INVESTIGATING CAUSES OF PAVEMENT DETERIORATION IN KHARTOUM STATE
Investigating Safety Impact of Raised Pavement … 1013/s12...Investigating Safety Impact of Raised...
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
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
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
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