SAFETY EFFECTS OF CROSS SECTION DESIGN ON URBAN AND SUBURBAN ROADS

摘要

Understanding the safety effects of cross section allocation on urban and suburban roads isimportant for agencies implementing complete streets philosophies. Research on urban andsuburban arterials, conducted as part of Highway Safety Manual development, found noconsistent relationships between lane width and safety. Other studies indicated that urban streetswith lanes narrower than 12 feet are not less safe, and may even be safer, than those with 12 footand wider lanes. These studies addressed geometric elements in isolation, potentially leading tothe inconsistent and counterintuitive findings. This paper takes a new look at the safety effects ofcross section allocation on urban and suburban road segments. The effects of cross sectionelement combinations and interactions, with a specific focus on lane width, shoulder width, andshoulder type, are considered as an inherent part of model estimation. The dataset for this studyincludes 1,577 urban street segments in Illinois, totaling nearly 380 miles in length and 3 years ofcrash data. Results show interactions between lane width and shoulder width that would not havebeen uncovered if these elements were considered in isolation. Shoulder width has a larger effecton safety when lanes are narrow and this safety effect decreases as lane width increases. Widerlanes do not always result in an increase in safety, particularly when shoulders are wider. Crashmodification factors for cross section design element combinations were developed usingmodeling results. Safety performance functions for urban and suburban road segments are alsopresented.