Speed Variation, Absolute Speed and Their Contribution to Safety, with Special Reference to the Work of Solomon

This conference paper, from the 2001 meeting of the Institution of Professional Engineers New Zealand (IPENZ) Transportation Group, considers how speed variation and absolute speed contribute to automobile crashes and their severity. The authors note that the effects of speed on crashes and their severity depends on a number of factors related to the driver, the road, and traffic characteristics. The authors review Solomon's work (1964), the first significant study that examined the relationship between vehicle speeds, their distribution, and crash risk. The authors conclude that the u-shaped curve of Solomon is more an artefact of the road conditions used in his study than evidence that driving below the mean speed is dangerous. However, the variation in speeds of vehicle is a crash factor only when vehicles are traveling closely enough to each other for interaction to occur. On New Zealand rural roads, which are often empty or relatively sparsely occupied by vehicles, there is a high percentage of single vehicle crashes (which increase in probability and severity with the absolute speed of the vehicle involved). The authors support speed reduction measures in both traffic and lone driver situations.