Abstract
The results of a study on the effects on injury accidents of 20 bypass road projects in Norway are reported. Effects were evaluated by means of an observational before-and-after study, controlling for the regression-to-the-mean effect and general areawide trends in the number of accidents. On the average, a statistically significant reduction of 19 percent in the number of injury accidents was found. The two confounding factors controlled in the study did not significantly affect the results. The results of the Norwegian study are consistent with the results of previous studies that evaluated the road safety effects of bypasses. A meta-analysis of 9 evaluation studies, containing a total of 93 estimates of the effects of bypasses, indicated an average reduction of 25 percent in the total number of accidents. Effects did not vary according to accident severity. Note, however, that none of the evaluation studies employed a sufficiently rigorous study design to rule out the possibility that confounding variables not controlled may have influenced the results.
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