By Michael Brenneis

Unlike other risk factors such as speed or impairment—where more is not safer—increasing the number of sharp curves on a road segment appears to lower the risk of crashing, according to a new paper in the journal Accident Analysis & Prevention. Curves carry a level of inherent risk—they’re more dangerous than a straight segment of road, independent of other risk factors. As this paper points out, a decrease in a curve’s radius can make it safer, as can an increase in the number of sharp curves on the road itself.

Each of the five studies reviewed in this paper describes a functional relationship between the spacing of curves on a road segment and the relative risk of crashing on a curve of a specified radius. For example, as the length of straight road preceding a curve increases, so does the crash rate on the curve. The crash rate was also found to increase as the distance between curves increases. Further, the risk of crashing in a curve, or between curves, is reduced as the distance between curves is reduced.

Using a 100m—radius curve (a sharp curve) all of the examined papers determined that crash rates increased as the mean distance between curves increased. Setting aside all other risk factors, and assuming a constant traffic volume, curvy roads make safer roads.

Why are more frequent curves safer? Traffic volume could be inherently lower on curvy roads if drivers seek out straighter roads that can accommodate higher speed. Drivers may slow down and pay more attention on curvy roads than they would on straighter roads with more infrequent curves. Or, drivers could be surprised by infrequent curves and not react quickly enough to prevent a crash.

Michael Brenneis is an Associate Researcher at SSTI.