The dynamic behavior of cars and heavy trucks is compared on the basis of the unintended responses which can ensue following braking and steering inputs. Such responses are discussed in the context of their potential for contributing to the production of accidents. Certain basic design characteristics of cars and trucks are shown to differ because of the differing operating missions of the two vehicle types. The contrasting design characteristics are then related to the "unintended" dynamic responses of interest such as the spin-out of unit trucks, the jackknife and trailer swing instabilities of tractor-semitrailers, the rollover of all vehicle types, and the amplified motions of trailers in doubles and triples combinations. Each response type is related to the corresponding accident experience, either through reference to existing data or by means of hypothesized accident scenarios.