of stronger materials, or materials with better energy absorption characteristics, should allow
weight reduction without compromising a vehicle’s crashworthiness, or even with an improvement
in crashworthiness, with proper design. In virtually all accidents, however, vehicle weight does
play an important role, because it determines the forces on the vehicles and their relative
decelerations.
In a head-on collision between two vehicles of different weights but identical designs, the
heavier vehicle will drive the lighter one backward, and the passengers in the lighter car will
experience higher decelerations. The precise balance of forces depends on how the car structures
collapse. If the heavier car is twice the weight of the lighter one, if they collide head-on while each
traveling at 30 mph and become entangled, the law of conservation of momentum dictates that the
heavier car would end up traveling 10 mph in the same direction it was going, while the lighter car
would wind up going backward at 10 mph. The change in speed of the lighter car (30+10, or 40
mph) would be twice that of the heavier one (30-10, or 20 mph). Because deceleration is
proportional to the change in velocity divided by the amount of time the velocity change requires,
the passengers in the lighter car would experience about twice the deceleration experienced by the
passengers in the heavier car. Consequently, passengers in light cars are at increased danger in
multi-vehicle collisions. Although a widespread shift to lighter vehicles will eventually lessen the
danger by reducing each vehicle’s exposure to heavier vehicles, the continued existence of freight-
carrying vehicles on roadways would prevent this problem from being cancelled out.
Light vehicles are also at a disadvantage in collisions with deformable obstacles. Deceleration
forces on passengers are directly proportional to the distance they travel during the deceleration--
this distance is the sum of the few inches an airbag may allow them to move forward, the foot or
so that the front end of the vehicle will crush in a controlled, relatively uniform manner,^85 and any
distance that the obstacle deforms. Because a heavier car will cause a larger deformation in an
obstacle than a lighter car (all else being equal), the distance of deceleration will be greater for the
heavier car--and the deceleration forces on the passengers will be smaller. This difference could be
dramatic, if the heavier car actually knocks over the obstacle (e.g., a collapsible light post or a
tree) and the lighter car is stopped by it.
This issue has great importance to the design of the many thousands of manmade roadside
objects--e. g., signposts, lampposts, cable boxes, and crash barriers--that can either pose hazards
or play a protective role to vehicles that have left the road. Current designs for these objects aim
at directing vehicles to safety or at breaking away in high-energy collisions. The existing array of
roadside objects, however, have been designed for the current and past fleet, and may pose
significant dangers to lightweight vehicles. In-fact, the fleet downsizing that followed the 1972 oil
embargo encountered significant problems with breakaway designs formulated for the pre-1972
fleet,^86 and these problems could easily be repeated with another round of fleet lightweighting,
unless significant planning is accomplished and capital investments are made.
Weight plays a role even in two-vehicle collisions where the weights of the vehicles are similar,
or in collisions into rigid, impenetrable barriers. In such collisions, the vehicles’ front structures(^85) Vehicle structurescannot collapse in a completely uniformmanner, so that deceleration-and deceleration forces on passengers-varies over the
brief period of the crash.. ,"Emerging Roadside Safety Issues,”TR News, vol. 177, March-April 1995.