Will You Stop in Time: Some Problems in Emergency Braking

The variability in emergency stopping distance of passenger vehicles is not recognized by some members of the general public and accident investigators. Many fail to realize the great influence of the driving environment on braking distance. Surface condition (wet/dry), vehicle mass and surcharge load are some of the more important variables which must be considered. A combination of wet surface and loaded vehicle could increase emergency stopping distance by up to 40 per cent. Thus, during field accident reconstruction, variables as vehicle mass, tyre type, amount of load and surface conditions, should be closely modelled. The study reported here considered the emergency stopping ability of a range of passenger vehicles from very small (835 kg), to large (1660 kg). The testing was carried out with vehicles in two different load configurations, on wet and dry asphaltic concrete. Tyre type and pressure were held constant for each vehicle during the study and braking distance was measured under emergency conditions with the vehicle held in a straight line. Speed at braking was measured using a Vericom VC.200 on-board computer and braking distance was measured using a `chalk gun' developed by the Queensland Police Force. There is also a belief that anti-skid braking systems (ABS) improve the emergency braking performance of vehicles. The project considered the effects of ABS on the performance of a large sedan under loaded and unloaded conditions on a wet asphaltic concrete surface. The effect of the ABS on steering performance under emergency situations was not evaluated.