THE FREQUENCY OF ROAD ACCIDENTS

An attempt has been made to find the way that the number and types of road accidents vary with the amount of road travel. The number of such occurring in any area in any period clearly depends on a number of factors including the climate, the traffic conditions, the kinds of vehicle in use and the behaviour of the road users. But superimposed on these factors there are some general tendencies. The numbers of accidents involving vehicles of various types occurring in any 'daylight' hour is, to a large extent, made up of two factors, one dependent on the traffic flow in the hour and the other dependent on the types of vehicle involved and on conditions which have changed over the years. The number of accidents involving single vehicles of any type in any hour is proportional to the distance travelled by vehicles of that type, whilst the number of two-vehicle accidents is proportional to the product of the distances travelled by the types of vehicles concerned. The 'constants' of the proportionality for accidents involving only cars and goods vehicles have been falling over the years, those for two-vehicles accidents falling much more than those for single vehicle accidents. The 'constants' for accidents involving two-wheeled vehicles only have remained rather more uniform, and, in some cases, have risen. The 'constants' for accidents involving cars or goods vehicles and two-wheeled vehicles have all been falling, but at a much lower rate than those involving cars and goods vehicles only. The number of pedestrian accidents in any daylight hour seems to be approximately proportional to the product of the distances travelled by the vehicles in that hour and the number of pedestrians crossing the road in that hour. The 'constants' of the proportionality have been falling over the years for car and goods vehicles-pedestrian accidents but have changed little for motor cycle-pedestrian accidents. An examination of data giving road fatalities in a year (D), numbers of motor vehicles registered (N), and population (P) in 23 countries over a long period of years found that 96 per cent lay within the range half to twice the number given by the formula D=0.0003 (cube root of (NP) squared). There are a number of known ways in which road accidents and fatalities may be reduced and these are not fully used in any country. This suggests that the number of road fatalities in any country may be regarded as the number of road deaths that the country is prepared to tolerate. The applicability of the formula suggests that the number of deaths the country is prepared to tolerate is given approximately by this formula.