• Stopping sight distance is the distance a motorist traveling at a particular speed needs to stop their vehicle once they detect an object on the road. To determine stopping sight distance, use actual operating speeds if they are available — not the design speed.
• The equation below is used to calculate stopping sight distance:

where:

t_RT = perception-reaction time, V = design speed (mph), a = deceleration level (ft/s²)

• Under favorable visibility conditions, motorists have direct, unobstructed of hazards. At night, good visibility is the result of hazards that are self-illuminated or retro-reflectorized and lighting that provides motorists a clear view.
• Under poor visibility conditions, motorists find it difficult to immediately perceive hazards because they blend into the background and are not directly in their light of sight. At night, poor visibility results from hazards not being self-illuminated or lacking retroreflectivity, poor or absent lighting, and glare.
• All designs must provide stopping sight distance at all points because any point on a road can become a hazard.