Speed and injury severity

For any given road, clear physical relationships lead to higher severity of injury outcomes as speed increases. When the collision speed increases, the amount of energy that is released increases as well. Part of the energy will be 'absorbed' by the human body. However, the human body tolerates only a limited amount of external forces. When the amount of external forces exceeds the physical threshold serious or fatal injury will occur. Hence, higher speeds result in more severe injury. This is particularly true for occupants of light vehicles, when colliding with more heavy vehicles and for unprotected road users, such as pedestrians and cyclists when colliding with motorized vehicles.

Higher speeds: more severe injury

Road safety effects of speed changes are directly related to the change in kinetic energy that is released in a collision. Based on this, Nilsson [44] developed the following formula to describe the effects of a speed change on the number of injury accident rates:

with A₂ as the number of injury accidents after a change; A₁ as the number of accidents before; v₁ as the average speed before a change, and v₂ as the average speed after.

Subsequently, Nilsson reasoned that the severe injury accident rate would be affected more by a change in speed than the overall accident rate. Based on empirical data of the effects on accidents after a speed limit change on Swedish roads, he increased the power of the function to calculate the number of severe injury (I) and fatal accidents (F) to respectively 3 and 4:

More recent empirical data appeared to fit these general formulas very well [45][19].