Iowa's Experience with Road Diet Measures

Abstract

A before-and-after study implemented from a Bayesian perspective to assess crash history reduction due to road diets in Iowa was conducted by the Iowa State University Department of Statistics in cooperation with the Iowa Department of Transportation's Office of Traffic and Safety. The study used both monthly crash data and estimated volumes obtained from TAS for 30 sites—15 treatment and 15 comparison—over 23 years (1982 to 2004). The sites had volumes ranging from 2,030 to 15,350 during this period and were largely located in smaller urbanized areas. The research objective was to assess whether road diets appear to result in crash reductions on Iowa roads. Crash data were analyzed at each site before and after the conversions were completed. Given the random, rare nature of crash events, a hierarchical Poisson model was fitted, such that log mean rate was expressed as a piecewise linear function of time period, seasonal effects, and a random effect corresponding to each site. Monthly traffic volumes were incorporated as exposures. Estimation of model parameters was conducted within a Bayesian framework. Results indicate a 25.2% reduction in crash frequency per mile and an 18.8% reduction in crash rate. This differs from a previously publicized study that reported a 6% reduction in crash frequency per mile and an insignificant indication for crash rate effects. The results from the Iowa study fit practitioner experience and agree with another Iowa study that used a simple before-and-after approach on the same sites.

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References

and Carriquiry

The Effect of Four-Lane to Three-Lane Conversion on the Number of Crashes and Crash Rates in Iowa Roads. Department of Statistics, Iowa State University, June 2005. www.dot.state.ia.us/crashanalysis/pdfs/iowa4to3laneconversion_fullbayes_june2005.pdf. Accessed Aug. 1, 2005.

Burden

and Lagerway

Road Diets: Fixing the Big Roads. www.walkable.org/download/rdiets.pdf. Accessed Aug. 1, 2005.

Huang

H. F.

Stewart

J. R.

and Zegeer

C. V.

Evaluation of Lane Reduction “Road Diet” Measures on Crashes and Injuries. In Transportation Research Record: Journal of the Transportation Research Board, No. 1784, Transportation Research Board of the National Academies, Washington, D.C., 2002, pp. 80–90.

Huang

H. F.

Stewart

J. R.

and Zegeer

C. V.

Evaluation of Lane Reduction “Road Diet” Measures and Their Effects on Crashes and Injuries. Highway Safety Information System. HSIS FHWA-HRT-04–082. http://www.hsisinfo.org/pdf/04-082.htm, 2004. Accessed Aug. 1, 2005.

Huang

H. F.

Stewart

J. R.

and Zegeer

C. V.

Summary of Evaluation of Lane Reduction “Road Diet” Measures and Their Effects on Crashes and Injuries (HSIS FHWA-HRT-04-082). ITE Journal, May 2005, insert.

Knapp

and Giese

Guidelines for the Conversion of Urban Four-Lane Undivided Roadways to Three-Lane Two-Way Left-Turn Facilities. Center for Transportation Research and Education, Iowa State University, April 2001. www.ctre.iastate.edu/reports/4to3lane.pdf. Accessed Aug. 1, 2005.

Knapp

and Giese

Speeds, Travel Times, Delays on US 75 4/3 Lane Conversion Through Sioux Center, Iowa, During the AM and PM Peak Periods. Center for Transportation Research and Education, Iowa State University, Ames, 1999.

Persaud

B. N.

and Hauer

Comparison of Two Methods for Debiasing Before-and-After Accident Studies. In Transportation Research Record 975, TRB, National Research Council, Washington, D.C., 1984, pp. 43–49.

Hauer

On the Estimation of the Expected Number of Accidents. Accident Analysis and Prevention, Vol. 18, No. 1 1986, pp. 1–12.

10.

Hauer

Detection of Safety Deterioration in a Series of Accident Counts. In Transportation Research Record 1542, TRB, National Research Council, Washington, D.C., 1996, pp. 38–43.

11.

Hauer

Statistical Test of Difference Between Expected Accident Frequencies. In Transportation Research Record 1542, TRB, National Research Council, Washington, D.C., 1996, pp. 24–29.

12.

Hauer

Observational Before–After Studies in Road Safety. Pergamon, Oxford, United Kingdom, 1997.

13.

Hauer

Kononov

Allery

and Griffith

M. S.

Screening the Road Network for Sites with Promise. In Transportation Research Record: Journal of the Transportation Research Board, No. 1784, Transportation Research Board of the National Academies, Washington, D.C., 2002, pp. 27–32.

14.

Persaud

B. N.

Retting

R. A.

and Lyon

C. A.

Crash Reduction Following Installation of Centerline Rumble Strips on Rural Two-Lane Roads. Accident Analysis and Prevention, Vol. 36, No. 6 2004, pp. 1073–1079.

15.

Statistical Evaluation in Traffic Safety Studies. ITE, Washington, D.C., 1999.

16.

Persaud

B. N.

NCHRP Synthesis of Highway Practice 295. Statistical Methods in Highway Safety Analysis: A Synthesis of Highway Practice. TRB, National Research Council, Washington, D.C., 2001.

17.

Highway Safety Manual. Home Page. www.highwaysafetymanual.org. Accessed Aug. 1, 2005.

18.

Raju

Souleyrette

and Maze

T. H.

Impact of the 65-mph Speed Limit on Iowa's Rural Interstate Highways: Integrated Bayesian Forecasting and Dynamic Modeling Approach. In Transportation Research Record 1640, TRB, National Research Council, Washington, D.C., 1998, pp. 47–56.

19.

Schluter

P. J.

Deely

J. J.

and Nicholson

A. J.

Ranking and Selecting Motor Vehicle Accident Sites by Using a Hierarchical Bayesian Model. The Statistician, Vol. 46, No. 3 1997, pp. 293–316.

20.

Davis

G. A.

and Yang

Bayesian Identification of High-Risk Intersections for Older Drivers via Gibbs Sampling. In Transportation Research Record: Journal of the Transportation Research Board, No. 1746, TRB, National Research Council, Washington, D.C., 2001, pp. 84–89.

21.

Pawlovich

M. D.

Evaluating Traffic Safety Network Screening: An Initial Framework Utilizing the Hierarchical Bayesian Philosophy. PhD dissertation. Iowa State University, Ames, 2003.

22.

Miaou

S.-P.

and Song

J. J.

Bayesian Ranking of Sites for Engineering Safety Improvements: Decision Parameter, Treatability, Statistical Criterion, and Cost Function.

Presented at 83rd Annual Meeting of the Transportation Research Board, Washington, D.C., 2004.

23.

Miaou

S.-P.

and Lord

Modeling Traffic Crash-Flow Relationships for Intersections: Dispersion Parameter, Functional Form, and Bayes Versus Empirical Bayes Methods. In Transportation Research Record: Journal of the Transportation Research Board, No. 1840, Transportation Research Board of the National Academies, Washington, D.C., 2003, pp. 31–40.

24.

Geurts

Wets

Brijs

and Vanhoof

Identification and Ranking of Black Spots: Sensitivity Analysis. In Transportation Research Record: Journal of the Transportation Research Board, No. 1897, Transportation Research Board of the National Academies, Washington, D.C., 2004, pp. 34–42.

25.

Simonoff

J. S.

Smoothing Methods in Statistics. Springer, New York, U.S.A., 1996.

26.

Gelman

Carlin

J. B.

Stern

H. S.

and Rubin

D. R.

Bayesian Data Analysis, 2nd ed. Chapman & Hill, London, 2004.

27.

Bayesian Inference Using Gibbs Sampling. www.mrc-bsu.cam.ac.uk/bugs. Accessed Aug. 1, 2005.

28.

Cowles

M. K.

Review of WinBUGS 1.4. Journal of the American Statistical Association, Vol. 58, No. 4 2004, pp. 330–337.

29.

Cowles

M. K.

and Carlin

B. P.

Markov Chain Monte Carlo Convergence Diagnostics: A Comparative Review. Journal of the American Statistical Association, Vol. 91, 1996, p. 434.

30.

Stout

Before and After Study of Some Impacts of 4-Lane to 3-Lane Roadway Conversions. Center for Transportation Research and Education, Iowa State University, March 2005. www.dot.state.ia.us/crashanalysis/pdfs/iowa4to3laneconversion_classicalbeforeafter_march2005.pdf. Accessed Aug. 1, 2005.