A study was conducted to quantify the effects of horizontal curve features on such operational variables as changes in vehicle speeds and vehicle encroachments over the centerline and edgeline. This information was considered important in determining curve design criteria that would lead to effective safety and operational improvements at current curve sites. The data included geometric, traffic, and operational measures from a data base of 78 curve sites in New York State. Various statistical procedures were used, including linear regression analyses, analyses of operations by differing groups of geometric conditions, residual analyses, and locally weighted nonparametric regression. It was found that, as curves become sharper, there is a proportionally greater increase in speed reduction and edgeline encroachments on the inside lane (i.e., the lane on a curve where the motorist must steer to the right). Centerline encroachments in the outside lane increase more drastically than those on the sharper curves to the right. These findings support the contention of drivers cutting the curve short, which can result in run-off-road crashes on the inside of the curve as well as head-on and opposite-direction-sideswipe crashes with oncoming motorists. Appropriate curve design guidelines are discussed that may help to minimise these operational and potentially accident-related trends.
Abstract