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Hypothesized mechanisms for US safety in numbers

June 9, 2016

Hypothesized mechanisms for “safety in numbers”

Safety in numbers is a cycling safety rule that says that the more people ride bikes, the safer each rider will be. Hypothesized mechanisms include (1) driver familiarity – because drivers more often see bikes on the road, they become better-trained to see them on the road and (2) driver empathy – because so many drivers also ride bikes, they are more aware-of/concerned-about bicycle safety issues. (Here’s a nice pile of pointers to papers, tracked down by a real live researcher.)

I think both of these mechanisms are entirely possible, but riding an actual bike in actual traffic in actual crowds of cyclists, I’ve noticed what looks like other ways that greater numbers provide safety. In at least one case I’ve captured it on video. The difference between these mechanisms and the others that are hypothesized is that they are extremely short term – “safety in numbers” can appear whenever there is a biking crowd and disappear as soon as it disperses. These are also somewhat more likely in crowded urban areas and depend somewhat on the existence of traffic jams.

The first mechanism I might call “schooling” (after Bike Snob’s “shoaling” and “salmoning”). Bikes riding in a line are schooling, and for several common cycling hazards, most of the risk is borne by the lead fish, and the rest get a free ride. If someone in a parked car is not looking for bikes and is about to open their door, but then a bike zips by, it’s not unreasonable that they would be startled, and maybe then look to see if it was clear – and if the bikes are schooling, all the followers get the benefit of that. The dooring risk is almost entirely on the lead cyclist. Similarly, cars pulling into or across traffic represent a threat only to the lead cyclist, and very little to the ones in the rear. A line of bikes is also somewhat protective against right hooks, since those usually occur when a driver thinks they can overtake a bike and turn right, or forgets the presence of a single bike. With a line of bikes, once the first is across the side street, it is obvious to the driver that a right turn is not possible.

A second method is less obvious, but safety decreases markedly in the range of speeds between the slowest and fastest typical commuters. A low-speed (below 10mph) crash is stupidly survivable; you can almost step off your bike as it falls down. A high-speed crash (above 20mph) is far more likely to send you to the hospital or worse. Bike lanes at rush hour tend to run single file for some distance, usually because the bikes are hemmed in between parked cars on the right and “parked” cars on the left. Inevitably, some riders will be slower than others, and the inability to pass then compels the would-be-faster riders behind to slow down until they can pass. This makes them safer, whether they like it or not. This, I’ve seen on video, where I play the role of impatient rider. The probability of this delay and the difficulty of passing both rise pretty quickly once there’s more than a couple of riders delayed behind a slow leader.

After dark, a school-of-fish also multiplies the effectiveness of any lights that cyclists might be using. Just considering use of lights and not, if an unlit cyclist pairs up with one using lights, they can obtain most of the safety benefit of the lights. When two cyclists both have lights, the variations in their movement or in the flashing style of their different lights will create additional visibility over a single cyclist; for example, one cyclist’s flashing light might draw attention, but the other’s steady light might allow a driver to accurately locate the pair. Not nearly as many cyclists ride at night, but bicycle lighting use in the US is not nearly as good as it should be, so there’s plenty of room for this to help.

I don’t know if I’m typical, but if I’m riding at night and overtake another cyclist without lights who’s not too much slower than me, I’ll slow down to give them the benefit of my lights. I’ve even done this with a (impressively fast and competent) rollerblader caught out too late on the local multi-use path.

The interesting (to me) thing about these is that they can work in the US, they take no time to work, and they take no change in driver empathy or enlightenment. And if a crowd of bikes disassembles, then the safety effects do as well. The effects should appear most often at rush hours, when the largest number of bikes are on the road and when they are most hemmed in by traffic.

A historical/hysterical note is where the idea for safety-in-numbers comes from, and why we assume its existence even when we’re not entirely sure how it works. Once upon a time, when Effective Cyclists were peddling their prescriptions for safer cycling (ride in the road, in traffic, just like the “vehicle” that bicycles legally are, and that legal status is a good thing for which the EC movement certainly deserves some credit) the counterexamples of “the Dutch” and “the Danes” came up, where many people often ride bikes on lanes entirely separate from auto traffic, with crash fatality rates 5 times lower than ours. The EC people were very good at finding and/or interpreting studies that “proved” that if only the Dutch would get rid of their separate facilities, they would be even safer than they are now, that in fact their extraordinary safety must have some other cause. (This might even be true, but nobody’s ever managed to get more than about 1% of the population to bike in an “Effective” style.)

And what was the obvious difference that might be the cause of that anomalous safety? “Numbers”. It must be “Safety in Numbers”, assumed to exist to fill a (huge) gap between theory and reality. This was convenient for the Effective Cyclists because they got to continue to feel correct about their prescriptions (“just you wait, once everyone here rides bikes, we’ll be the safest cyclists on the planet!”) but now this same hypothesized mechanism is used to justify creation of cycling-specific infrastructure that Effective Cyclists hate (“we’re tired of waiting, EC is phenomenally unpopular and we’ll never get the numbers that give us the safety we want if we do it your way. And by-the-way, global warming, particulate pollution, pedestrian deaths, urban congestion delays, traffic noise, and public health, we need this now. Infrastructure will get butts in saddles and safety-in-numbers ‘proves’ that they’ll be safe.”)

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