Safety Impacts of Differential Speed Limits for Trucks and Passenger Cars on Rural Interstate Highways: A Modified Empirical Bayes Approach
Publication: Journal of Transportation Engineering
Volume 132, Issue 1
Abstract
To compare the safety effects of a uniform speed limit (USL) for all vehicles as opposed to a differential speed limit (DSL) for cars and heavy trucks, detailed crash data for six states were obtained for rural interstate highways for the period 1991–2000. The states were categorized into four policy groups based on the speed limit type employed during that decade: maintenance of a uniform limit, maintenance of a differential limit, a change from a uniform to a differential limit, and a change from a differential to a uniform limit. A modified empirical Bayes framework was used to evaluate crash frequency changes without presuming a constant relationship between crashes and traffic volume. Aggregate results showed no consistent safety effects of DSL as opposed to USL. The reason for this finding is that within each state, the modified empirical Bayes methodology suggested that crash risk increased regardless of speed limit policy. The crash estimation models (CEMs) that were developed for each state varied substantially from one state to another such that use of a single state’s CEM for other states could lead to an erroneous conclusion regarding DSL versus USL safety impacts. However, by looking at results for multiple states, this study shows how that problem is avoided. Two considerations for future applications of the empirical Bayes methodology that proved helpful for this investigation are noted. First, it is feasible to use a site as its own control group in a before/after study if the geographical variation from state to state is greater than the temporal variation from the before to the after period. Such a situation arises not only in the case of statewide speed limit policies, but also for other transportation-related policies enacted on a statewide basis. Second, results were consistent between conventional approaches and the modified empirical Bayes approach. The latter technique showed the crash frequency increasing regardless of whether a state changed from DSL to USL, changed from USL to DSL, maintained USL, or maintained DSL, leading one to conclude that speed limit policy has no consistent impact on safety. This conclusion is similar to that reached when using conventional methods, which had shown that in each state, speed limit policy (DSL versus USL) usually had no significant impact on crash rate (crash frequency divided by vehicle miles traveled).
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Acknowledgments
The writers gratefully acknowledge the many persons who provided data, insights, assistance, and other contributions to this effort. Any person who has worked with state crash and speed data will recognize that obtaining these data, especially data from as far back as a dozen years ago, is no small undertaking. Assistance from K. Bonds (Ark.), J. Williams (Ariz.), C. Haack and J. Benton (Calif.), N. Brizzi (Id.), L. Rust (Ind.), W. Baldwin, G. Groat, M. Pawlovich, and T. Welch (Iowa), J. Schaefer (Mo.), P. Jomini and V. Gonzales (Mont.), S. Cosper and A. Wyatt (N.C.), L. Evans, D. Gardner, J. Hausman, and S. Shokouhi (Ohio), S. Reed (Ore.), R. Collins and M. Moore (Tex.), B. Siddiqui (Utah), T. Rickman and K. Tanner (Wash.), J. Legg, R. Lewis, and R. Russell (W. Va.) proved essential to this study. Insights from D. Lord and E. Hauer also proved essential for application of the empirical Bayes method. Comments from D. Warren and R. Kremmes (Federal Highway Administration) and F. Council (N.C.) were also useful. Finally, the team also could not have accomplished this work without the data reduction assistance of L. Woodson (Va.). Inclusion of the above names and states does not imply, however, agreement with the results of this study.
References
Bonneson, J. A., and McCoy, P. T. (1993). “Estimation of safety at two-way stop-controlled intersections on rural highways.” Transportation Research Record. 1401, Transportation Research Board, Washington, D.C., 83–89.
Brown, H. C., and Tarko, A. P. (1999). “The effects of access control on safety on urban arterial streets.” Presented at the 78th Annual Meeting of the Transportation Research Board, Transportation Research Board, Washington, D.C.
Bureau of Transportation Statistics (BTS). (2000). “The changing face of transportation.” BTS00-007, BTS, Washington, D.C.
Bureau of Transportation Statistics (BTS). (2003). BTS 2003 pocket guide to transportation highlights growth of air freight, BTS, Washington, D.C., ⟨http://www.bts.gov/PressReleases/2003/bts001_03.html⟩.
Council, F. M., Duncan, C. S., and Khattak, A. J. (1998). “Applying the ordered probit model to injury severity in truck-passenger car rear-end collisions.” Transportation Research Record. 1635, Transportation Research Board, Washington, D.C., 63–71.
Freedman, M., and Williams, A. F. (1992). “Speeds associated with 55 mph and 65 mph speed limits in northeastern states.” ITE J., 2(2), 17–21.
Garber, N. J., and Gadiraju, R. (1989). Effect of truck strategies on traffic flow and safety on multilane highways, AAA Foundation for Traffic Safety.
Garber, N. J., and Gadiraju, R. (1991a). Impact of differential speed limits on highway speeds and accidents, Dept. of Civil Engineering, Univ. of Virginia, Charlottesville, Va.
Garber, N. J., and Gadiraju, R. (1991b). Impact of differential speed limits on highway speeds and accidents, AAA Foundation for Traffic Studies, Washington, D.C.
Garber, N. J., Miller, J. S., Yuan, B., and Sun, X. (2003a). “The safety impacts of differential speed limits.” Transportation Research Record. 1830, Transportation Research Board, Washington, D.C., 56–62.
Garber, N. J., Miller, J. S., Yuan, B., and Sun, X. (2003b). The safety impacts of differential speed limits on rural interstate highways, Federal Highway Administration, Washington, D.C.
Hall, J. W., and Dickinson, L. V. (1974). An operational evaluation of truck speeds on interstate highways, Dept. of Civil Engineering, Univ. of Maryland.
Harkey, D. L., and Mera, R. (1994). Safety impacts of different speed limits on cars and trucks, Federal Highway Administration, Washington, D.C.
Hauer, E. (1992). “Empirical Bayes approach to the estimation of ‘unsafety’: The multivariate regression method.” Accid. Anal Prev., 24(5), 457–477.
Hauer, E. (1997). Observational before-after studies in road safety, estimating the effect of highway and traffic engineering measures on road safety, Dept. of Civil Engineering, Univ. of Toronto.
Hauer, E., Ng, J. C. N., and Lovel, J. (1988). “Estimation of safety at signalized intersections.” Transportation Research Record. 1185, Transportation Research Board, Washington, D.C., 48–61.
Hauer, E., and Persaud, B. N. (1988). “How to estimate the safety of rail-highway grade crossings and the safety effects of warning devices.” Transportation Research Record. 1114, Transportation Research Board, Washington, D.C., 131–140.
Idaho Transportation Department Planning Division. (2000). Evaluation of the impacts of reducing truck speeds on interstate highways in Idaho—Phase III, Final Rep., National Institute for Advanced Transportation Technology, Univ. of Idaho.
Mountain, L., Fawaz, B., and Jarrett, D. (1996). “Accident prediction models for roads with minor junctions.” Accid. Anal Prev., 28(6), 695–707.
Persaud, B. N. (1991). “Estimating accident potential of Ontario road sections.” Transportation Research Record. 1327, Transportation Research Board, Washington, D.C., 47–53.
Persaud, B., Hauer E., Retting, R., Vallurupalli, R., and Muxsi, K. (1997). “Crash reductions related to traffic signal removal in Philadelphia.” Accid. Anal Prev., 29(6), 803–810.
Persaud, B. N., and Lord, D. (2000). “Accident prediction models with and without trend: Application of the Generalized Estimating Equations (GEE) procedure.” Presented at the 79th Annual Meeting of the Transportation Research Board, Transportation Research Board, Washington, D.C.
Persaud, B. N., Retting, R. A., Gardner, P. E., and Lord, D. (2000). Crash reduction following installation of roundabouts in the United States, Insurance Institute For Highway Safety, Ryerson Polytech Univ., Toronto.
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© 2006 ASCE.
History
Received: Jun 2, 2004
Accepted: Mar 18, 2005
Published online: Jan 1, 2006
Published in print: Jan 2006
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