Does “photo traffic enforcement” really save lives, as the MPD claims?

Photo enforcement saves lives The Metropolitan Police Department (MPD believes that their traffic cameras are responsible for much of the sharp decline in traffic fatalities in the District: “Whereas nationally the fatality/100K population has decreased from 2001 to 2011 by 28%, DC’s rate has decreased by 69%”. The implication is that most of the decrease in traffic fatalities in DC since 2001 is due to MPD “photo enforcement”.

Is this so? If red-light cameras are reforming driver behavior at inter­sections, and speed cameras are cauing lower vehicle speeds, one should expect to see substantial reductions in traffic collisions, and in pedestrian fatalities. But neither is in fact observed.

While the traffic death count in DC dropped by half during the 2000-2009 decade, the total number of traffic collisions decreased by only about 9%. Between 2007 and 2009, the number of collisions actually increased by 5%, even as the traffic fatality count decreased by 30%.

traffic collisions

How can a modest decrease in collisions result in a large, 50% reduction in fatalities? How can the fatality count continue to decrease, even when the number of collisions is increasing? It seems clear that safer cars are responsible for this reduction in road fatalities. In 1989, Federal law began mandating passive restraints in passenger cars, and recent model cars now feature front and side air bags, crash-absorbing bodies, and passenger-protecting cage structures to improve passenger survivability in collisions. So, as older cars are retired and replaced by newer, safer cars, traffic fatalities decline, in DC and nationwide.

These safety features are certainly more effective at the moderate-speed collisions of the city than at highway speeds, a plausible explanation for the larger decrease in fatalities in the urbanized District than nationwide.

Also, despite the proliferation of red-light and speed cameras in the District since 2001, pedestrian deaths in DC have hardly changed: 11 in 2001, 13 in 2010. The figure shows pedestrian deaths in DC since 2000 (National Highway Traffic Safety Administration data), and total traffic deaths with pedestrian deaths subtracted, for an approximate count of vehicle occupant deaths (motorcycle and bicycle deaths will be included). Clearly the big decrease in recent years has been in vehicle occupant deaths, whereas pedestrian deaths have hardly changed.

This is consistent with the hypothesis that the reduction in traffic fatalities is due largely to safer cars. Air bags and other passive restraints that protect vehicle occupants do nothing for pedestrians.

pedestrian vs car occupant deaths

The static count in pedestrian deaths, while automobile fatalities were decreasing, led to the recent worried observations that more than half of D.C. traffic deaths in 2010 were pedestrians. That wasn't because pedestrian deaths were up, but because vehicle occupant deaths have gone down, while pedestrian deaths have remained constant.

Traffic cameras might conceivably play a contributing role to fatality reduction, by lowering speeds at which collisions happen. That's evidently what the MPD wants to believe; if traffic accidents aren't much decreased, then it must be lower traffic speed that is reducing traffic deaths. But the record on actual speed reductions due to speed cameras and other enforcement measures is, at best, mixed.

Easily the best explanation for the decreasing fatality count on DC streets is increased survivability of collisions by the occupants of cars, thanks to the safety measures brought about stringent Federal Government requirements. This explains why fatalities, predominantly among vehicle occupants, are down sharply, though collisions are not much decreased, and pedestrian fatalities remain unchanged.

The MPD claim that their efforts are to be credited for much or most of the fatality decrease on DC roads is just not credible, given the relatively small decrease in the number of actual vehicle collisions, and the absence of a significant decrease in pedestrian deaths.

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Page created October 5, 2012