Safety Effects of the Conversion of Rural Two-Lane Roadways to Four-Lane Roadways
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Safety Effects of the Conversion of Rural Two-Lane Roadways to Four-Lane Roadways

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      Given the lack of funds for new highway construction and the ever-increasing traffic flows, the issue of conversion from two lanes to four lanes is of increasing importance. The best way to assess the safety effects of these conversions would be to develop a model that would take a set of preexisting two-lane conditions and predict the benefit of conversion to a second set of four-lane conditions. This would require a database of geometric and crash information for a massive number of conversions in which many different sets of "before" conditions were converted to many different sets of "after" conditions. An alternative source of such information is predictive cross-sectional models for different before/after conditions. This analysis bases its results not on actual changes, but on differences between two sets of data. This cross-sectional analysis approach was used to develop an initial estimate of the safety effects of conversion of typical sections of two- to four-lane roads in four states--California, Michigan, North Carolina, and Washington--and to determine whether such effects would be similar across states. Conversion from "most typical" two-lane sections to most typical four-lane divided sections resulted in a crash per kilometer reduction of 40-60%. For conversions of more extreme configurations, crash reductions varied from 10% to 70%. Thus, conversion to four-lane divided sections resulted in significant safety benefits. However, the effects of conversion from two-lane to four-lane undivided roadway are still open to debate. Single-state estimates ranged from a 20% reduction to a slight increase in crash rate, depending on annual average daily traffic.
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