Using the AASHTO calculation method, what is the stopping sight distance for a road with a design speed of 60 mph and level grades?

The calculation of stopping sight distance (SSD) is critical in roadway design to ensure that drivers have sufficient distance to perceive and react to obstacles or changes in the road conditions. According to the AASHTO standards, the stopping sight distance accounts for both perception-reaction time and the braking distance required to stop a vehicle traveling at a given speed.

For a design speed of 60 mph on level grades, the stopping sight distance is influenced by several factors, including the perception-reaction time (typically assumed to be 2.5 seconds) and the brake deceleration rate. AASHTO provides a specific formula to compute the SSD, which combines these elements effectively.

When calculating for a speed of 60 mph, the resulting SSD typically amounts to approximately 570 feet for level grades, as this encompasses the distance a vehicle travels during the perception and reaction phases as well as the braking distance. This substantial distance ensures safety by allowing drivers adequate time to stop their vehicles when encountering unexpected situations.

This value aligns with best practices for road safety and visibility requirements as established in AASHTO guidelines, emphasizing the need to provide sufficient sight distance for drivers to respond appropriately.