This is the first of a series in which I combine statistics from various sources in an effort to find out something mildly interesting.
It's pretty common, when cycling comes up, that someone will mention that cyclists are a menace and that they frequently endanger pedestrians. It is certainly true that cyclists do hit pedestrians, sometimes through careless cycling and sometimes not. On rare occasion these collisions are fatal for the pedestrian and/or sometimes for the cyclist. But are cyclists really the menace they're made out to be? Some people claim they're more scared of cyclists. Does that make sense?
Pedestrian crashes provide a pretty good means of determining the relative safety with which cyclists and drivers operate their vehicles. Assuming that pedestrians behave the same around cyclists as they do drivers, they can behave as a control group. That assumption may be flawed, as, for example, it may be that pedestrians rely too heavily on their sense of hearing and so step out in front of cyclists more often. Nonetheless, a comparison between crash ratios and exposure ratios can give an idea if cyclists really are more reckless than drivers.
Determining the reported crash ratio is pretty easy. DDOT did a study of bike and pedestrian crashes from 1997-99 and according to that report there were 29 reported bicycle-pedestrian crashes (BPC) and 1785 reported car-ped crashes (CPC) during that period. This gives a ratio of 1:62.
Of course, this only considers reported crashes. It is well documented that crashes, even ones with injuries, are under-reported. A study in Sweden determined that as few as 37 percent of all injury causing accidents are actually recorded. It has also been shown that the rate of under reporting is inversely related to the severity of the crash. Since a crash with a cyclist will be, on average, less severe than a crash with a car, it is reasonable to believe that there is more under reporting in BPC. Since there is no way to know this ratio, I'll use a variable U, where U/62 gives us the actual ratio of BPC to CPC [U=(% of actual CPC the reported CPC represents/% of actual BPC the reported BPC represents)]. Later we can guess as to what value of U makes sense.
This is a little tougher. What we have is a study that calculates the exposure of cyclists, pedestrians and drivers to drivers as expressed in miles for 2007 in DC. These are 37.2 million miles, 82 million miles and 362 million miles respectively.
One problem is that "miles" is the wrong measurement. Exposure by time makes more sense. I saw the tree across from my old place get hit by cars on two separate occasions. It traveled 0 miles in all that time, but it was certainly exposed to traffic. So these have to be translated into time. That can be done by using the average speed of each mode (3 mph for pedestrians, 12 mph for cyclists and a conservative 20mph for drivers*) Dividing by those speeds gives us the exposure as a measure of time. Note that the assumption here is that drivers on average go slower than the speed limit. This is not the case on area highways, and if too low it will understate the exposure of pedestrians to drivers.
Exposure as time (million hours)
Pedestrians - 27.3
Cyclists - 3.08
Drivers - 18.1
Using these numbers the ratio becomes 1:6.
Another problem is that we're using 2007 exposure data and 1997-99 crash data. There were more bikes on the road in 2007 than in 1997-99, so we need to reduce the exposure. Though it is not a perfect proxy, we can use the commuting rates to give an estimate of how much to reduce exposure. In 2007 the percentage commutes by bike in DC was 1.7%. We can extrapolate the percentage of bike commuters in 1997 by using the values from 1990 and 2000 which were 0.75% and 1.16% respectively and this gives us 1.04%. Which means we have to decrease the exposure of cyclists by about 40%. I'm going to assume that driver exposure is unchanged.
That changes the bicycle exposure to 1.88 million hours and the ratio to 1:9.6
However, that's still not accurate. Another problem is that these numbers are based on "exposure to cars" instead of "exposure to pedestrians" which is what we're considering here. Many of the hours for drivers are on highways where there are no pedestrians - and thus no exposure. Since not every minute of driving exposes a driver to a pedestrian, the driver number should be less than 18.1 million hours. Furthermore, pedestrians and cyclists can interact at more places than just crosswalks and parking lots. There are also sidewalks and trails, but these aren't considered in the exposure numbers. So the cyclist number should be more than 1.88 million hours.
While it is difficult to say how much the two are off by, it is almost surely true that the crash ratio is far less than 1:9.6
The ratio of reported BPC to reported CPC is 1:61 and the ratio of bike/ped exposure to car/ped exposure is no more than 1:9.6. Which means that each car on the roads of DC is at least 6 times more likely to be in a reported crash with a pedestrian than each bicycle.
U, mentioned above, would need to be slightly larger than 6.35 for BPC and CPC to be equal per capita. Or, for example, if 75% of car-pedestrian crashes are reported - as estimated in one study dealing only with injury-causing crashes, no more than 11.8% of bicycle-pedestrian crashes can be reported. This seems too low to me.
I think we can conclude the following from this:
It makes sense, because the lack of crash-protection equipment for cyclists, means they have more to lose in a crash than drivers do. Put another way, making cars safer has made driving less safe due to the Peltzman effect. It's reasonable to conclude that the sense of danger people have on bikes causes them to behave safer to compensate.
This analysis is not perfect. Some potential flaws:
1. The study that determined exposure had identified errors in their methods.
2. The ratio of unreported bike-ped crashes to unreported car-ped crashes is unknown
3. I used 1997-99 crash numbers with 2007 exposure numbers and my method for correcting this may be inaccurate.
4. My estimates for average bike and car speed are based on limited facts.
5. The exposure numbers were for exposure to cars, not to pedestrians.
*During congestion cars move at 14mph, but we're not always congested.