Friday, April 4, 2014

Lies, damn lies, and statistics

Richard Hall - a person with opinions from Marin - wrote a blog post and sent me the link.

The article is called Cyclists Disregarding Red Lights a Major Cause of Accidents

He goes on to describe this and gives us some statistics he gleaned from the San Rafael Police Department. He then comes to the following conclusion:

This reinforces the need for two programs here in San Rafael: - SRPD enforcement of traffic laws for cyclists, at red lights and stop signs.

Interesting. Let's look at his statistics. He starts off with a set of statistics that show that there were 30 cyclist/motorist collisions from March 2013 to March 2014. The stats indicate that 50% of the accidents were the fault of the cyclist. Thus, "Cyclists disregarding red lights are a major cause of accidents at red lights."

Of course, this stat is for all accidents involving cyclists and motorists, not just the ones at red light. He does then go on to give the stats for bike/car accidents at red lights. There were two . And they were both caused by the cyclist. So the question going back to his original premise - does two accidents equal "Major Cause of Accidents"?

We don't know. The fact that both bike/car accidents were caused by cyclists means they were the ONLY cause, not just the "Major" cause ... of bike/car accidents at red lights. The sample size is pretty small, and one could pick nits with that, but the real reason that Hall's conclusion is bogus is because he didn't give us the numbers for Car vs. Car accidents at red lights. If cyclists caused 100% of the two bike/car accidents, but motorists ran into each other thousands of times, the primary cause of any accidents at red lights would be cars (I think it's safe to assume in a car vs car accident at a red light, a driver of a car was at fault).

There is this conclusion, at least amongst anonymous (and Richard Hall) internet comment board members that cyclists are doing all sorts of crazy things. So Hall has made the conclusion that we need to crack down on the rampant cyclist misbehavior. To prevent two collisions annually, a stat which is based on one year's statistics. And maybe by magic this specific program will do something about the other 13 collisions that apparently didn't happen the red lights that SRPD will be focusing on.

Any police department has limited resources, and they should be targeted at what causes the most problems. Hall has cherry picked the data and is looking for who causes the most problems in a specific circumstance, but police resources should be focused based on overall data, not just for one kind of circumstance. This is even before we discuss the magnitude of damage caused by the offender, with motorists causing a lot more damage per incident like driving through the front of a restaurant.

Thinking about that last link, maybe I need a blog post "Motorists Disregarding the Front of the Building a Major Cause of Accidents" and show that 100% of car vs restaurant accidents are caused by Motorists, ergo the SRPD needs to enforce traffic laws for motorists, at the front of Home Cooking Restaurants. Admittedly the sample size is small, but in 2103 it happened again the ever popular Motorist Careens down Sidewalk, Guide Dog saves Pedestrian, Car smashes through Store Window That gives us two pieces of Car on Building violence in 2 years in San Rafael alone.

Please, SRPD, think of the buildings!