Research Report
Darren J. Torbic, Lindsay M. Lucas, Douglas W. Harwood, Marcus A. Brewer, Eun Sug Park, Raul Avelar, Michael P. Pratt, Akram Abu-Odeh, Elizabeth Depwe, and Kimberly Rau
Jan-17
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.
Design speed
Highway design
Interchanges
Ramps (Interchanges)
HSIS Summary Reports are two to eight pages in length and include a brief description of the issue addressed, data used, methodology applied, significant results, and practical implications.
A variety of research studies have been performed using data from HSIS. Many of the final reports prepared are now available electronically.
Research reports are often summarized in executive summaries, technical briefs, or other abbreviated formats. Included here are those road safety summaries that involved research using HSIS data.
In addition to conducting research, HSIS resources are also used to develop products that can be used by practitioners in the analysis of safety problems.
HSIS data are sometimes used in research studies that result in other types of finished products, such as dissertations, theses, and conference proceedings.
