Stopping sight distance is defined as the distance needed for drivers to see an object on the roadway ahead and bring their vehicles to safe stop before colliding with the object. The distances are derived for various design speeds based on assumptions for driver reaction time, the braking ability of most vehicles under wet pavement conditions, and the friction provided by most pavement surfaces, assuming good tires. A roadway designed to criteria employs a horizontal and vertical alignment and a cross section that provides at least the minimum stopping sight distance through the entire facility.
Stopping sight distance is influenced by both vertical and horizontal alignment. For vertical stopping sight distance, this includes sight distance at crest vertical curves , headlight sight distance at sag vertical curves , and sight distance at under crossings .
For crest vertical curves, the alignment of the roadway limits stopping sight distance . Sag vertical curves provide greater stopping sight distance during daylight conditions, but very short sag vertical curves will limit the effective distance of the vehicle’s headlights at night. If lighting is provided at sag vertical curves, a design to the driver comfort criteria may be adequate. The length of sag vertical curves to satisfy the comfort criteria over the typical design speed range results in minimum curve lengths of about half those based on headlight criteria.
For horizontal curves, physical obstructions can limit stopping sight distance . Examples include bridge piers, barrier, walls, back slopes, and vegetation.
Stopping sight distance is influenced by both vertical and horizontal alignment. For vertical stopping sight distance, this includes sight distance at crest vertical curves , headlight sight distance at sag vertical curves , and sight distance at under crossings .
For crest vertical curves, the alignment of the roadway limits stopping sight distance . Sag vertical curves provide greater stopping sight distance during daylight conditions, but very short sag vertical curves will limit the effective distance of the vehicle’s headlights at night. If lighting is provided at sag vertical curves, a design to the driver comfort criteria may be adequate. The length of sag vertical curves to satisfy the comfort criteria over the typical design speed range results in minimum curve lengths of about half those based on headlight criteria.
For horizontal curves, physical obstructions can limit stopping sight distance . Examples include bridge piers, barrier, walls, back slopes, and vegetation.
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