Low Cost Safety Improvements Pooled Funds Study Safety of Lane/Shoulder Width Combinations on Two-Lane Rural Roads Dr. Frank Gross, Vanasse Hangen Brustlin (VHB), Inc Dr. Paul P. Jovanis, Penn State University

Overview u Introduction u Objective u Study Design u Methodology u Data Collection u Preliminary Results u Conclusions u Future Research

Background on Strategy u Shoulder Paving/Widening l Proven strategy u Pavement Width l What lane/shoulder width produces lowest crash odds? l Identified at Technical Advisory Committee June 2006 u Target crashes l Head-on l Run-off-road l Sideswipe u Potential Difficulties l Confounding Variables u Key to Success l Flexible modeling approach

Literature Review u Crash Modification Factors in Highway Safety Manual l Key studies: Zegeer et al. (1981); Zegeer et al. (1988); Griffin and Mak (1987) Recommended CMF for Shoulder Width (Harwood et al., 2000)

Literature Review u Few Studies Address Allocation of Total Width l “Road diets” change total number of lanes Burden and Lagerwey (2001); Welch (1999) l Reallocation of width on urban freeways Add lane by reducing lane and shoulder width McCasland (1978); Urbanik and Bonilla (1987) u Evaluate Re-allocation Without Other Changes

Objective u Estimate Safety Effectiveness l For a given pavement width, what allocation of lane/shoulder width produces the lowest crash odds? u Secondary Questions of Interest l Do effects vary by: Traffic volume? Speed limit? l For a given lane width, do effects vary as shoulder width increases? l Is the treatment economically feasible?

Methodology u Case-Control Methodology l Cases: crash-involved segments for a given year l Controls: non-crash-involved segments for a given year u Matching Variables l ADT and Segment Length u Additional Covariates l Speed, District, Unpaved Shoulder, Curvature, and Grade

Methodology PairADT Segment length SW: 0 SW: 1 SW: 2 SW: 3 SW: 4 SW: 5 LW: 9 LW: 10 LW:1 1 LW:1 2 Outcome SW means shoulder width and LW means lane width

Methodology u Case-Control Methodology l Allows answer to primary and secondary questions l Regression-to-the-mean not an issue l Accounts for confounding variables Matched design Model covariates

Study Design u Required Sample Size l Minimum: 15,094 segment-years Detect 10 percent reduction in total crashes with 90 percent confidence l Desirable: 57,576 segment-years Detect 5 percent reduction in total crashes with 90 percent confidence u Assumption l 50 percent discordant pairs u How Does Assumption Hold? l PA discordant pairs: 70 percent (LW) and 80 percent (SW) l WA discordant pairs: 66 percent (LW) and 84 percent (SW)

Data Collection-1

Data Collection-2 u Crash Data l 5 years of PA data l 6 years of WA data u Roadway Data (PA and WA) l Number of Lanes l Area Type l AADT l Segment Length l Speed Limit l Surface Width l Paved Shoulder Width (WA only) l Horizontal Curvature l Vertical Curvature (PA only) l Unpaved Shoulder Width l District

Data Collection-3 Variable PA Cases (total crashes) PA Controls (total crashes) WA Cases (total crashes) WA Controls (total crashes) PA Cases (target crashes) PA Controls (target crashes) WA Cases (target crashes) WA Controls (target crashes) Length (ft) AADT Speed (mph) Lane Width (ft) Shoulder Width (ft)

Data Collection -4 Pavement Width Lane Width Shoulder Width PA Sample (Total crashes) WA Sample (Total crashes) PA Sample (Target crashes) WA Sample (Target crashes) 26' 10‘3'4, , ' 11'2'4,4432,0013, ' 12'1' ' 10'4'4, , ' 11'3'6,7562,5814, ' 12'2'1, ' 10'5' ' 11'4'10,1562,3886, ' 12'3'2,1561,4791, ' 10'6' ' 11'5'2, , ' 12'4'4,8592,3583, ' 10'7' ' 11'6'2,6771,1901, ' 12'5'1, ' 10'8' ' 11'7' ' 12'6'1,5771, Total 48,79517,32832,5794,435

Evaluation Results (PA Total Crashes) Lane Width Category (ft) Crash Risk 26' PW 32' PW 36' PW 34' PW 30' PW 28' PW

Evaluation Results (PA Target Crashes) Lane Width Category (ft) Crash Risk 26' PW 32' PW 36' PW 34' PW 30' PW 28' PW

Evaluation Results (WA Total Crashes) Lane Width Category (ft) Crash Risk 26' PW 32' PW 36' PW 34' PW 30' PW 28' PW

Evaluation Results (WA Target Crashes) Lane Width Category (ft) Crash Risk 26' PW 32' PW 36' PW 34' PW 30' PW 28' PW

Example Odds Ratio Total Paved Width (ft) LW (ft) SW (ft) Odds Ratio Standard Error P value Lower Confidence Level. Lower Confidence Level ****

Conclusions u Within Pavement Width l PA: Not many significant changes Particularly for total crashes l WA: General decrease in crashes for narrow lane and wide shoulder Target crashes in particular Be aware of small samples u Within Lane Width l General decrease in crashes as shoulder increases Supports model results (consistent with prior studies) u Preliminary Results! l Need to explore outliers l Answer secondary questions

Future Research u Field-verify sites in PA l Preliminary data verification using PA video logs u Evaluate Anomalies l Met with PennDOT to discuss results l Review PennDOT and WSDOT design guides u Secondary Questions l Do effects vary by: Traffic volumes? Speed limit? u Economic analysis

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