Enhanced Design Guidelines for Interchange Loop Ramps

Abstract

The objective of this research was to develop improved design guidance for interchange loop ramps. Despite their long use, there is little research on the design, safety, and operational characteristics of loop ramps. An observational field study was conducted to investigate the relationship between speed and lane position of vehicles and design elements of the ramp proper, and differences in performance between single-lane and multi-lane loop ramps. An assessment was also undertaken to determine how well crash prediction methods in the Highway Safety Manual (HSM) represent the safety performance of two ramp types with distinctly different geometrics: loop and diamond ramps. Based on the research results, recommendations regarding the design of the loop ramp proper, applicable at service interchanges are as follows: (a) for a given radius and design speed, recommended lane and shoulder widths for loop ramps are provided that are expected to induce speeds at or below the ramp design speed, result in similar levels of safety, and encourage drivers to stay within their intended travel lane; (b) regarding multi-lane loop ramps, outside lane widths of 12 ft for entrance ramps and 14 ft for exit ramps are sufficient to accommodate traffic comprised primarily of passenger vehicles, but if the outside lane is expected to accommodate moderate to high volumes of trucks, the outside lane width should be increased; and (c) when implementing the HSM ramp crash prediction methodology, calibration factors should be calculated separately for diamond and loop ramps.