FAQ A6

Why is it safer to travel in larger vehicles?

It is safer to travel in larger vehicles because they are rarely involved in higher-severity crashes. Crash severity is typically measured as the change in speed that the vehicle experiences during an impact event. This change in speed is often referred to as the crash “delta V” and is a measure of the kinetic energy that generates the forces on the occupant that can result in injury. Because engergy is proportional to delta V * delta V, the energy in a 30-mph delta-V crash is more than twice that of a 20-mph delta V crash (i.e., 900 is more than twice 400).

The delta V experienced by a vehicle during a crash depends on how fast it is moving, as well as the mass of the vehicle or object that it srikes. If two vehicles of equal mass impact head on, they will both experience aboutthe same delta V, which will be close to the speed of each vehicle at the time of impact. However, when a large vehicle impacts with a smaller vehicle, the smaller vehicle will experience a greater delta V than the larger vehicle. The small vehicle could actually reverse direction during the impact, whereas the larger vehicle could continue to move in the same direction at a somewhat lower speed. The mix of large and small vehicles on the roadways, plus the laws of physics, mean that the distributions of real-world crash severities for large vehicles are significantly different and less severe than that of smaller passenger-size vehicles. For these reasons, it is generally considered safer to travel in larger vehicles. Using larger vehicles to transport heavier occupants and heavy wheelchairs is a good countermeasure to concerns about the disparity between WTORS strength and forces generated by higher wheelchair and occupant masses.