BEFORE-AFTER ACCIDENT ANALYSIS FOR TWO SHOULDER UPGRADING ALTERNATIVES

The Texas State Department of Highways and Public Transportation has tried several techniques to improve operating conditions on rural two-lane highways. One common treatment has been the addition of paved shoulders. An innovative treatment that provides additional capacity at a minimum cost has been the conversion of two-lane roadways with full-width paved shoulders to undivided, four-lane roadways without shoulders. Although both treatments improve traffic operations, their effect on safety has not been fully quantified. A study was conducted to establish the consequences related to safety whenever these two treatments are implemented. An accident frequency comparison for accident type by class of roadway was made for the before and after improvement time periods. Separate comparisons were made for both all accidents and nonintersection accidents. To supplement this analysis, a paired t-test was used to determine significant changes in either accident type or severity. The findings of the analysis are as follows: (a) Addition of full-width paved shoulders to a two-lane roadway is effective in reducing the total number of accidents, (b) conversion of a paved shoulder to an additional travel lane results in fewer total accidents only if traffic volume is greater than 3000 vehicles/day, and (c) the type of accident change varies with type of roadway and volume level. (Author)