Feb 1, 2012 10:45 AM
Dear Sheffield University Cyclists,
Here's a debate that's been doing the rounds for a while. At times it seems that opinion is split between "pro helmet" and "anti helmet" camps, with your camp of choice being largely dependent on whether or not you wear a helmet! As always, the discussion is more complex than that, so a few of us have been bouncing emails back and forth about the evidence. Beyond the ambiguous question "should cyclists wear a helmet?" (highly context dependent and subjective), there are a number of other questions about helmets that have been answered in the literature:
Is there evidence that helmets can be a barrier to cycling uptake (either due to cost or convenience or appearance)? For children (the most vulnerable group), apparantly so 
Does mandatory helmet wearing reduce the rate of cycling? It appears so, based on evidence from Australia 
Is it true that cycling becomes safer the more people cycle? Apparently so 
Is there evidence that helmets reduce the severity of head injuries due to linear acceleration of the skull? Yes 
Is there strong anecdotal evidence of the efficacy of helmets? Most definitely - many cyclists have a "helmet saved my life" story, or some more or less exagerated variation on the theme - sor example I have a friend who urges people to wear them after hitting a bus.
Is there controversy about the efficacy of helmets to protect the head in the scientific literature? Yes 
Do some people limit the potential benefit of helmets by wearing them incorrectly? Yes - around 15% according to one study 
Do head injuries account for a large proportion of cyclist deaths? Yes - "61.7 percent of motor vehicle collision deaths were due to head injury." 
What proportion of cyclist injuries are to the head? - Can't find information on this - any ideas? My gues is ~20%
The thing that amazes me is the amount of high quality information about specific questions. I had little idea about all this data, and there is much more out there, so hope this is of use to others involved in helmet debates. Please add more evidence!
For what its worth (not a lot) my opinion is that the fewer barriers to people cycling the better. Cycling is not a single activity - cycling to the shops is not the same as going mountain bike riding or road riding in the peaks - and helmet recommendations should vary accordingly. Cycling only has an environmental benefit if it replaces fossil fuel transport in some way and, these types of trips may not always benefit from wearing a helmet (e.g. riding to the pub and back - not drinking would have a far greater impact or young commuters seeing helmets as a sign that cycling is a dangerous activity not riding because they don't have a helmet or don't like wearing one).
However, there is strong evidence to suggest that well designed helmets worn properly can reduce the impact of certain types of crashes and this is undeniably a good thing. The additional sense of security and visibility offered by helmets are other benefits - a confident cyclist is less likely to get hit. Therefore my advice is
'wear whatever you feel most at confident in for the specific trip you are undertaking'.
Regarding spending money on a helmet, I suspect that in Sheffield investing in a professional service of the brakes (new brake pads, inner cable, possibly outer cable, lubrication), tires (new tires suited to the terrain and cyclist behaviour), wheels (is the rim wall about to explode?), LIGHTS (2 rear, 2 front at least) and other critical components (simply buying a track pump or new brake blocks can increase safety margins) and good cycle training is 9/10 money better spent than a $100 lump of moulded petrochemicals that you strap to your head!
What advice would you give?
1. "Children are more likely to simply fall off their bicycles and may therefore derive more benefit from wearing a helmet. However, the cost—between £12 and £90—and the necessity of replacing helmets every few years as the child grows may be prohibitive." http://www.bmj.com/content/318/7197/1505.2
2. "The Australian state of Victoria made the use of helmets compulsory in 1990, and in the following year deaths and head injuries among cyclists fell between 37%and 51%However, 40%fewer adults and 60%fewer children continued to cycle after the introduction of the laws. http://www.bmj.com/content/318/7197/1505.2
The full article states that "This suggests the greatest effect of the helmet law was not to encourage cyclists to wear helmets, but to discourage cycling. In contrast, despite increases to at least 75% helmet wearing, the proportion of head injuries in cyclists admitted or treated at hospital declined by an average of only 13%." http://www.sciencedirect.com/science/article/pii/0001457596000164
From the same article: "The benefits of cycling, even without a helmet, have been estimated to outweigh the hazards by a factor of 20 to 1 (Hillman 1993; Cycle helmets—the case for and against. Policy Studies Institute, London)"
3. See Fig. 12 of this paper ( http://ubuntuone.com/2mK9tYl8s5AmuXLwXihihp ). There is a clear inverse relationship between cyclist deaths
(per km) and the distance cycled in the Netherlands. This relationship near universal, suggesting that simply encouraging cycling may be a good safety measure (on a per km cycled basis - which is how the rate should be reported due to the disproportionate health benefits of cycling):
"The phenomenon of ‘safety in numbers’ has consistently been found to
hold over time and across cities and countries. Fatality rates per
trip and per km are much lower for countries and cities with high
bicycling shares of total travel, and fatality rates fall for any
given country or city as cycling levels rise (Jacobsen, 2003).
Figure 12. Inverse trends in cycling fatality rates and annual
kilometres cycled per inhabitant in the Netherlands (1950–2005)."