Summary: Dr Ian Walker's measurements show that under some conditions British drivers leave 3.3 inches more passing distance if the cyclist is not wearing a helmet, and another 2.2 inches if the cyclist is wearing a wig. The average passing clearance for all three cases was more than four feet. The cyclist's position on the road changed everything, canceling the difference at times. A new study in 2013 supports our contention that Walker had misinterpreted his data.
Note: the full study was published in Accident Analysis and Prevention 39 (2007) 417–425.
The BBC reported on September 11, 2006, that a study conducted in Salisbury and Bristol, UK showed with very precise measurements of 2,500 passing cars that the test cyclists were given 8.5cm (3.3 inches) more clearance by cars if they were not wearing helmets.
When the researchers donned female wigs they got more clearance, 14cm (5.5 inches) more than apparent males in helmets. They did not report on what a skirt and helmet combination would do. The author was hit by a bus and a truck during the experiment, and was wearing a helmet both times.
The BBC article on the study speculates on the psychology involved. The researchers think that cyclists in helmets are perceived as experienced cyclists who merit less passing clearance. Dr. Walker, a traffic psychologist from the Bath University's Department of Psychology, and author of a book on vulnerable road users, is quoted: "This study shows that when drivers overtake a cyclist, the margin for error they leave is affected by the cyclist's appearance." The current discussion is based mostly on a press release and the BBC report, since the study had not yet been published.
No argument there, since an erratic child would get more clearance than any other rider, helmet or no. But here are some more quotes the BBC attributes to Dr. Walker:
"We know helmets are useful in low-speed falls, and so definitely good for children, but whether they offer any real protection to somebody struck by a car is very controversial.
"Either way, this study suggests wearing a helmet might make a collision more likely in the first place," he added.
Dr Walker thinks the reason drivers give less room to cyclists wearing helmets is because they see them as "Lycra-clad street warriors" and believe they are more predictable than those without.
The study measures something that might not be readily apparent to most cyclists--an average of 3.3 inches more clearance. The extra distance would make quite a difference if the average passing distance were 1.5 feet and a lot less if the full recommended 3 feet were the rule. In a Web paper discussing his findings, Dr. Walker notes that the average passing distance was 4.5 feet for black cars and 4.1 feet for white trade vans. The difference of 3.3 inches fades to insignificance with passing clearances that good. With lanes of 8.5 feet, the motorists were giving him a half-lane of clearance whether or not he had a helmet or a wig on. In most cases they were shifting over the line to the other lane to pass. Some of the effect is probably due to the "shy distance" from the curb on the opposite side of the road--the driver's side--as they passed. Cyclists in the US would be very pleased with that much consideration by motorists.
Other findings from the study make it plain that the cyclist's position on the roadway (distance from the edge) made more difference than the wearing of a helmet or a wig. When the cyclist is 1.25 meters (4.1 feet) out--the center of the lane--there was a substantial difference in motorist behavior attributed to the helmet. But with the cyclist riding one meter (3.3 feet) from the side of the road--just 10 inches toward the curb--there was virtually no difference. Both findings are certainly food for thought, as is the information that the roads in question were two lane with 8.5 foot lanes. In the US our lanes are generally wider than 10 feet and often 12 or more. But our vehicles are typically wider as well.
It is curious that the BBC report on the study focused just on helmet wearing as the main factor affecting drivers' responses. The position on the roadway variable was ignored. We note that in the BBC photo, the test cyclist is riding a bicycle with an upright position and large black panniers. That would probably have an effect on car drivers, since the panniers add extra width and very likely increased passing distances. Users of side-mounted flags report the same effect. And an upright cyclist might indeed be perceived as more likely to be unsteady, helmet or no. We speculate that perhaps the clearance would be even less for a rider who appeared to be a racer and seemed more likely to be holding a steady road position. But that might have nothing at all to do with whether or not the cyclist was wearing a helmet. Children weaving along the road here get more clearance than other riders, whether or not they wear a helmet. So the study is interesting, but the reporting on it would be a lot more useful if it included the findings on some of the other factors instead of trying to marginalize the helmet as "good for kids" and useful only in "low speed falls." We have enough experience here with helmets and car crashes to have convinced the cycling community that the protection offered even in a car crash is real and not controversial. And we disagree with the author's conclusion that anything he measured indicates that the helmeted cyclist is more likely to be sideswiped.
It seems unlikely that average passing clearance was the best way to test Dr. Walker's hypothesis. A better way would be to establish a danger zone and look at the percentage of "violations" of that space for the helmeted and unhelmeted riders.
We hope that somebody will replicate this work in the US to see if our driver psychology matches, and that they will test for some of the other variables in addition to helmets. Research on driver psychology could help us understand what is needed to encourage drivers to give all cyclists adequate passing clearance. We would be really happy with the four feet the British cyclists were getting. We don't think it is likely to be as simple as who wears a helmet, and given the safety tradeoffs we don't think that taking your helmet off or wearing a wig is the best way to get you 3.3 inches more clearance from passing motorists who are already giving you more than four feet of space. It would appear that at least in the UK cities of Salisbury and Bristol, riding exactly 3.3 feet from the edge of the road has more effect.
Response from Dr. Ian Walker:
Thanks for getting in touch. The full study had a lot of detail which obviously couldn't be included in a press release, and several other factors as well as helmets were considered. Your point about absolute distances is a good one, so you might be interested in one analysis in particular. I compared the group of overtakers who came closest with the group who left the most room, and you can see their data in Figure 2 in the attached document (you can also see exactly how close drivers came from Figure 1 too). What figure 2 shows is the ratio of particularly close overtakers to generous overtakers in each category - a score over 1 means there are more people passing close than leaving plenty of room. Note that the helmet effect varies with riding position, but leads to almost twice as many people getting particularly close in the 1.25m condition when the helmet was on, which is pretty interesting I think. Feel free to mention this on your page.
The study is due to appear soon in Accident Analysis and Prevention.
One thing I should add (as it has come up in a discussion elsewhere)
is that our roads here in the UK are quite narrow. A typical urban
road is about 5.2m across, so each lane is about 2.6m. This means
that the 1.25m position was about the centre of the lane.
2013 Study by Olivier and Walter:
Walker mistinterpreted his data
The previously observed significant association between passing distance and helmet wearing was not found when dichotomised by the one metre rule. Other factors were found to be significantly associated with close passing including cyclists' distance to the kerb, vehicle size and city of observation (Salisbury or Bristol, UK). P-values from bootstrap samples indicate the significance of helmet wearing resulted from an overly large sample size.
After re-analysis of Walker's data, helmet wearing is not associated with close motor vehicle passing. The results, however, highlight other more important factors that may inform effective bicycle safety strategies.
For an entirely different perspective, see this study of bike lanes done in Texas. They add bike lanes to their streets to persuade cyclists to get away from the curb, and to reduce the "swerve" of cars going around bicyclists. That would seem to reduce the passing clearance by design, without accounting for the requirement wherever there is parking for bicycles to use the extreme edge of the lane to avoid being doored by parked cars. What a difference a few thousand miles and a few cultural dissimularities can make.
New York City Study
New York's numbers show that the safety tradeoff would be particularly bad there if you left your helmet at home. They found that 97% of the cyclists killed in traffic there had no helmet. We have more on our Stats page.