Trade Off Considerations In Highway Geometric Design PDF Download

Geometric design standards have not been developed specifically for low volume roads. Design standards and criteria are extrapolated from higher standard roadways that are often not relevant to low volume roads. Safety is a primary criterion used for selection of design standards along with economic, financial, social and environment factors. This study attempts to determine trade-offs of various design standards and designs based on safety and economic criteria. It also summarizes existing geometric design standards that are employed by federal, state and county governments. A methodology for selection of design standards for low volume roads employing safety criteria is developed. The study approach is based on data and relationships from previous research. Accident costs cited by National Safety Council are adopted for analysis. Procedures are developed to determine the balance between design standards and accident costs. Ten-foot lane width and two-foot shoulder width are found to be reasonable for low volume situations. Horizontal and vertical alignments affect safety, vehicle running costs and travel times. Guardrail warrants of low volume roads differ from that of high volume roads. The balance and trade-offs between guardrail installation cost and accident cost expected for a given embankment geometry are evaluated. The balance between a clear zone from the edge of pavement and the accident cost savings has also been developed. It is recommended that a comparison between the accident savings due to roadway improvement and the incremental construction cost must be made to evaluate safety and economy effectively. An approach to evaluate the level of safety relative to design is developed for decision making.