Sight distance

Speeds and visibility

To give an account of the speeds practiced by the users, one uses with the international level V₈₅ speed in lower part of which 85% of the users roll, in fluid circulation term.

This speed is function of the geometrical characteristics of the site, particularly of the curves and the slopes. Thus for a two-track road:

V₈₅ Speed according to the ray: V₈₅= 92/(1+346/R^1,5)

V₈₅ Speed according to the slope (>250 m): V₈₅= 92/(1+0,31/p ²)

The sight distance necessary generally depends on the speed applied, of the variable, reaction time according to the type of event and the type of reaction (at the wheel, to the pedals, etc), and of the distance necessary to the operation (braking, modification of trajectory, starting, etc).

V₈₅ speed to be taken into account can be chopped on the level of speed limit (90 km in the general case) except as regards the calculation of the visibility on the level of the accesses and crossroads.

Visibility on a turn

The driver must have with the approach of the turn a visibility such as it can perceive the turn and modify its behavior (trajectory, possibly speed) in time. The distance necessary can be estimated in the current cases at a distance corresponding to 3 seconds traversed at the V₈₅ speed practiced upstream of the turn (at the point considered), that is to say d_virage = 3 X V₈₅ with V₈₅ is expressed in m/s.

This distance is measured between the point of observation: the eye of the driver (height 1m, 2 m of the flat rim of the roadway) and the point observed (height 0 m on the axis of the carriageway at the beginning of the circular part of the turn).

Visibility on an obstacle located on the roadway

It is necessary to ensure the visibility a distance making it possible the driver to stop before a " obstacle" (vehicle stopped, pedestrian, etc). The sight distance must be higher than the stopping distance.

The Distance stopping D is made up of the Distance braking (distance covered during the effect braking which makes pass the speed of V₈₅ to 0 under conventional conditions of wet roadway) increased Distance of perception-reaction, distance covered during the reaction time (taken equal to 2 seconds in the case of a reaction of braking in front of an unexpected obstacle).

In curve, the Distance braking is raised of 25%.

In France

Measure distance

The distance is measured between the eye of the driver (height 1m, 2m of the edge of the carriageway) and the obstacle fixes potential on the roadway.

According to the highway type, it is up to the originator to determine the height obstacle to be taken into account. Generally it is the height of the rear light most favorable of a vehicle stopped on the roadway which is taken into account (minimum lawful: 0,35m ) but for specific cases such as the zones with stone fall, this height can be brought back to 0,15m.

On roads with twice two made uneven ways, insofar as the principal risk relates to the presence of a vehicle stopped on the roadway, the ICTAAL retained the height of the rear lights of the vehicles, and, starting from a specific study on the vehicles circulating on this type of network a height of 0,60m instead of the usual 0,35m retained.

Value of the stopping distance in alignment

The Distance stopping retained in France for the design of the roads is, according to speeds V₈₅, the following one:

Value of the stopping distance in curve or in the event of masks

In the interior of the curves, if the desirable side releases to ensure the visibility the stopping distance lead to disproportionate costs, one will seek to obtain a sight distance corresponding to the stopping distance in straight line.

In addition, if the desirable side releases to ensure the visibility the stopping distance lead to disproportionate earthworks, one can plan to reduce these releases to a value to in no case lower than 3 m, counted starting from the edge of the carriageway, and ensuring a visibility allowing an operation of side avoidance of the obstacle. One has then:

d_obstacle = 3,5 X V₈₅ with V₈₅ is expressed in m/s.

In all the cases, when the visibility corresponding to the stopping distance in curve is not offered, it is advisable to envisage a local reduction authorized speed limit.

Comparison between standards

The stopping distance is function of the coefficient of longitudinal friction, itself variable according to speed. However this coefficient is variable according to the standards of the various countries. It varies thus from 0,31 to 0,46 for France and from 0,28 to 0,38 for Canada, which gives different distances stopping and definitely longer for Canada:

Visibility in an ordinary plane crossroads or an access

The nonpriority access or road user must have the time necessary to get informed about the presence of another user on the priority road, to decide his operation, to start and carry out his operation of crossing, before an emergency vehicle initially masked does not occur.

It is necessary for that which it sees at a distance corresponding to 8 seconds (preferably, if not 6 S constitutes an imperative minimum) at the V₈₅ speed practiced on the main roads. That is to say:

d_carrefour = 8 X V₈₅ (mini 6 X V₈₅)

On a road with 3 ways, or twice two ways comprising a central installation (of a width not exceeding 5 or 6 m), these values are to be changed respectively to 9 and 7 seconds).

d_carrefour = 9 X V₈₅ (mini 7 X V₈₅)

with V₈₅ is expressed in m/s.

Visibility for the going beyond

This point relates to new installations of roads with 2 ways, or the roads with 3 ways whose center lane is not assigned to a direction of circulation.

In practice, only the sight distances about 500 m and more make it possible to ensure, for an appreciable percentage (30 to 50%) of situations (relative speeds of the involved vehicles, etc), possibilities of sure going beyond.

It is reasonable to seek to ensure such distances sight (> 500 m) on a proportion from at least 25% length of the project (while avoiding if possible concentrating these 25% out of only one section of the layout). Apart from these zones, no constraint relating to the visibility of going beyond is to be taken into account.

This objective (visibility higher than 500 m out of 25% of the layout) requires a percentage of right alignments in general much more important, right alignments being able to suffer from limitations of visibility because of the profile longitudinally (or to contain crossroads with central installation oú the going beyond is not possible).

When the proportion of 25% is not reached, of the crenels of going beyond can allow to offer complementary possibilities for the going beyond.

Other elements to be taken into account

It is also advisable to take into account:

side masks

Any object located in the environment of the road near a turn (panels, slope, trees, cultures, etc) is likely to mask the visibility. The manager often not controlling the evolution of this environment, it agrees to make the systematic assumption of it except whenever the appearance of masks in the long term seems improbable (presence of a ravine, a water level, etc). One then defines a Triangle of visibility and the manager can impose a Servitude of visibility.

convex elements (ray in salient angle) of the profile longitudinally;
mobile masks in curve on the right.

Internal bonds

Sources

"Installation of the Principales" Roads; - Recommendations for the design and the geometry of the road - SETRA - 1994,
To include/understand the principal parameters of geometrical design of the roads” - Sétra - January 2006.

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