James R. Schmidt, Jr., BSME, Sr. Collision Reconstruction Engineer ::::
A vehicle traveling 50 miles per hour (73.3 feet per second) emergency brakes to a stop on a dry roadway in 119 feet. That same vehicle traveling down a 5% grade, for example, takes longer to stop, as the downgrade effectively reduces the friction. The braking distance for that vehicle would be 128 feet. When traveling uphill the opposite is true, as the uphill grade (again, assuming 5%) effectively increases the friction, resulting in a braking distance of 111 feet. The higher the friction, the quicker a vehicle can stop. The lower the friction, the longer it takes a vehicle to stop.
Speed from video can be an easy thing to evaluate. Of course, we must be able to see the vehicle of interest and the video needs to be of a good quality with a constant/known frame rate. Speed is simply distance divided by time. For example, if a vehicle travels 100 feet in 1.37 seconds, its speed is 73 feet per second (or approximately 50 mph). Alternatively, if the view of the vehicle is limited to something small, or just a “blip”, the time it takes the vehicle to travel its wheelbase (i.e. distance from front axle to rear axle) can be used. For example, a 2007 Honda Odyssey minivan passes through the field of view of a surveillance camera, and in doing so it travels its wheelbase of 118 inches (9.83 feet) in 4 frames of a 30 frame per second video (0.133 seconds). Speed is calculated at 73.8 feet per second (or approximately 50 mph). Each case is unique, and evaluations ranging from simple to complex, using traditional to advanced 3D computer-aided methods, can be performed to determine speed and many other parameters that may be of interest.