Why we fight about black spots
- Correspondence to: P L Jacobsen 2771, 14th Street, Sacramento, California 95818, USA;
- Accepted 4 October 2006
Injury prevention—need for a consensus between epidemiologists and traffic engineers
In this issue, Morency and Cloutier1(see page 360) report that, over a 5-year period, motorists injured pedestrians at exactly one quarter of all intersections in central Montreal. Those of us alarmed over unsafe streets will not be surprised by this finding. But I suspect that many of those setting traffic policies will be.
That is because the roads agency in Montreal operates under a policy of fixing collision “black spots”, where a “black spot” is an intersection with eight or more collisions in a 5-year period. Amazingly, seven motorist–pedestrian collisions in 5 years—that’s one in every 260 days—is not enough to brand an intersection as dangerous. The black spots approach fixes only 1% of the intersections where a motorist hits a person.
The unspoken premise of this policy seems to be that a high frequency of collisions is evidence of design problems unique to that intersection. However, Morency and Cloutier’s report suggests that faults in the road design are not site-specific but are nearly universal.
Morency and Cloutier approach the data from the perspective of epidemiologists and present the data in terms that make the problem particularly vivid: one quarter of all intersections were the site of a collision over a relatively short period of 5 years. By contrast, traffic engineers identify 22 black spot intersections for fixes.
These two different presentations may result from the different perspectives of the different professions. …