Creation of bypass roads

International studies indicate that the installation of a bypass road, which leads non-local (through) traffic around a town or business district, reduces traffic accident rates on both the new and the old roads. The decrease in the overall accident rate varies between 19% and 66%. However, most before-after studies did not meet the requirements of a good before-after design: a selection bias may be present in the studies which may have led to some overestimation of the intervention effects. In general, safety effects of bypass roads are expected to be greater when the old road through town has a relatively high accident rate, when more traffic is shifted to the bypass road, when no extra traffic to either the old or the new road is generated, when speed-reducing measures are used to control for possible increases in speeding on the old road network, and when intersections between the old and new bypass road have a safe design. The general principle that bypass roads can lead traffic away from roads with a relatively high crash rate makes it a road capacity/road safety measure that will be effective in and transferable to most traffic conditions.