The speed a driver is traveling contributes both to the risk of a car accident occurring and to the severity of that accident.
Pedestrian accidents are one place where we can clearly see the relationship between speed and severity of an accident. If a pedestrian is struck by a car traveling at 20 mph has only a 5% chance of being killed, but a pedestrian struck by a car traveling at 30 mph has a 45% chance of being killed. That risk rises to about 100% at 60 mph.
However, speed is also associated with increased severity of car accidents. The main factor in car accidents is how much a vehicle’s speed changes at the time of the accident. If the car’s speed changes only 1-10 mph, there is only a 4.5% risk of injury and only a 1% risk of serious injury. However, with a change of 50 mph, there is a 69% risk of injury, and a 52% risk of serious injury.
Fatality risk also increases with speed changes. At about 70mph speed change in a crash, the fatality rate approaches 100%.
Do Slow-Traveling Cars Cause Accidents?
Early research on speed and accidents showed a U-shaped crash curve, with vehicles both below and above the average speed having increased accident risk. However, further research has shown that the increased risk of slow-traveling vehicles is relatively minor, and that the highest risk occurs to vehicles that are traveling 15 mph more than the median speed.
When single-vehicle accident risks and injury severity are incorporated, the risk posed by slow-driving traffic virtually disappears.