Over 1 million people die in traffic accidents every year. According to the WHO Global Status Report on Road Safety 2023, the figure was 1.19 million in 2021 alone. The numbers have been declining in recent years, reflecting improvements in road and vehicle safety. But the human toll remains unacceptably high. As of 2019, road traffic crashes were still the leading cause of death among children and young people between 5 and 29 years of age, and the 12th leading cause of death overall.
No matter how much road and vehicle safety improve, the human factor will always be harder to control. And there are reasons to think that this particular dimension may be getting worse.
The reason is the increase in extreme heat events around the world.
In recent posts, we discussed how heat clouds people’s thinking, resulting in poor decisions, and lowers impulse control, resulting in more violent crime. Put those two facts together, and the implications for driving become clear. When judgment is impaired and impulses take over, the consequences behind the wheel can be severe.
Sometimes, phrases like “impaired judgment” or “impulsive decision-making” sound too abstract. Consider what they look like in traffic: speeding; turning without signaling; impatient overtaking; ignoring blind spots; road rage; reversing without checking; tailgating; jaywalking. Less abstract now?
Driving Under the Influence of Heat
The effects of extreme heat on driving have been known for decades. Research from the 1970s and 1980s already showed that high temperatures reduce attention span and increase driver fatigue, sometimes leading to drowsiness. Extreme heat impairs cognitive functions and increases risky behaviours such as speeding and aggressive driving. It also decreases reaction times and makes drivers more prone to distraction. Further, sun glare can reduce visibility.
All these factors add up and increase the risk of accidents. What has changed in recent years is not the relationship between extreme heat and traffic accidents. It is simply that extreme heat days are becoming more frequent, making the problem more severe. Ongoing improvements in vehicle and road safety may be masking this trend.
More Traffic Fatalities on Hot Days
Research studies looking at traffic accidents under different weather conditions can tell us whether high temperatures have a relevant effect on traffic accidents and fatalities. And those studies speak a clear language. For example, research reported by Resources for the Future, a nonpartisan think tank focused on environmental and resource economics, sheds light on the scale of the effect. Leard and Roth (2017) studied weather and traffic accidents using records from over 46 million police-reported accidents, including over a quarter million fatalities, in the United States. They found that on days with temperatures above 80°F (27°C), traffic fatality rates were 8.6% higher compared to days in the 50–60°F range (10–16°C).
Not all of this is due to drivers becoming more reckless, though. A large part of the effect in the data of Leard and Roth could be attributed to greater exposure: on hot days, more pedestrians, cyclists, and bikers are out, and they are less protected. The rest of the effect, however, is likely due to increased driver recklessness. Both factors are also likely to compound, as greater exposure means that the consequences of every additional accident due to driver error are likely to be more severe.
A study of over 100,000 motor vehicle crashes in Spain, by Basagaña and coauthors (2015), found that crash risk increased by 2.9% during heat wave days overall. But when looking only at crashes caused by driver errors, the increase was 7.7%, confirming that reduced driver performance is a major driver behind the problem.
In the same study, the risk of a crash due to driver error increased by 1.1% for each 1°C rise in maximum temperature. Similar results have been found worldwide. Park and coauthors (2021), using data from South Korea between 2012 and 2017, found that traffic accidents increased by approximately 0.6% per 1°C rise in daily maximum temperature. Gariazzo and coauthors (2021) used high-resolution road crash and temperature data from Italy between 2013 and 2015 and found that crash risk escalated markedly on extreme hot days, defined roughly as the hottest 25% of days. Those “extreme” days of the 2010s, however, may soon become the new normal.
What Can You Do?
Once it is clear that heat clouds judgment and increases impulsivity, it follows that driving on extreme-heat days or during heat waves is riskier than usual. Even if you stay cool, hydrated, and use air conditioning sensibly, other drivers, pedestrians, and cyclists around you may not. Their impairment raises your risk too.
Awareness is the first step toward safety. Whether you drive, ride, or walk, on hot days, assume that everyone around you has a shorter attention span and slower reactions. Give people more space. Do not assume they have seen you, or that they will brake in time.
Exposure compounds the problem. If you ride a motorbike, bicycle, or electric scooter, wear a helmet. Regardless of what the law requires, choosing not to wear a helmet when you are that exposed in traffic is not a lifestyle preference: it is a risk you should not take. The temptation to skip it is strongest when it is hot, but that is also when the danger may be highest.
Finally, assume that you, too, are operating with a reduced attention span and slower reactions. Increase the distance to the car in front of you, in case you need to brake. Reduce your usual speed by around 20%. If the heat is severe and you commute by car, speak to your employer about working from home or shifting your hours to avoid peak traffic.
Most drivers believe they are better than most other drivers (which is a statistical impossibility, but never mind that). Even if it were true, heat does not discriminate: you are a worse driver when it’s hot.
Safe driving, everyone.