Study Details
Study Title: Safety Evaluation of Geometric Design Criteria for Entrance-Exit Ramp Spacing and Auxiliary Lane Use
Authors: Le, T. Q., and Porter, R. J.
Publication Date:JAN, 2012
Abstract: Geometric design decisions regarding interchange and ramp spacing on freeways have traditionally taken a nominal approach to safety. This approach oversimplifies driver behavior and complex interactions between roadway geometrics, traffic operations, and safety and tends to promote a "one size fits all" approach to evaluating design alternatives. The objective of this paper is to quantify the relationship between ramp spacing and freeway safety. Data for this study included freeway geometric features, traffic characteristics, and crash counts for 404 freeway segments in California and Washington State. The relationship between ramp spacing and safety was explored using a negative binomial regression modeling approach. Results indicated that expected crash frequency increased as ramp spacing decreased. The expected proportion of crashes resulting in a fatality or injury appears to decrease as ramp spacing decreases. The presence of an auxiliary lane was associated with lower expected crash frequencies for any given ramp spacing; safety benefits of providing an auxiliary lane diminished as ramp spacing increased. The model results related to ramp spacing and auxiliary lane presence were transformed into crash modification factors and validated through comparisons with an independent research study on weaving areas in Texas. The safety findings were generally consistent with current design criteria related to ramp spacing and auxiliary lane presence in the Green Book. The ability to quantify the expected safety impacts of ramp spacing associated with new or modified interchanges will assist transportation agencies in making well-informed assessments of the overall benefits, impacts, and costs of freeway access decisions.
Study Citation: Le, T. Q., and Porter, R. J. "Safety Evaluation of Geometric Design Criteria 3 for Entrance-Exit Ramp Spacing and Auxiliary Lane Use." Presented at the Transportation Research Board 91st Annual Meeting, Paper No. 12-2153, January 22-26, 2012, Washington, DC.
Study Report: Download the Study Report Document
CMFs Associated With This Study
Category: Access management
Countermeasure: Decrease freeway ramp spacing from infinity to S (ft) with/without auxiliary lane
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|---|---|---|---|---|---|
 | |  | Not specified | All | Principal Arterial Interstate | Not specified |
| |  | Not specified | K,A,B,C | Principal Arterial Interstate | Not specified |
| | | Multiple vehicle | All | Principal Arterial Interstate | Not specified |
