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 Geometric Design: Theory and Concepts

 
Decision Sight Distance

Normally, the stopping sight distance is an adequate sight distance for roadway design. However, there are cases where it may not be appropriate. In areas where information about navigation or hazards must be observed by the driver, or where the driver’s visual field is cluttered, the stopping sight distance may not be adequate. In addition, there are avoidance maneuvers that are far safer than stopping, but require more planning by the driver. These may not be possible if the minimum stopping sight distance is used for design. In these instances, the proper sight distance to use is the decision sight distance.

The decision sight distance is the distance traversed while recognizing an object or hazard, plotting an avoidance course, and making the necessary maneuvers. Unlike the stopping sight distance, the decision sight distance is quite complex. Various design values for the decision sight distance have been developed from research. The table below gives a few values for the decision sight distance (AASHTO, 1994).

Design Speed (km/h) Decision Sight Distance (meters)
Stop
Rural
Road		 Stop Urban Road Adjustment Rural
Road		 Adjustment Suburban Road Adjustment Urban Road
50 75 160 145 160 200
80 155 300 230 275 315
90 185 360 275 320 360
110 265 455 335 390 435

It is up to the engineer to decide when to use the decision sight distance. Providing the extra sight distance will probably increase the cost of a project, but it will also increase safety. The decision sight distance should be provided in those areas that need the extra margin of safety, but it isn’t needed continuously in those areas that don't contain potential hazards.