Use of caffeinated substances and risk of crashes in long distance drivers of commercial vehicles: case-control study
“Use of caffeinated substances and risk of crashes in long distance drivers of commercial vehicles: case-control study” by Lisa Sharwood and colleagues (BMJ 2013;346:f1140, doi:10.1136/bmj.f1140).
Objective To determine whether there is an association between use of substances that contain caffeine and the risk of crash in long distance commercial vehicle drivers.
Design Case-control study.
Setting New South Wales (NSW) and Western Australia (WA), Australia.
Participants 530 long distance drivers of commercial vehicles who were recently involved in a crash attended by police (cases) and 517 control drivers who had not had a crash while driving a commercial vehicle in the past 12 months.
Main outcome measure The likelihood of a crash associated with the use of substances containing caffeine after adjustment for factors including age, health disorders, sleep patterns, and symptoms of sleep disorders as well as exposures such as kilometres driven, hours slept, breaks taken, and night driving schedules.
Results 43% of drivers reported consuming substances containing caffeine, such as tea, coffee, caffeine tablets, or energy drinks for the express purpose of staying awake. Only 3% reported using illegal stimulants such as amfetamine (“speed”); 3,4 methylenedioxymethamfetamine (ecstasy); and cocaine. After adjustment for potential confounders, drivers who consumed caffeinated substances for this purpose had a 63% reduced likelihood of crashing (odds ratio 0.37, 95% confidence interval 0.27 to 0.50) compared with drivers who did not take caffeinated substances.
Conclusions 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.
Why do the study?
Sleepiness at the wheel and reduced arousal are a common cause of road traffic incidents. It is estimated that sleepiness underlies about 20% of road crashes in Europe. The World Health Organization acknowledged this and launched the “Decade of Action for Road Safety 2011-2020” to start worldwide initiatives to reduce the number of road traffic incidents. Without action, road traffic incidents will become the fifth leading cause of death in 2030. Given the yearly increasing number of car crashes and the frequent association with sleepiness at the wheel, it is vital to examine ways to counteract drowsy driving. Hence the authors of this study deal with an important scientific subject with direct relevance to society as a whole.
Consumption of caffeinated beverages such as coffee and energy drinks is a popular strategy to maintain alertness while driving. This strategy is supported by evidence from double blind placebo controlled experimental studies on drivers in driving simulators, on public highways, and using tests that examine driving related skills and abilities. It is, however, important to note that an improvement in driving performance in an experimental setting does not automatically mean that drivers are less likely to have crashes in the real world. So this case-control study is important because the researchers examined whether there is an association between the use of caffeinated substances and the risk of a crash in real life.
What did the authors do?
The authors aimed to determine whether there is an association between use of substances that contain caffeine and the risk of a road crash in long distance commercial vehicle drivers. They also looked at whether the use of illegal stimulant drugs such as cocaine, amfetamines, and ecstasy influenced the likelihood of a crash.
The study was conducted among Australian commercial long distance drivers. The study design was a case-control study. In this type of study design, participants are not randomised to a condition or group, but it is observed in which group they fit (according to a set of criteria) and then the groups are compared. In this study “cases” were commercial long distance drivers who had a crash during the past 12 months (table 1). Case drivers (n=530) were identified from police records and recruited into the study by way of a letter of invitation. Control drivers were eligible to participate if they matched the same inclusion and exclusion criteria as case drivers, except that they did not have a crash during the past 12 months (table 1). Control drivers (n=517) were approached and interviewed at truck stops along major long distance routes in Australia, to ensure that they came from the same population as the cases.
|Characteristics||Cases (n=530)||Controls (n=517)||Unadjusted odds ratio (95% CI)||P value|
|Mean (SD) body mass index||29.7 (5.4)||30.8 (5)||0.97 (0.95 to 0.99)||0.002|
|Overweight*||203 (38.3)||188 (36.4)||1.02 (0.85 to 1.39)||0.52|
|Obese†||234 (44.2)||261 (50.5)||0.78 (0.61 to 0.99)||0.05|
|Mean (SD) hours of sleep||7.22 (1.3)||6.76 (1.4)||1.29 (1.17 to 1.42)||<0.001|
|Regular consumption of alcohol||357 (67.3)||408 (78.9)||0.48 (0.42 to 0.73)||<0.001|
|Mean (SD) No of standard drinks consumed per occasion||5.7 (4.3)||6.5 (6.0)||0.97 (0.94 to 0.99)||0.03|
|Regular exercise taken||221 (41.7)||230 (44.5)||1.1 (0.87 to 1.43)||0.36|
|Mean (SD) days of exercise/week||3.4 (1.8)||3.3 (2.1)||1.03 (0.94 to 1.13)||0.43|
|Previous crash‡||119 (22.5)||75 (14.7)||1.67 (1.21 to 2.29)||0.002|
|Mean (SD) km driven in past week||3178 (1585)||4384 (1701)||0.99 (0.99 to 0.99)||<0.001|
|Uses substance to stay awake:|
|No||368 (69.4)||227 (43.9)||1|
|Yes||162 (30.6)||290 (56.1)||0.34 (0.27 to 0.44)||<0.001|
|None||414 (78.1)||266 (51.4)||—||—|
|Low||26 (4.9)||29 (5.6)||—||—|
|Moderate||20 (3.8)||30 (5.8)||—||—|
|High||70 (13.2)||192 (37.1)||—||—|
|Consumption of energy drinks:|
|None||452 (85.2)||407 (78.7)||—||—|
|Low||16 (3)||16 (3.1)||—||—|
|Moderate||17 (3.2)||15 (2.9)||—||—|
|High||34 (6.4)||71 (13.7)||—||—|
|Often/sometimes used illegal stimulant¶||10 (1.9)||20 (3.9)||0.47 (0.22 to 1.03||0.06|
The case-control design is a suitable methodology to identify factors that contribute to the observed difference between the groups—that is, having or not having a crash. The researchers were particularly interested in whether consuming caffeinated beverages and illegal stimulant drugs are such factors. They also gathered information on a variety of other factors that may contribute to having a crash, such as driving experience, age, hours driven, hours of sleep before the trip, and number of breaks. In the statistical analyses, using logistic regression, the researchers controlled for these factors. This is important, because these factors can all contribute to having a crash.
The likelihood of having a crash was expressed using odds ratios. An odds ratio of 1 means that crashes are equally likely in cases and controls. An odds ratio greater than 1 indicates that crashes are more likely in cases, whereas an odds ratio less than 1 indicates that crashes are less likely in cases.
What did the study find?
Table 2 summarises the results of the study. The researchers showed that consuming caffeinated substances reduces the likelihood of crashing among long distance drivers by 63% (odds ratio 0.37, 95% confidence interval 0.27 to 0.50). By contrast, the use of stimulant drugs was not associated with a reduction in the likelihood of having a crash.
|Use of stimulant substances||Cases (n=530)||Controls (n=517)||Adjusted* odds ratio (95% CI)|
|No†||368 (69.4)||227 (43.9)||1.00|
|Yes||162 (30.6)||290 (56.1)||0.37 (0.27 to 0.50)|
|Never†||520 (98.1)||497 (96.1)||1.00|
|Often/sometimes||10 (1.89)||20 (3.87)||0.68 (0.27 to 1.67)|
|Previous crash§||119 (22.5)||75 (14.5)||1.81 (1.26 to 2.62)|
What are the strengths and limitations of this study?
The authors acknowledge that data from driving simulators and experimental studies are available, but they should always be critically examined to determine to what extent testing under these artificial circumstances (that is, no real risk of injury or death when having a crash) has relevance to actual driving in the real world. This real life evidence was obtained in the current case-control study.
Another strength of the study is that the researchers included commercial drivers. Although driving is an over-learnt daily activity of many adults, daily driving time differs significantly, as does driving experience and quality. By including only commercial drivers used to travelling long distances it can be assumed that the observed crashes in this study are not likely to be caused by a lack of driving experience. The fact that 99% of participants were men should not be seen as a limitation of the study, as this reflects the actual sex differentiation among commercial long distance drivers.
Experimental studies showed that a significant improvement in driving was evident after consuming relatively low dosages of caffeinated beverages—that is, 80 mg caffeine, found in one cup of coffee, or a 250 mL can of energy drink. It would have been interesting to examine if caffeine dosage has an influence on the risk of a crash. Although the researchers divided drivers into three groups according to the amount of caffeine that was consumed: low (<200 mg), moderate (200-400 mg), and high (>400 mg), they did not compare the crash risk of these groups. Instead, for their statistical analyses the authors used a binary division (caffeine consumed was divided into “yes” and “no” groups). Unfortunately, the sample of stimulant drug users was too small to draw reliable conclusions.
What does the study mean?
In line with experimental studies, this case-control study adequately showed that consumption of caffeinated beverages such as coffee and energy drinks improves alertness, which has a positive impact on potentially dangerous activities such as driving a car. These findings have direct relevance for traffic safety. Long distance drivers should be advised to have regular breaks during trips, especially when they experience sleepiness at the wheel. The results of this study suggest that during these breaks, consumption of caffeinated beverages may further improve drivers’ alertness and reduce the chances of having a crash.
Evans J. The perils of pedalling. BMJ 2013;346:f3360.
1Division of Pharmacology, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Utrecht, Netherlands, 2Centre for Human Psychopharmacology, Swinburne University, Melbourne Victoria, Australia
Correspondence to: J.C.Verster@uu.nl
Competing interests: Over the past five years JV has received research support from Takeda Pharmaceuticals and Red Bull and has acted as a scientific adviser for Takeda, Sanofi-Aventis, Transcept, Sepracor, Red Bull, Canadian Beverage Association, Deenox, Trimbos Institute, and Centraal Bureau Drogisterijbedrijven.
Provenance and peer review: Commissioned; not externally peer reviewed.
Cite this as: Student BMJ 2013;21:f3696