A simplified analysis is made of the directional stability of vehicle combinations, such as a tractor combined with one or two semitrailers or a truck and full trailer. Vehicle combinations are treated as linear dynamic systems with two degrees-of-freedom for each unit. The equations of motion are derived considering the influence of braking and acceleration, and a characteristic equation for traveling at constant speed is obtained.The Routh criteria are examined to distinguish between stability and instability, considering oscillatory and aperiodic motions separately. Special attention is given to the behavior of the vehicle during braking, with discussion of the influence of various design parameters and the distribution of the braking forces applied to the wheels. Finally, a new design of fifth wheel device is presented in order to stabilize tractor-semitrailer combinations.