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  • 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

    20,000AADT 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

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