High temperatures increase crash risk [7] [18] [19].
In China [19], high temperatures were found to increase the risk of an injury crash by 20% compared to normal temperatures (relative risk 1.20, 95% confidence interval 1.01 - 1.4). In Italy [18], high temperatures were found to increase the risk of a fatal crash or injury crash by 12% compared to normal temperatures (relative risk 1.12, 95% confidence interval 1.09-1.16). In the United States [7], the relationship between heat waves and the number of fatal crashes was examined: on days during a heat wave, an increase in fatal crashes of 3.4% (95% confidence interval 0.9 - 5.9%) was found.
A major reason why high temperatures lead to a higher crash rate is that heat affects road user behaviour, for instance through poorer sleep, decreased reaction time and concentration, and getting annoyed faster (see, for example, [20]). In addition, heat increases the risk of a blowout, especially if tyres are too soft.
