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Friday, August 20, 2010

Useless Statistic, Caught in the Wild

Schweyen and city traffic engineer Gary Shannon said a traffic signal is no panacea. A city study of pedestrian accidents between 2002 and 2007 recorded only one accident at Broadway and Third Street, in 2005, Shannon said. Several signalized intersections had many more pedestrian accidents in the study period — including six at South Broadway and Fourth Street and five at South Broadway and Second Street, Shannon said.

— From a Rochester Post-Bulletin newspaper article

Presumably, this article paragraph is saying that a signal may not be needed on Third Street because some other intersections with signals have more accidents than this one. But hold up: does this matter? Is this statistic even meaningful?

An intersection can have a high rate of accidents for a variety of reasons, ranging from usage, to bad lines of sight, to user error. A traffic light can help with some problems, but a very busy intersection will still have accidents.

In fact, this is a sort of a false dichotomy. The choice being made isn’t where to spend money between these specific intersections, it’s simply whether to add a light on one of them. Whether other intersections that already have lights have more accidents or not isn’t really a useful datum.

What we really ought to be told is how much the accident rate has improved (we hope it’s improved) at those intersections after they added the signals and how much it would help (projected) to add one on Third Street. Or, comparing apples to apples, what other intersections are problems where that same money might be spent to alleviate accidents. (Either by adding or by upgrading lights.)

posted by John Weiss at 13:23  

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