As congestion on two-lane rural roads increases, there is increasing interest in conversion to a four-lane divided facility within the same corridor. There is a surprising lack of research aimed at precisely estimating the safety benefits of these conversions. This lack is due in large part to the difficulty of conducting such research, because the best answer would result from analysis of a large sample of converted locations covering many different before-and-after configurations. The authors have attempted to estimate the benefits of such conversions by developing cross-sectional models producing crash rates for typical sections of two- and four-lane roadways in four different states. Predicted crash reductions for conversion from most typical two- to four-lane divided sections ranged from 40-60 percent. The reduction due to conversion to a four-lane undivided configuration is much less well defined, ranging from no effect to perhaps a 20 percent reduction. Continuing research needs include (a) verification of the undivided four-lane results, (b) additional information on the effects of driveways, (c) estimates for higher levels of two-lane average daily traffic, (d) expansion of the outcome variable to include crash severity, and (e) verification of all results by before-and-after studies of actual conversions.

Forrest M. Council, and J. Richard Stewart. Safety Effects of the Conversion of Rural Two-Lane to Four-Lane Roadways Based on Cross-Sectional Models, Transportation Research Record 1665, TRB, National Research Council, Washington D.C., 1999.