Performance-Based Safety Evaluation of Requests for New Access or Modifications to Existing Access on Freeways

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Freeway access management activities have traditionally taken a nominal approach to safety. Acceptable safety performance is presumed to result from attaining some desired interchange or ramp spacing. This approach oversimplifies driver behavior and complex interactions between roadway geometrics, traffic operations, and safety. The objective of this paper is to quantify the relationship between ramp spacing and freeway safety, with safety defined as number of accidents, or accident consequences, by kind and severity, expected to occur during a specified time period. Data for this study include freeway geometric features, traffic characteristics, and crash counts collected from over 1,600 directional miles of freeways in California and Washington State. The relationship between ramp spacing and safety was explored using a negative binomial regression modeling approach. Results indicate 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 is associated with lower crash frequencies for any given ramp spacing. The safety benefit derived from the presence of an auxiliary lane diminishes as ramp spacing increases. 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 ability to quantify the potential safety impacts of a new or modified interchange on the freeway mainline will assist transportation agencies in making well-informed assessments of the overall benefits, impacts, and costs of freeway access decisions.

National Technical University of Athens
School of Rural and Surveying Engineering
Zografou Campus, Heroon Polytechniou 9
Athens 15780 Greece

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