Low Cost Safety ImprovementsPooled Funds Study
Safety of Lane/Shoulder WidthCombinations on Two-Lane Rural Roads
Dr. Frank Gross, Vanasse Hangen Brustlin (VHB), IncDr. Paul P. Jovanis, Penn State University
Overview Introduction Objective Study Design Methodology Data Collection Preliminary Results Conclusions Future Research
Background on Strategy Shoulder Paving/Widening
Proven strategy Pavement Width
What lane/shoulder width produces lowest crash odds?
Identified at Technical Advisory Committee June 2006
Target crashes Head-on Run-off-road Sideswipe
Potential Difficulties Confounding Variables
Key to Success Flexible modeling approach
Literature Review Crash Modification Factors in Highway Safety Manual
Key studies: Zegeer et al. (1981); Zegeer et al. (1988); Griffin and Mak (1987)
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
0 500 1000 1500 2000 2500
Average Daily Traffic Volume (veh/day)
0' ShoulderThis factor applies to single-vehicle run-off-the-road, multiple-vehicle same direction sideswipe accidents, and multiple-vehicle opposite direction sideswipe accidents.
2' Shoulder
4' Shoulder
6' Shoulder
8' Shoulder
Recommended CMF for Shoulder Width (Harwood et al., 2000)
Literature Review
Few Studies Address Allocation of Total Width “Road diets” change total number of lanes
• Burden and Lagerwey (2001); Welch (1999) Reallocation of width on urban freeways
• Add lane by reducing lane and shoulder width• McCasland (1978); Urbanik and Bonilla (1987)
Evaluate Re-allocation Without Other Changes
Objective Estimate Safety Effectiveness
For a given pavement width, what allocation of lane/shoulder width produces the lowest crash odds?
Secondary Questions of Interest Do effects vary by:
• Traffic volume?• Speed limit?
For a given lane width, do effects vary as shoulder width increases?
Is the treatment economically feasible?
Methodology Case-Control Methodology
Cases: crash-involved segments for a given year Controls: non-crash-involved segments for a given year
Matching Variables ADT and Segment Length
Additional Covariates Speed, District, Unpaved Shoulder, Curvature, and Grade
Methodology
Pair ADT Segment length
SW: 0
SW: 1
SW: 2
SW: 3
SW: 4
SW: 5
LW: 9
LW: 10
LW:11
LW:12 Outcome
1 2 5 0 0 1 0 0 0 0 1 0 0 1
1 2 5 0 0 0 0 0 1 0 0 0 1 0
2 4 3 1 0 0 0 0 0 1 0 0 0 1
2 4 3 0 1 0 0 0 0 0 1 0 0 0
SW means shoulder width and LW means lane width
Methodology Case-Control Methodology
Allows answer to primary and secondary questions Regression-to-the-mean not an issue Accounts for confounding variables
• Matched design• Model covariates
Study Design Required Sample Size
Minimum: 15,094 segment-years• Detect 10 percent reduction in total crashes with 90 percent
confidence Desirable: 57,576 segment-years
• Detect 5 percent reduction in total crashes with 90 percent confidence
Assumption 50 percent discordant pairs
How Does Assumption Hold? PA discordant pairs: 70 percent (LW) and 80 percent (SW) WA discordant pairs: 66 percent (LW) and 84 percent (SW)
Data Collection-1
Data Collection-2
Crash Data 5 years of PA data 6 years of WA data
Roadway Data(PA and WA)
Number of Lanes Area Type AADT Segment Length Speed Limit Surface Width Paved Shoulder Width
(WA only) Horizontal Curvature Vertical Curvature
(PA only) Unpaved Shoulder Width District
Data Collection-3
Variable
PA Cases (total crashes)
PAControls (total crashes)
WACases (total crashes)
WAControls (total crashes)
PACases (target crashes)
PAControls (target crashes)
WACases (target crashes)
WAControls (target crashes)
Length (ft) 2598 2578 1936 1874 2628 2609 2059 1996
AADT 3921 3701 4693 4398 3530 3363 4133 3970
Speed(mph) 47.36 48.20 51.04 51.18 47.61 48.26 51.76 51.65
LaneWidth (ft) 11.21 11.19 11.55 11.61 11.02 11.10 11.49 11.61
ShoulderWidth (ft) 2.96 3.14 4.95 5.23 2.82 3.02 4.81 5.33
Data Collection -4PavementWidth
LaneWidth
ShoulderWidth
PA Sample (Total crashes)
WA Sample (Total crashes)
PA Sample (Target crashes)
WA Sample (Target crashes)
26' 10‘ 3' 4,838 352 3,550 60
26' 11' 2' 4,443 2,001 3,134 522
26' 12' 1' 196 173 148 53
28' 10' 4' 4,024 225 2,803 20
28' 11' 3' 6,756 2,581 4,601 686
28' 12' 2' 1,485 591 994 161
30' 10' 5' 567 84 393 17
30' 11' 4' 10,156 2,388 6,622 526
30' 12' 3' 2,156 1,479 1,420 429
32' 10' 6' 406 63 250 12
32' 11' 5' 2,960 778 1,932 206
32' 12' 4' 4,859 2,358 3,107 640
34' 10' 7' 84 4 54 1
34' 11' 6' 2,677 1,190 1,667 277
34' 12' 5' 1,242 906 720 242
36' 10' 8' 75 61 42 14
36' 11' 7' 294 403 188 115
36' 12' 6' 1,577 1,691 954 454
Total 48,795 17,328 32,579 4,435
Evaluation Results (PA Total Crashes)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
10 11 12
Lane Width Category (ft)
Crash Risk 26' PW
32' PW
36' PW
34' PW
30' PW
28' PW
Evaluation Results (PA Target Crashes)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
10 11 12
Lane Width Category (ft)
Crash Risk26' PW
32' PW
36' PW
34' PW
30' PW28' PW
Evaluation Results (WA Total Crashes)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
10 11 12
Lane Width Category (ft)
Crash Risk 26' PW
32' PW
36' PW
34' PW
30' PW
28' PW
Evaluation Results (WA Target Crashes)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
10 11 12
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)
OddsRatio
StandardError P value
Lower Confidence
Level.
LowerConfidence
Level
32 10 6 1.000 * * * *
32 11 5 1.419 0.197 0.012 1.081 1.863
32 12 4 1.366 0.185 0.021 1.047 1.783
)β,Cov(β*2-)Var(β)Var(β)βse(β j1
i1
j1
i1
j1
i1
)βexp(βRatio(ij) Odds j1
i1
)]βse(β*1.96)βexp[(βC.I. 95% j1
i1
j1
i1
Conclusions
Within Pavement Width PA: Not many significant changes
• Particularly for total crashes WA: General decrease in crashes for narrow lane and wide
shoulder• Target crashes in particular• Be aware of small samples
Within Lane Width General decrease in crashes as shoulder increases
• Supports model results (consistent with prior studies)
Preliminary Results! Need to explore outliers Answer secondary questions
Future Research
Field-verify sites in PA Preliminary data verification using PA video logs
Evaluate Anomalies Met with PennDOT to discuss results Review PennDOT and WSDOT design guides
Secondary Questions Do effects vary by:
• Traffic volumes?• Speed limit?
Economic analysis
QUESTIONSor
COMMENTS
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