The Transportation Research Board’s (TRB) Geometric Design Committee awarded its 2015 Best Paper in Geometric Design Award to Professor Richard J. Porter, Michael Scott Shea, and Thanh Le. The title of the paper is, A Combined Crash Frequency – Crash Severity Evaluation of Geometric Design Decisions (15-5881).

On January 13th their paper was honored in Washington D.C. at the Geometric Design Committee meeting.

Title: A Combined Crash Frequency – Crash Severity Evaluation of Geometric Design Decisions: Entrance-Exit Ramp Spacing and Auxiliary Lane Presence
Authors: M. Scott Shea, Thanh Q. Le, R.J. Porter
Abstract: This paper quantifies the effects of freeway ramp spacing and auxiliary lane presence on crash frequency and crash severity. Crash frequencies are predicted using a safety performance function and crash severities are estimated using a “severity distribution function.” The paper then demonstrates how to combine quantitative knowledge related to the effects of ramp spacing and auxiliary lane presence on both crash frequency and severity into a framework for assessing the overall crash cost for different ramp configurations. Geometric features, traffic characteristics, and crash data were collected for 404 freeway segments in California and Washington State. Negative binomial regression models and multinomial logit regression models are used to estimate the effects of ramp spacing and auxiliary lane presence on expected crash frequencies and crash severities, respectively. Results show that expected multi-vehicle crash frequency increases when ramp spacing decreases. Meanwhile, there is a decrease in the proportion of severe crashes (fatal, incapacitating injury) with a decrease in ramp spacing, even though the overall frequency of these severe crashes remains relatively unchanged. Providing an auxiliary lane is expected to decrease crash frequency, although this reduction appears to be primarily in less severe crashes (possible injury and property damage only). The findings appear to effectively capture the complex relationships between geometric design designs and operations and the high sensitivity between speed and crash severity. The paper provides quantitative tools for making informed freeway and interchange design decisions where ramp spacing is a consideration.
MPC Project Information: MPC 406, http://www.mountain-plains.org/research/projects/downloads/2012_mpc_406.pdf