Can caffeine really cut crash risk?

“Lorry drivers who drink coffee cut their crash risk”, reports BBC News.

This headline came from a large study of long-distance lorry drivers in Australia. It found drivers who had recently been involved in a crash were less likely to have consumed caffeine products, such as coffee or caffeine pills, to stay awake than those who hadn’t had a crash.

The researchers involved boldly concluded that consuming caffeine can “significantly protect against crash risk”, however, this claim is arguably too strong.

While it might seem to be common sense that caffeine may make you a safer driver by keeping you awake and alert, there are many other possible explanations for the results.

Those reporting a crash may have been the generally poorer and less experienced drivers and the link to caffeine consumption may have been coincidental.

Also, drivers who crashed may have been less likely to report having caffeine to stay awake for fear of implying they were tired and shouldn’t have been driving (this is known as recall bias).

Caffeine-consumption is no substitute for taking regular breaks during long-distance driving. The Department for Transport advises a 15-minute break every two hours on a long journey.

Where did the story come from?

The study was carried out by researchers from universities based in Australia and was funded by the Australian Research Council and numerous Australian transport institutes.

The study was published in the peer-reviewed British Medical Journal as an "open access" article meaning it is free to all to read.

Both BBC News and the Metro reported the study accurately, but also repeated the conclusions of the authors, which were slightly overblown, and did not discuss any of the limitations of the study.

The BBC coverage included useful detail of the number of participants in the study as well as common sense warnings that coffee is no substitute for sleep.

What kind of research was this?

This was a case control study to determine where there was an association between use of substances that contain caffeine and the risk of crash in long-distance commercial vehicle drivers in Australia.

The authors explain how long-distance drivers of commercial motor vehicles routinely experience monotonous and extended driving periods in a stationary position. This, when combined with the disruption to sleep cycles linked to the common requirement of night driving, has been associated with drowsiness at the wheel. This can obviously increase the risk of crashes.

As alertness is crucial to road safety, the researchers wanted to understand the role of caffeine on alertness and crashes in long-haul lorry drivers. A case-control study compares the histories of two groups of people with (the ‘cases’) or without (the ‘control’) a particular condition.

In this study:

the cases were lorry drivers who had had a crash
the controls were lorry drivers who had not had a crash

Through this process, they could identify differences between the two groups (such as caffeine consumption) and identify factors that potentially caused the condition of interest.

This type of study is particularly useful for studying rare diseases, but may also be used for studying rare events, such as lorry crashes.

A cohort study, which followed a population over time, would not be expected to detect a sufficient number of rare events to allow reliable comparison between groups.

What did the research involve?

The researchers recruited 530 long-distance drivers of commercial vehicles who were recently involved in a crash attended by police (the cases) and compared them against 517 drivers who had not had a crash while driving a commercial vehicle in the last 12 months (the controls).

The participants were drivers of commercial vehicles, presumably mainly lorries, because those driving passenger coaches or buses were excluded.

Cases were identified from a police crash database and had to have crashed their vehicle while on a long-distance trip, defined as over 200km (124 miles) from base.

Similarly, controls also had to be on a long distance trip defined as at least 200km from base to the point they were interviewed for the study, which was by a face-to-face interview at a truck stop.

Case drivers were interviewed over the phone, generally within four weeks of the crash.

All interviewees were assured the interview was anonymous to minimise selection bias – without the guarantee of anonymity, drivers engaged in risky or illegal behaviour would probably be unwilling to take part.

The 40-minute interviews gathered information on a large variety of topics relating to the specific journey of interest (the crash journey or control journey) as well as more general information.

This included:

the driver’s demographics (such as age, places of residence, income)
use of medications
health behaviours over the past month (such as patterns of alcohol consumption)
sleep quality and quantity
intake of caffeinated stimulant substances such as tea, coffee, energy drinks or caffeine tablets

Drivers were asked what (if any) substances they consumed for the purpose of staying awake (including illegal stimulants such as amphetamine) while they were driving during the previous month as well as the type and frequency used.

The likelihood of a crash associated with the use of substances containing caffeine was adjusted for other key factors (confounders) that could also influence the risk of a crash. These included:

age
health disorders
sleep patterns
symptoms of sleep disorders
kilometres driven
hours slept
breaks taken
night driving schedules

The analysis was appropriate and took the reasonable step of accounting for the key factors identified above.

What were the basic results?

The study recruited 1,047 drivers, 99% were men, 43% of drivers reported consuming substances containing caffeine for the purposes of staying awake and 3% reported using illegal drugs such as amphetamine (speed), ecstasy or cocaine.

At the start of the study, compared to control drivers, case drivers (the ‘crashers’):

were slightly younger (1.9 years younger on average)
had significantly less driving experience (4.8 years less on average)
drank less fluids (non-caffeinated)
were more likely to have had a crash in the previous five years (excluding the current crash)
drove less distance in the previous week
were less likely to have used substances to stay awake

Somewhat surprisingly, the crashers were found to have had more hours sleep.

After adjustment for the main confounders, drivers who consumed caffeine to stay awake were 63% less likely to crash compared with drivers who did not take caffeinated substances (odds ratio (OR) 0.37, 95% confidence interval (CI) 0.27 to 0.50).

This was adjusted for age, distance driven, hours of sleep, night driving, breaks taken and the Australian state where the crash (or recruitment) took place.

Interestingly, case drivers were also 81% more likely to have been in a crash in the previous five years (excluding the current crash) compared to controls (OR 1.81, 95% CI 1.26 to 2.62) suggesting the controls were the generally safer drivers.

How did the researchers interpret the results?

The researchers concluded that “caffeinated substances are associated with a reduced risk of crashing for long-distance commercial motor vehicle drivers. While comprehensive mandated strategies for fatigue management remain a priority, the use of caffeinated substances could be a useful adjunct strategy in the maintenance of alertness while driving.”

They went on to state that “consumption of caffeinated substances can significantly protect against crash risk”. This seems an overly bold conclusion to take from the research.

Conclusion

This study provides support for a common sense feeling that drinking coffee may keep you awake at the wheel and lead to fewer crashes. However, due to the case-control design of the study, it cannot prove cause and effect and there are other potential reasons for the association found. The following limitations and alternative explanations should be considered:

The results are most relevant to Australian lorry drivers travelling distances of over 200km from base. These findings may not necessarily translate into other countries where road types and conditions, as well as general road safety and laws governing how many breaks truckers should take, may vary significantly.
A case-control study can never prove cause and effect. This study found that people who had crashed their lorry were less likely to report consuming caffeine to stay awake. It does not show that drinking caffeine causes fewer crashes or protects from crashes. This is an important distinction, especially as it is easy to relax into believing this explanation as it makes some common sense. It could be that those who crashed may have generally been the poorer or less experienced drivers (suggested by the findings that they had less driving experience and were more likely to have reported crashing in the past five years than control drivers), and just so happened to drink less coffee. The two may not be causally linked.
In the same vein, any increase in safer driving associated with caffeine consumption could be mediated through having to take more toilet breaks at the roadside as coffee does have diuretic properties. This could potentially have had more influence than any stimulant effect of the caffeine. Hence, it could be other factors, such as more regular breaks, or generally safer driving, that are really associated with fewer accidents.
The study made some effort to account for the effects of regular stops, hours of sleep and other strongly influencing factors but there may be some residual effect from these. This could account for some of the link found between caffeine intake and crash risk.
The study used self-reported measures of caffeine consumption via interview that are prone to some error in recall. Particularly, those who crashed may be less likely to report drinking coffee to stay awake in an effort not to appear at fault for their accident by implying they were tired and needed the caffeine kick to keep safe. Those in the control group know they haven’t crashed recently so may be more likely to say they use caffeine to stay awake as they have a history of safe driving on their side. This could have biased the results to show a link where there may not be one.
One way to reduce this bias would be to regularly survey truck drivers for their caffeine habits prospectively and wait for instances of crashing before comparing the measures. This would be quite labour intensive, time-consuming and expensive which is probably why the pragmatic approach of using a case-control trial was taken.

Caffeine consumption may be associated with safer driving as the headlines imply, but this study does not prove it. There are other explanations to the findings of this particular research, and the limitations suggest it would be wise to interpret the findings with caution.

While caffeine consumption may give you a short-term boost to attention – excessive consumption can lead to lack of concentration, restlessness and irritability which could adversely impact your driving ability.

If you are planning to drive long-distance, it is recommended that you take a 15-minute break every two hours. You should not rely on coffee alone to ‘get you through’.