There is a strong link between crashes and traffic volume. The higher traffic volume, the higher crash risk and, usually, the more crashes [5] [7]. Research, however, shows that the increase in the number of crashes is not proportional to the increase in the number of vehicles (traffic volume). Because the number of crashes increases less rapidly, crash risk for individual road users is lower at higher traffic intensities than at lower intensities [8].
Results of research into the precise link between traffic volume and road safety are mixed. Some studies indicate that the number of crashes increases when traffic volume [i] increases, but that crashes are less serious [9]. Research by Lord et al. [10] finds a decrease in crash risk at increased traffic densities, while research by Wang et al. [11] does not find a link between crashes, crash severity, and traffic volume.
[i] Often expressed as I/C: the ratio between actual traffic volume (I) and theoretical capacity (C). The higher this ratio is, the higher the risk of congestion.
