Brake tests were run on tractor-trailers with differing brake circuits and under differing driving conditions. The investigation shows that the single-circuit arrangement of brake systems for tractors and trailers can represent a considerable danger due to poor braking effectiveness in the case of tractor-brake failure and poor driving stability in the case of trailer-brake failure. Even the usual partitioning of brake circuits between front and rear axles compare poorly to other possible arrangements. Duo-Duplex brakes with dual-circuit control of activating components appear to be preferable. In general, the danger of a sudden and total failure of air brakes is less than for hydraulic brakes, due to the lower pressure levels and the continuous replenishment of the control medium. However, many important questions concerning brake security and design have not yet been addressed by any experimental findings. The relationships between system-failure and accident frequency as well as driver response upon brake failure are very poorly understood and should be the objects of future investigations.