HPT

10.4Differential Speed:

Differential speed is the principal value in predicting automobile and human carnage during traffic accidents. It is the velocity difference between two moving objects or one fixed and one moving object. On a typical two lane roadway, two cars traveling in opposite directions at 55 mph would have a differential speed of 110 mph. A car traveling at 55 mph which runs into a bridge abutment or tree collides with a differential speed of 55 mph. Accident damage, and to some extent, death and injury, can be reasonably predicted based on the mass of the objects closing at various differential speeds and the amount of deformable structure incorporated within a vehicle, and/or in the path of a fixed object. Many high speed racing tracks have flexible safety walls or a series of tires tied together that absorb the impact of an errant car. Guard rails, for example, absorb some differential speed during impact, however, are placed primarily to guide vehicles back into the flow of traffic in the event of control loss in order to minimize differential speed.