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Effectiveness of speed cameras in preventing road traffic collisions and related casualties: systematic review

BMJ 2005; 330 doi: https://doi.org/10.1136/bmj.38324.646574.AE (Published 10 February 2005) Cite this as: BMJ 2005;330:331

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confounding factors

The authors of this article mentioned the possibility of "confounding factors" affecting the apparent results of their analysis. I would like to suggest that there is a significant confounding factor in all the studies done on this subject. By way of doing so, let me relate a story.

While a medical resident, I commuted over a distance of 12 miles by bicycle and on weekends went on long recreational rides in the country. Once, I was asked by someone who saw me on a very convoluted, forested country lane why I would ride in such a dangerous area. I pointed out that I was never endangered on that road by a car because the drivers perceived that it was a dangerous road and were extremely alert.

Therefore, they always saw me and avoided me. Conversely, I was struck by cars on two occasions (one car struck me three times without the driver becoming aware of me) on a broad avenue with two traffic lanes and a full width shoulder in each direction. In all cases I was well onto the shoulder and "sideswiped" (fortunately without injury) as the car came onto the shoulder. My conclusion was that the driver's state of alertness and traffic awareness was the key factor in avoiding a collision. People driving on a wide, straight road tend not to pay much attention and use their spare attention to drink, eat, shave, and converse on the phone with their friends. Since that time, this conclusion has been reinforced many times. I would submit that the knowledge of being observed by "the law" heightens awareness and results in fewer collisions. The camera is simply the device that causes this phenomenon. I suspect that a policeman on foot at the roadside with a bullhorn admonishing drivers to pay attention to their surroundings would have the same effect at considerably less expense. Of course, it would fail to produce the revenues the cameras do, which would handicap the idea in the minds of both British and american politicos.

Competing interests: None declared

Competing interests: No competing interests

02 May 2005
James B Norwood
physician
Reno, NV 89513