We’ve all been there. You’re in the car trying to pull out of someone’s driveway, and it is next to impossible to see vehicles traveling down the street. The sweat starts to bead on your brow and visions of an impending accident race through your mind. Inevitably, most of us just floor it and hope for the best. But nonetheless, this is a very dangerous circumstance that can be avoided in the future by designing safer intersections and driveway access points using some good old sight distance calculations.
There are several types of sight distance calculations used in road design, but we’ll only focus on two, sight line distance and safe sight stopping distance. The first is the sight line (visibility) calculation. Basically, this is the viewpoint of the person pulling out onto a road looking in both directions. This calculation determines how much distance is needed for someone to identify an oncoming vehicle and safely pull out onto a roadway. The Department of Transportation has guidelines on this calculation based on design speed limit, the slope of the road, and reaction time. The general rule of thumb is for every 1 mph, the motorist should be able to have an unobstructed view of 10ft. For example: on a 35 mph road, the motorist should be able to see approximately 350 ft. in both directions.
The second calculation is safe sight stopping distance. This is from the viewpoint of someone driving along the road. Based off of two phases (driver perception/reaction time and braking distance of an average vehicle) this calculation is used to determine how much distance a motorist traveling down a roadway needs to identify an obstruction in the road and be able to stop completely. So if someone pulls out in front of a car driving down the road, the car can safely avoid a collision by coming to a complete stop. While also taking into account the design speed limit and slope of the road, this calculation will allow plenty of space for even the most inexperienced of drivers (Denny Howell).
So if you are proposing to build a new driveway, modify an old one, or even constructing a street, call D.L. Howell & Associates first to check if you will have proper safe sight distance and we’ll see (pun!) what we can do.