Investigating Safety Impact of Raised Pavement … 1013/s12...Investigating Safety Impact of Raised...

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Investigating Safety Impact of Raised Pavement Markers on Freeways in Louisiana Xiaoduan Sun, PhD, PE Subasish Das University of Louisiana

Transcript of Investigating Safety Impact of Raised Pavement … 1013/s12...Investigating Safety Impact of Raised...

Investigating Safety Impact of Raised Pavement Markers on

Freeways in Louisiana

Xiaoduan Sun, PhD, PE Subasish Das

University of Louisiana

Outline

• Introduction

• Objectives

• Data analysis

• Conclusions

Raised Pavement Marker (RPM)

• A raised pavement marker (RPM) is widely used as a safety device on roadways

Specific function of RPM

• To improve preview distance

• To provide all weather visibility

• To provide an audible and tactile warning to

drivers when traversed by the vehicle.

• To improve road safety by providing

directional cues via the reflective color.

Background

• The Louisiana Department of Highways began

using raised pavement markers (RPM) in 1966 on

an experiment basis for replacing painted lines.

• RPM was first implemented at a large scale in

1967 on the I-10 Mississippi River Bridge in Baton

Rouge.

• At present time, all freeways in the state have

RPM installed

Setting

(Road Type)

Traffic Volume

(AADT)

Crash Type

(Severity) CMF Std. Error

≤ 20,000 1.13 0.2

20,001-60,000 0.94 0.3

>60,000 0.67 0.3

Rural

(Four-lane Freeways)

Nightime

All Types

(All Severities)

The need to study Raised Pavement Markers (RPM) in Louisiana

CMF from the HSM

Ref: Bahar, G., C. Mollett, B. Persaud, C. Lyon, A. Smiley, T. Smahel, and H. McGee. National Cooperative Highway Research Report 518: Safety Evaluation of Permanent Raised Pavement Markers. NCHRP, Transportation Research Board, National Research Council, Washington, DC, 2004.

Should the state continue the practice?

Number of Segments vs. AADT (all freeways in Louisiana)

0

10

20

30

40

50

60

70

80

90

AADT

AADT Distribution

Objective

• To investigate the safety effect of RPM on Louisiana freeways

Outline

• Introduction

• Objectives

• Data analysis

• Conclusions

Data Analysis

• Data for 893 miles of freeways (in 257 segments)

– Annual RPM and striping ratings (nine years from 2002-2010)

– Crash record

• Analysis

– Average Crash Rate Analysis

– Statistical Testing (t-test)

– With and Without Analysis

– Crash Characteristics Analysis

• Three condition ratings:

– ‘G’ as Good

– ‘P’ as Poor

– ‘F’ as Fair

• Rating ‘C’ as Construction

Ratings

2002 2003 2004 2005 2006 2007 2008 2009 2010

Control Section

Section Length

450-91 2.54 G G P G G F F F P

450-92 1.36 F F G G G F F F P

450-93 3.40 F F G G G F F F P

450-94 1.17 F F G G G F F F P

450-95 0.13 F F G G G F F F P

450-96 0.38 F F G G G F F F P

Summary of Ratings

Freeway

Number of Segments in Each Rating Group

GG GF GP FG FF FP PG PF PP

Rural 606 85 171 63 110 140 75 31 285

Urban 1,028 189 280 156 214 266 141 88 734

Total 1,634 274 451 219 324 406 216 119 1,019

Average Crash Rate Analysis

• Due to the difference in segment length and AADT, crash

rate (crashes per million VMT) was calculated for each

segment.

• The analysis was conducted for rural and urban freeways

separately because of the difference in freeway design

and operation .

• The focus of the analysis was only on the cases with both

ratings in the same category.

Average Crash Rate by Combined Ratings on Rural freeways

Rural and night hours

0.159 0.163

0.196

0

0.05

0.1

0.15

0.2

0.25

GG FF PP

Striping and RPM rating

Avg

. C

rash

Ra

te

Rural and 24 hours

0.666

0.7600.817

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

GG FF PP

Striping and RPM rating

Avg

. C

rash

Ra

te

23% increase 23% increase

Average Crash Rate by Combined Ratings on Urban freeways

Urban and 24 hours

2.1132.005 2.077

0

0.5

1

1.5

2

2.5

GG FF PP

Striping and RPM rating

Avg

. Cra

sh R

ate

Urban and Night hours

0.3840.406

0.369

0

0.1

0.2

0.3

0.4

0.5

GG FF PP

Striping and RPM rating

Avg

. Cra

sh R

ate

Average crash rate by single rating on rural freeways

Rural and 24 hours

0.6580.692 0.706

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

G F P

RPM rating

Avg

. Cra

sh R

ate

Rural and night hours

0.1520.165 0.168

0

0.05

0.1

0.15

0.2

0.25

G F P

RPM rating

Avg

. Cra

sh R

ate

Rural and night hours

0.1610.180 0.178

0

0.05

0.1

0.15

0.2

0.25

G F P

Striping rating

Avg

. Cra

sh R

ate

Rural and 24 hours

0.6750.724

0.760

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

G F P

Striping rating

Avg

. Cra

sh R

ate

Striping RPM

Statistical Testing

• t-test was performed to examine the significant difference between good and poor rating of RPM and striping.

• The ratings from each year on all rural freeway segments were used in the statistical test as one independent data sample instead of the segment averages.

• t-test was done at three AADT level.

Results of Statistical Test (Average Crash Rate between Good and Poor)

Roadway Type

Feature Period

t-test for Equality of Means

t df Mean

Difference Std. Error

95% Confidence Interval of the Difference

Lower Upper

AADT ≤ 20,000

Rural RPM Night -1.781 489 -0.033 0.018 -0.069 0.003

Rural RPM 24 Hrs -1.101 489 -0.065 0.059 -0.181 0.051

Rural RPM+Striping Night -2.603 309 -0.063 0.024 -0.110 -0.015

Rural RPM+Striping 24 Hrs -2.591 309 -0.212 0.082 -0.373 -0.051

20,000≤AADT ≤ 60,000

Rural RPM Night -2.665 816 -0.038 0.014 -0.066 -0.010

Rural RPM 24 Hrs -3.249 816 -0.142 0.044 -0.228 -0.056

Rural RPM+Striping Night -2.285 492 -0.047 0.020 -0.087 -0.007

Rural RPM+Striping 24 Hrs -2.840 492 -0.168 0.059 -0.284 -0.052

AADT > 60,000

Rural RPM Night -2.128 1339 -0.025 0.012 -0.049 -0.002

Rural RPM 24 Hrs -2.573 1339 -0.102 0.040 -0.180 -0.024

Rural RPM+Striping Night -2.800 889 -0.045 0.016 -0.077 -0.013

Rural RPM+Striping 24 Hrs -3.504 889 -0.186 0.053 -0.289 -0.082

Results • The statistical testing results show the safety effect of RPMs slightly varies

by AADT

• Crash rate difference is statistically significant RPMs alone and RPMs plus striping for AADT larger than 20,000

• For AADT under 20,000 – the probability of getting a positive safety effect is 0.26 with a CMF of

1.13 and a standard error of 0.2 from HSM

– the probability of a positive safety effect is 0.97 with the crash rate difference of -0.033 and a standard error of 0.018

• For AADT between 20,000 and 60,000 – the probability of getting a positive safety effect is 0.58 from the HSM

– the probability of a positive safety effect is 1.00 from this study

x=CMF

f(x)

1

0.2578

1.13

Probability distributionMean=1.13, Standard Error=0.2

x=Crash rate difference

f(x)

0

0.9666

-0.033

Probability distribution Mean=-0.033, Standard Error=0.018

Probability of positive safety effect of RPM For AADT less than 20,000

From HSM From the Louisiana Study

“With” and “Without” Analysis • Two adjusting factors are calculated by the following

equation:

Where

wjA = average AADT of “with” group for segment j

wTjA = average AADT of “without” group for segment j

NWj = number of years under “with” group for segment j

NWj = number of years under “without” group for segment j

WTj

wja A

Ajr )(

WTj

Wjs N

Njr )(

Results

Crash Analysis for Rural Freeways at Nighttime

Expected Crashes

Feature TypeNumber of

Sections

With

(Good)

Without

(Poor)

Expected

Crash

Reduction

% Reduction

RPM 114 641 675 34 5.30%

Striping 77 476 477 1 0.20%

Estimated Safety Effectiveness

Highway

Type Feature

Crash

Hour Rating N Mean CMF

AADT≤ 20,000

Rural RPM Night Good 291 0.139 0.81

Poor 200 0.172

Rural RPM 24 Hrs Good 291 0.635 0.91

Poor 200 0.7

Rural RPM+Striping Night Good 225 0.138 0.69

Poor 86 0.201

Rural RPM+Striping 24 Hrs Good 225 0.644 0.75

Poor 86 0.856

20,000 ≤ AADT≤ 60,000

Rural RPM Night Good 436 0.141 0.79

Poor 382 0.179

Rural RPM 24 Hrs Good 436 0.596 0.81

Poor 382 0.738

Rural RPM+Striping Night Good 329 0.148 0.76

Poor 165 0.195

Rural RPM+Striping 24 Hrs Good 329 0.602 0.78

Poor 165 0.77

AADT≤ 60,000

Rural RPM Night Good 745 0.153 0.86

Poor 596 0.178

Rural RPM 24 Hrs Good 745 0.655 0.87

Poor 596 0.757

Rural RPM+Striping Night Good 606 0.155 0.78

Poor 285 0.2

Rural RPM+Striping 24 Hrs Good 606 0.655 0.78

Poor 285 0.841

• Crash rate is used for the analysis

• Only “Good’’ ratings and “Poor” ratings are considered

• Nine years data is used for both ratings

Crash Type

0%

10%

20%

30%

40%

50%

60%

Good Poor Fair

Percen

tag

e (%

)

RPM Rating

Crash Type

Non-Collision W/MV

Rear End

Sideswipe

Driver Condition

0%

10%

20%

30%

40%

50%

60%

Good Poor Fair

Percen

tag

e (%

)

RPM Rating

Driver Condition

Normal

Inattentive/Distracted

Others

Outline

• Introduction

• Objectives

• Data analysis

• Conclusions

Results Discussion

• RPM does offer safety benefit to the state rural freeways based on all analysis methods

• The RPM benefits vary by AADT

• Because of combined effects of two ratings, it is hard, if not impossible, to accurately estimate CMF for RPM independently

• No safety benefit of RPM is detected on urban freeways

Recommendation

• Continue the current practice on RPM on all freeways in Louisiana

Reducing crashes on rural freeway

Thank You and Questions