The science behind taking regular breaks when driving

Driver fatigue has long been highlighted as a leading cause of road traffic accidents. In the USA, the National Highway Traffic Safety Administration’s latest figures suggest that there are around 90,000 drowsy-driving road crashes annually in the USA, resulting in 50,000 injuries and 800 deaths.

VicRoads, an Australian road safety organisation, estimates that 20%  of road crashes are sleep-related. Meanwhile, in Germany, a study of motorway accidents in Bavaria estimated that 35% of fatal motorway crashes were due to driver inattention and fatigue.

We know that sleep-related accidents peak in the early hours of the morning, between 2-6am, and in the mid-afternoon, between 3-4pm, due mainly to circadian rhythms. Studies have shown that the risk is three times as great between 3-4pm than at 10am. However, the greatest danger is at night. Drivers are 50 times more likely to fall asleep at the wheel at 2am than 10am. 

The blood glucose effect

In terms of the science of fatigue, our brains are powered by blood glucose. When we are fatigued, there is a reduction in the amount that reaches our brains, affecting some regions more than others. For example, the parietal lobe, responsible for integrating sensory data, is significantly impacted by sleep loss and by fatigue. The same applies to the thalamus, which controls our ability to maintain alertness and vigilance. 

Likewise, the prefrontal cortices, which are responsible for problem solving, our ability to control our mood, our reasoning capabilities, our ability to control our emotions, and our ability to communicate are impacted by sleep loss, and fatigue. 

“Fundamentally,” psychologist Dr Paul Jackson told a Global Fleet Champions webinar on driver fatigue recently, “fatigue has a significant impact on our ability to focus our attention, which means we’re much more easily distracted by the irrelevant or unimportant information. This leads to a range of consequences in terms of our cognitive performance. 

“We all know that our alertness is one of the first things to be affected when we're suffering from fatigue. As such, we make more vigilance and concentration errors. But in addition, when we're fatigued, we find it more difficult to follow the rules.

“When fatigued, we see an increase in ‘intentional non-compliance errors’. This happens because our brains are trying to reduce effort and workload. “We see slips and lapses. We're less likely to pay attention to checking our mirrors, indicating and keeping to the speed limits. It may also result in us driving more slowly.”

Mandating breaks

There’s strong evidence to show that crash risk increases as driving time increases. This becomes apparent after five hours with the highest crash odds in the 11th hour of driving. In fact, moderate sleep deprivation produces effects that are equivalent to low levels of alcohol intoxication. After 18 hours without sleep, for example, a driver’s reaction times, response times and cognitive skills are equivalent or worse than an individual with a .05 BAC (blood alcohol content). 

It’s no surprise therefore, that many countries mandate that breaks should be taken at regular intervals. EU rules, for example, state that drivers should have a break of at least 45 minutes after a maximum of 4 hours and 30 minutes driving. It’s also forbidden to drive for more than nine hours in day, although this can be extended to 10 hours twice a week. Furthermore, there is a maximum of 56 hours driving over the course of a week and 90 hours in any two consecutive weeks. Many organisations err on the side of caution and state that their employees should take a short break every two hours.

Unfortunately, fatigue isn’t an exact science. We know that risk factors include night driving, the duration of wakefulness, inadequate sleep, sleep disorders, food and alcohol consumption, circadian rhythms and prolonged working hours (including time spent performing non-driving tasks), and breaks are a way to counter these risk factors.

Truck drivers: 10 hours max with two breaks

A 2014 paper in the Journal of Safety Research reported on the effects of driving hours and rest breaks on commercial truck driver safety. Its first major finding was that during the first 10 hours of driving, there were no significant variations in crash risks. However, starting from the 11th hour of driving, crash risk becomes much more likely. They found the crash odds were more than three times greater than those of the first hour. 

However, when rest breaks were introduced, they found that they “played a positive role” in reducing truck-driver crash risks. Compared with the crash odds for trips without any rest breaks, one break over 11 hours reduced the crash odds of a trip by 68%, two breaks reduced it by 83% and three by 85%. 

With the third break bringing relatively little extra benefit, the authors argued that drivers may not want to take too many rest breaks during a 10-hour drive as it would disturb their work schedule. However, this study didn’t consider the length of the break, or the point at which it was taken – nor did it consider the age, experience or gender of the driver.

What kind of break?

The UK’s Royal Society for the Prevention of Accidents (Rospa) recommends that drivers break for at least 15 minutes at least every two hours. But what kind of break? A study in the journal of the Society for Psychophysiological Research compared the effects of a short nap (less than 15 minutes), coffee (150mg of caffeine) and a placebo on a monotonous early-afternoon drive in a car simulator. 

It was discovered that caffeine and a nap both significantly reduced driving impairments, subjective (perceived) sleepiness and EEG activity indicating drowsiness. Taking a break by itself proved ineffective and caffeine brought more consistent results than a nap. 

Taking this a step further, a potential improvement on this solution is the ‘coffee-nap’. Researchers at Loughborough University recommended a combination of caffeine and a short sleep. Its success comes from the fact that it takes up to 20 minutes for our bodies to respond to the effects of caffeine: By taking a short nap immediately after a coffee, the stimulant effect kicks in just as you are waking up.

Ultimately, Rospa recommends only three strategies to prevent tiredness.

  • Stop driving and have a rest.

  • Take a short nap (15-20 minutes).

  • Consume caffeine equivalent to two cups of coffee

It seems that science does not support other steps to improve alertness when sleepy, such as opening a window or listening to the radio. “Their efficacy has not been demonstrated,” says Rospa.

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