Bike helmets are less effective than we think
Previous research may have overestimated the protective effect of bicycle helmets, new analysis reveals.
Bicycle helmets reduce the risk of head injuries, but nowhere near as much as previous studies have shown.
According to an Australian meta-study from 2001 of the efficacy of bicycle helmets, a helmet can reduce the risk of head injuries by 60 percent.
However, a new re-analysis of this study suggests that cycling helmets do indeed reduce the risk, but far from as much as the previous studies showed.
The new study was conducted by Norwegian traffic researcher Rune Elvik, the chief research officer at the Institute of Transport Economics, Oslo, Norway, and a professor at Aalborg University, Denmark.
Previous studies overestimated safety benefits
One would think that the earliest bike helmets that were made would be less effective than the ones that are made today, but we’re actually seeing an opposite trend: it has become less safe to wear a bike helmet over the years, and that is surprising.
According to Elvik, the 2001 study contains two errors, which lead to inflated results about the efficacy of cycling helmets:
- There was a tendency towards publication bias, i.e. there was a tendency that academic journals are less likely to publish studies that go against the prevailing view – in this case, studies that show more injuries with helmets than without, or that the helmets only have a limited effect.
- The most recent studies featured in the 2001 meta-study showed a smaller effect of the helmets than the older studies did.
Limited effect in recent studies
Adding another four studies to the equation did not change much: the effect in the recent studies was simply smaller than in the older studies, and this reduced the overall assessment of the efficacy of the helmets.
Looking at the overall effect of the helmets on injuries to the head, face and the neck, Elvik argues that the helmets reduce the risk by 33 percent, which is almost half as much as the 2001 study found.
If we look only at the studies published after 2000 and take into account the apparent fact that academic journals are less likely to publish studies that point towards a limited effect of the helmets, the overall effect of the helmets disappears altogether, argues the researcher.
Fewer head injuries but more neck injuries with a helmet
However, if we look at the individual injuries, he continues, there are indications that the helmets provide better protection against serious injuries than against less serious ones.
”In terms of head injuries, there is a clear reduction in injuries when people wear a helmet. But if we look at e.g. the neck and the back of the neck, then all studies indicate a certain risk of injury when wearing a helmet,” he says, adding that neck injuries are usually not as serious as head injuries.
Have bike helmets become less safe?
Why is it, then, that bike helmets appear to have become less safe over the years?
“One would think that the earliest bike helmets that were made would be less effective than the ones that are made today, but we’re actually seeing an opposite trend: it has become less safe to wear a bike helmet over the years, and that is surprising.”
One explanation, he says, may be that the older studies predominantly used helmets with a hard shell, whereas the more recent studies mainly used helmets with a softer shell, which have been shown to be less effective:
”The earliest helmets were modelled on motorcycle helmets and covered the entire head. The bicycle helmets that people use today have ventilation holes to keep the head cool. Studies that have compared these two types of helmets have shown that the new type provides less protection than the old one.”
Read the Danish version of this article at videnskab.dk
- Publication bias and time-trend bias in meta-analysis of bicycle helmet efficacy: A re-analysis of Attewell, Glase and McFadden, 2001. Accident Analysis & Prevention. http://dx.doi.org/10.1016/j.aap.2012.12.003
- Bicycle helmet efficacy: a meta-analysis. Accident Analysis & Prevention. Volume 33, Issue 3, May 2001, Pages 345–352. http://dx.doi.org/10.1016/S0001-4575(00)00048-8
- Helmets for preventing head and facial injuries in bicyclists. The Cochrane Library. DOI: 10.1002/14651858.CD001855
- Drivers overtaking bicyclists: Objective data on the effects of riding position, helmet use, vehicle type and apparent gender. Accident Analysis & Prevention. Volume 39, Issue 2, March 2007, Pages 417–425. http://dx.doi.org/10.1016/j.aap.2006.08.010