speaking about the first version of the highway code, stresses that the pedestrian had the right to be informed on arrival of a car by an aural signal and that him in return, has the duty to leave him (the car of course) the passage. To improve the circulation terms inside the agglomeration it is advisable to support the installation of the roadway systems on their hierarchisations; Hierarchisation which is conceived according to two criteria namely:

· The function of the way according to the type of supported traffic (transit, exchange, internal service road) and the characteristics of its installations.

· The urban infrastructurecrossed.

* ⁴ Indeed, there exist classically microscopic models and the macroscopic models of study of traffic. The first endeavor to model the road traffic by individual evolution of each vehicle. In this model, the dynamics of the vehicle is given by a law of continuation and the speed of a vehicle is directly function of the distance which separates it from the vehicle right in front of him, modulo a time lag, which is the reaction time of the driver. On the largest scale (called macroscopic scale), the simplest model used is the hydrodynamic model of traffic, which supposes that the density of vehicle satisfies a law of conservation. Here the speed of the vehicles is supposed to be a function of the density of vehicles.