The hazards related to the motion of heavy goods vehicles are important issues of traffic safety. The aim of this work is to show how selected deviations of geometric design parameters can impact the selected quantities that describe the safety of the braking process (braking distance, deceleration, instability indications). A quasistatic model of the rectilinear movement of the vehicle combination (tractor-semitrailer), as well as models of key subsystems (pneumatic brake system and the tangential tyre-road forces), were adopted. Before starting the actual tests, model validation was carried out in which the results of the original program were compared with the results of road tests. Then, simulations were made for the chosen tractor-semitrailer combination for different conditions – nominal and several changes in the longitudinal position of the fifth wheel. A comparison was made for mainly braking distances and forces in the connection. The presented results indicate that a considered geometric deviation, affects the measures of braking efficiency. These conditions may also significantly contribute to changes in the force in the connection, thus reducing or increasing the risk of jackknifing. It is worth emphasizing that the valuable results were obtained using a relatively simple calculation method.