While the positive relationship between traffic crashes and extreme summer conditions is certainly not unheard of, it is rarely used in practice when designing policies or issuing roadway safety warnings.
A recent study in Accident Analysis and Prevention shows that heat waves have a significant effect on the frequency of traffic crash fatalities. The study examined fatal traffic crash reports obtained from the Fatality Analysis Reporting System provided by NHTSA in the continental U.S. for May-September of2001-2011. The geospatial information from the crash reports were linked to North American Land Data Assimilation System (NLDAS-2) grid cells to determine whether the crash dates were in a heat wave day or not.
The research not only studied the relationship between fatal traffic crash frequency and heat wave, it also stratified the result by driver gender, age, and BMI, as well as day of the week, time of day, speed limit, rural/urban roadways, and meteorological factors like solar radiation and precipitation. The study found that there was a 3.4 percent increase in fatal traffic crashes on heat wave days versus non-heat wave days for all drivers, and the effect was stronger in females (5 percent) than male drivers (3 percent). The association was found highest for the age group 56-65 (8.2 percent increase) and as per BMI, obese drivers were found to have a 3 percent higher risk. Regarding roadway characteristics, the study shows that on a heat wave day rural roadways (6.1 percent increase) with speed limits between 30 and 55 miles per hour (4.8 percent increase) show the highest positive association.
In a nutshell, this study indicates that extreme high temperatures, accompanied by low precipitation and high solar radiation, can result in certain groups being at higher risk of fatal traffic crash and that certain roadway characteristics are less favorable for driving.
Many studies in the past have also shown a decrease in driver performance due to extreme weather conditions. In a similar study in Spain in 2015 examining the association between traffic crashes and high ambient temperature, the authors concluded that traffic crashes involving driver performance–associated factors increased by 7.7 percent in association with heat waves and increasing temperature. In 2017, the Insurance Institute for Highway Safety published an article saying that traffic accident numbers in the U.S. are highest in the months of July, August, and September.
With climate change resulting in extreme temperatures all around the world, these studies highlight a pressing need for policy and safety interventions to adapt to changing conditions. While there is definitely a need for more in-depth research to understand what kind of mitigation measures would be most useful, it is something cities and DOTs could look into for improving roadway safety.
Saumya Jain is a Senior Associate at SSTI.