The objective of this research was to develop improved design guidance for interchange loop ramps. An observational field study was conducted to investigate the relationship between speed and lane position of vehicles and key design elements of the ramp proper, and the difference in performance on the ramp proper between single-lane and multi-lane loop ramps. Also, comparisons of predicted and observed crash frequencies were performed to validate the Highway Safety Manual (HSM) crash prediction method for loop ramps.

Based on the research findings, the primary recommendations regarding the design of the loop ramp proper, applicable to loop ramps at service interchanges in both urban and rural areas, are as follows: (a) For a given radius and design speed, recommended lane and shoulder widths for entrance and exit loop ramps are provided that are expected to induce speeds at or below the ramp design speed. For a given radius and ramp design speed, the recommended lane and shoulder widths are also expected to result in similar levels of safety, and vehicles are expected to stay within their intended travel lane. (b) Outside lane widths of 12-ft for multi-lane entrance loop ramps and 14-ft for multi-lane exit loop ramps are sufficient to accommodate traffic comprised primarily of passenger vehicles. If the outside lane is expected to accommodate a moderate to high volume of trucks, the outside lane width should be increased. (c) When implementing the HSM ramp crash prediction methodology, separate calibration factors should be calculated for diamond ramps and loop ramps.