Roadway widths for low-traffic volume roads

In recent years, maintaining and reconstructing the two-lane highway system--which represents 80% of the total U.S. highway mileage--has emerged as a serious problem. Much of this mileage has relatively low traffic volume with approximately 80% having an average daily traffic of less than 400 vehicles per day. Further, a great proportion of the two-lane rural road system is 30 years old or more, necessitating investment to replace pavement, repair shoulders, and address other problems. This report describes the results of comprehensive safety, operational and economic analyses regarding minimum roadway widths for low volume roads. The objective of this research was to develop an engineering analysis procedure for determining optimal roadway widths for the construction and reconstruction of low volume roadways. This research involved (1) a critical review of the literature pertaining to the safety, operations, and geometrics of low volume roads; (2) the collection of data related to traffic volume, vehicle speed, percent trucks, functional classification, level of service, and accidents; (3) a thorough analysis of the data leading to the formulation of a model for the analysis of traffic and environmental conditions that affect the safety and operation of low volume roads; and (4) an extensive economic analysis using data gathered from the states and case studies of actual low volume roadway improvement projects completed during the last 5 years. The research investigated the effects of various combinations of lane width and shoulder width. For 10-ft lanes, accident rates were lower by 0.98 per million vehicle miles (MVM) when accompanied by shoulders of greater than 4 ft versus shoulders of 4 ft or less. For 11- and 12-ft lanes, shoulder widths of 3 ft or greater produced accident rates 0.56 per MVM lower than lanes with shoulder widths less than 3 ft. The accident model developed in this study produced identical accident benefits for 11- and 12-ft lanes. It was concluded that narrower roadways could function safely and effectively allowing highway improvement funds to be stretched over a greater proportion of the two-lane road network. It was also concluded that narrower width combinations than those called for by the current AASHTO Policy on the Geometric Design of Streets and Highways appear appropriate. In particular, where total roadway widths (i.e., lanes plus shoulders) currently exceed 30 ft, there are many cases where narrower widths could be used resulting in lower costs with no apparent degradation in safety. The recommended changes in design standards would produce meaningful savings in construction and reconstruction costs associated with the low volume rural system. These results cover most low volume roadways because the samples included paved and unpaved roadways, roadways in various terrains, a range of traffic volumes and mix, and roadways from different regions of the nation.