Tuesday, August 17, 2010

Traffic, NYC, NYT and the mysterious missing denominators

The NYT covers a report on pedestrian accidents on NYC streets from NYC's transportation department.

Here is a bit from the NYT summary:

Pedestrians would be well advised to favor sidewalks to the right of moving traffic — left-hand turns were three times as likely to cause a deadly crash as right-hand turns — and to stay particularly alert at intersections, where three-quarters of the crashes occurred.

Could it possibly be that more accidents happen at intersections because intersections are almost always where the pedestrians are out in the street with the cars?

Some denominators sneak in during a discussion of taxis, but prove a bit complex and confusing:
In Manhattan, about 16 percent of pedestrian crashes that led to death or serious injury involved a taxi or livery cab. Taxis account for only 2 percent of vehicles registered in the city, but at some times of day, they can make up nearly half of Manhattan’s traffic, according to some estimates — challenging the widely held perception of cabbies as the scourges of city streets.
Of course, the correct denominator is vehicle-miles, not vehicles registered, as taxis presumably get driven quite a lot more than the average vehicle, particularly in NYC. Thus, the numbers stated here are consistent with both taxis having safe, professional drivers who pose less of a danger than others and with them being the scourge of the streets. There just is not enough information to know.

Here is the wise transportation commissioner:
“One crash is one crash too many,” said Ms. Sadik-Khan, who said that Monday’s report would help her department “solve the riddle of why people are dying, and where they are dying, in the city.”
Of course, the optimal number of crashes is not in fact zero but rather the positive number at which the marginal cost of avoiding additional accidents equals the marginal cost from doing so. I guess there are no economists and no denominators at NYC's transportation department.

Not a very impressive performance from either the "newspaper of record" or NYC's transportation department.

Hat tip: Jesse Gregory (whose email said (correctly): "seems like the kind of article you love to hate")

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