Anthonares

This month’s edition of Physics Today has an article by Marc Ross, Deena Patel, and Tom Wenzel that throws a little scientific light on vehicle accident safety. In 2004, 42,636 people were killed in traffic accidents in the U.S. This makes traffic accidents the number one non-medical cause of death.

Given these staggering numbers, it is not surprising that great strides have been made in increasing vehicle safety. Modern vehicles are equipped with advanced airbags and seat belts that are designed to decrease the maximum deceleration experienced by passengers. Passenger compartments are tougher and better resist penetration by vehicle components. Increasingly, vehicle frames are designed to absorb a significant amount of the impact energy.

Yet, despite all of these advances, progress in reducing death rates in the US has slowed considerably, and total deaths are on the rise. The year at which progress slowed and deaths began to increase was 1992, the same year that SUV and light truck ownership began to increase significantly.

SUVs and light trucks are dangerous to passenger vehicles for a number of reasons: 1) they impact a vehicle higher up than cars do, so a side-impact from an SUV might penetrate the window rather than hitting the door, 2) the larger SUVs (as opposed to smaller crossover utility vehicles, XUVs) are built with the body-on-frame design that has been used for trucks since the 1920s (see the picture on the left, it’s of a 1984 Ford). This design means that the weight of the vehicle is transferred through two very stiff steel beams, and makes trucks a particularly lethal projectile. Here is a table from the Ross and others article that illustrates this well:

Annual Deaths Per Million Vehicles
(Vehicle type is the impacting vehicle in a front-side collision, impacted vehicle is a car of any type, numbers have been normalized to the number of registered vehicles of each type, data from 1997-2001)

By the way, 1/2 of all vehicle deaths and serious injuries in the US are as a result of front-side collisions. Clearly, the risk of death from body-on-frame type vehicles is much higher than from large cars. By redesigning truck front-ends to impact vehicles closer to the ground, survivability could be improved in these cases. The picture at the top-right is of a nissan sentry struck by a 3/4 ton pickup truck. Notice that the location of the impact vertically is above the widest (and strongest) part of the door.

Additionally, rollovers cause approximately 1/4 of all traffic deaths. Higher center of mass vehicles such as SUVs and trucks are much more prone to rollover than cars or XUVs. This fact, combined with the lethality of SUVs and trucks as an impactor, provides clear evidence that their standards of design must be improved. One relatively simple design alternative that the authors suggest is to provide for variable-height suspensions that lower trucks for on-road driving, but raise them for off-road clearance.

The authors also note that by improving the designs of passenger compartments, safety belts, and airbags even further, many more deaths could be prevented. For instance, 13 percent of deaths or serious injuries result from instances when the seat belt or airbag are used appropriately. Also, testing standards in the U.S. need to be improved to gather more useful data, as do post-crash data gathering standards. Only with more quantitative information can the safety of vehicles continue to improve, reducing risk of death or injury to passengers on increasingly crowded roadways.