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

Source: Federal Highway Administration

"Given the lack of funds for new highway construction and the ever increasing traffic flows that must be handled in existing roadway corridors, the issue of conversion from two lanes to four lanes is of increasing importance to the State and local transportation departments, the Federal Highway Administration (FHWA), and the public. There is also increasing interest in better defining the safety effects of such conversions, since quite often the conversion becomes highly politicized.

"The best way to assess these safety effects would be to develop a model that would take a set of pre-existing ("before") two-lane conditions, no matter how extreme, and predict the benefit of conversion to a second set of "after" 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. Since no such dataset is available anywhere, judgment of the safety effects of conversion of rural two-lane roads to a greater number of lanes must be based on other sources of information.

"An alternative source of such information is predictive cross-sectional models for different before/after conditions. Unlike the stronger before/after methodology, where an actual change has occurred, the alternative cross-sectional analysis bases its results not on actual changes, but on differences between two sets of data -- undivided roadways. This cross-sectional analysis approach was used in this study to develop an initial estimate of the safety effects of conversion of typical sections of two- to four-lane roads, and to determine whether such effects would be similar across multiple States."

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