Sunday, February 13, 2011

Safety in Numbers? Or safety in good cycling practices?

Steve Avery generated this plot with a random number generator
Steve Avery has some good comments about "Safety in Numbers" on his web page. I won't reinvent the wheel here, just refer you to his post.

I do worry about taking the Minneapolis safety assertions without a grain of salt. Plotting commuters against crashes (second figure) makes the assumption that commuters are a proxy for cyclists exposed to the risk of  crashes or of total cyclists.  However, if you look at actual data, turns out that most crashes are during the evening rush hours while few are during the morning rush hours. If commuters really are the population that is crashing, why are they only crashing while going home? I would want to test whether after-work or after-school recreational riders are crashing, thus helping create the bulge in the return rush hour numbers.
Actual Minneapolis data. Source document in pdf is here
Do we track those recreational rider numbers? The largest group of injured bicyclists are still in the 10-14 year old age group. So do we really know whether there are more cyclists on the roads, or simply a higher estimate of commuter cyclists (these are self-reporting data from surveys, not actual bike counts) driving down the so-called crash rate? Or for that matter, do we know how well crashes or injuries are counted?  In 2009, for example, there were fewer reported (957) bike crashes than (973) reported bike casualties (including 10 fatals). More tandems??? Or, counting errors?

Smeed's law, as well as mechanistic attempts to explain his numerical observations, go back a long way. I am sure there are advantages to having more cyclists on the roads, i.e., motorists have more experience interacting with bicyclists, there is an expectation of seeing cyclists, more money is spent on bicycling education and awareness, etc.. But I wouldn't count on numbers alone to keep one safe. This article for example suggests just the opposite of "safety in numbers" for motorcyclists (and based on how I saw or read about young servicemen riding their motorcycles in Honolulu, I can understand why--young riders were often engaging in high risk behavior, were riding beyond their competence, often on extremely high performance vehicles, or were drunk.)

Although one doesn't have 50-100 horsepower to abuse on a bicycle, take it from my personal experience: doing something dumb on either a motorcycle or on a bicycle is an excellent way to get hurt, no matter how many other people are riding today. Don't let "safety in numbers" convince you to be complacent. Its not simply more butts on bikes that makes for safety, but more brains on bikes.


Steve A said...

I retrospect, I wish I had labeled the abscissa as "Collision Rate" rather than "Accident Rate." However, maybe its appropriate, since the data in the plot is entirely invented anyway. The way those random numbers fell was entirely and really accidental!

SiouxGeonz said...

In my experience, my ride home is more hazardous than the a.m. ride. I'm not sure why, but it's busier and more frenetic.

Steve A said...

SiouxGeonz - I've counted the number of interactions I have per mile with motorists and there are at least 4X as many on the ride home as on the ride to work. That would be consistent with your experience.

Khal said...

I never counted, but can think of more memorably bad experiences in the morning. We have a denser morning rush hour vs. a less packed afternoon rush hour on the roads I use.

But that does sound like something worth investigating: asymmetric risk factors in the a.m. vs. p.m. commute. Thank you!

Khal said...

It might also be worth looking at crash patterns in Minneapolis. I have, for the last thirty two years, mysteriously lived east of where I work (on Long Island, in Honolulu, and in New Mexico). No, I never planned that, either.

But the result is I almost never have the sun in my eyes when I am commuting home or to work. That increased ability to see clearly probably puts me at an advantage, as well as ensuring that following motorists are able to see me clearly. I watch carefully for oncoming traffic at intersections, since sun-blinded or squinting motorists could be a hazard I have to consider.