Ok, I am sure the Man-Who-Give-Us-All-The-Finger-With-Every-Post will join us again soon. Why would I wear a helmet while cycling and not while driving? I think the answers already given are sensible and adequate. But then I started thinking about the odometer readings on my cars and bikes. I do about 12k miles a year in the cars, average over the last 6 years. I started biking again last year after at least a decade's absence because I lost the shuttle service between my work rail station and work itself. I don't really have any valid long term data on my cycling mileage. I was easily going to hit 1500 miles this year before my accident and I think that is still a very likely total for the year. It is easy enough to go through the math and calculate the relative risks of cycling versus driving for someone who drives/cycles either the same mileages or drives/cycles in the same ratio, 8 to 1.

For driving I am using the 2008 data from the DOT on motor vehicle crashes. I am adjusting the death rate for motor vehicles down to 45% of the actual value because 55% of those who die in motor vehicles were not wearing seat belts. Get ready for a big surprise everyone, I always wear a seat belt while driving! In order to accurately calculate the MV death rate per VMT I am adjusting the given VMT for 2008 to 82% since the seat belt usage rate is given as 82% for 2008. So I am using the death rate for MV occupants who wear seat belts in my calculations. For the cycling data I am using a combination of DOT and CPSC data. The DOT only collects data for MV accidents, the CPSC collects data from hospitals to get the total rates for activities like cycling. The CPSC finds that MV's are involved in 90% of cycling deaths (another big surprise) so the DOT numbers have to be adjusted upwards accordingly to account for the 10% of cycling deaths that are due to other factors. The CPSC finds that MV's are involved in only 10% of cycling injuries. I have to say that figure surprises me yet it apparently holds true over long periods of time. I would have thought that MV's were involved in a much larger percentage of cycling injuries. Accordingly I multiplied the DOT injury rate by 10 to get the total cycling injury rate for 2008. Well cycling helmets are designed to prevent fatal injuries within specified impact speed limits so I am including the injury rates mostly for completeness. Cycling helmets are not as effective as they should be at preventing non-fatal head injuries and I have found no data that establishes how effective they are at that. Both the Carpenter and Stehr report mentioned above and the DOT observation that only 9% of bicycle fatalities in 2008 wore helmets suggest that in real world accidents cycling helmets cut the death rate for those that wear them by half and possibly by two thirds. The only estimate I have for cycling VMT is the 2001 data that says it is between 6 and 21 billion miles so that is what I used for the calculations.

Crunching all the numbers I find that for seat belt wearers who drive and cycle in an 8:1 ratio like myself your chances of dying on a bicycle versus dying in a car in a given year are between 0.78 and 2.7. So you are somewhere near just as likely to die while cycling to nearly three times as likely to die while cycling. Does it make sense now to wear a helmet while cycling? It cuts your total chance of dying on the road by between 22 and 36% if you accept that cycling helmets have about a 50% success rate in real world impacts. Just for completeness your chances of injury on a bike are 3.4 to 12 times as high as your chances of injury in your car under the same circumstances. With no data on how effective cycling helmets are at preventing non-lethal head injuries it is impossible to estimate what effect they have on your total chance of a road injury. I would guess they are no better at preventing injury than death even though the chances of injury on a bike are considerably higher than in a car since very many injuries on a cycle do not involve the head and a helmet can do nothing for them.

I would imagine that many forum stalwarts cycle far more in a year than I, but it is also likely that many of you drive more than I do too. If your ratio of cycling to car driving is very much larger than mine the benefit to you from wearing a helmet while cycling but not while driving is even larger. If your ratio is smaller your benefit is smaller and you might achieve more by wearing a helmet while driving. We still have the problem that we have absolutely no information that tells us what helmet is needed while driving or whether such a helmet even exists or what its real world impact on death rate is likely to be. Without that information it is impossible to implement the notion with any confidence anyway. The information on cycling helmets on the other hand is widely available.

So that is why you wear a helmet while cycling and not while driving.

Ken