cable median barrier with inside shoulder rumble strips on divided roads raghavan srinivasan, bo...
DESCRIPTION
Objectives Develop crash modification factors (CMFs) for combination of cable median barrier and inside shoulder rumble strips Specific combination may vary depending on the state Disaggregate analysis to determine if CMFs are associated with: AADT Median width Expected crashes in the before periodTRANSCRIPT
Cable Median Barrier with Inside Shoulder Rumble Strips on Divided RoadsRaghavan Srinivasan, Bo Lan, & Daniel Carter,UNC Highway Safety Research Center
Background Cable median barriers introduced in the
United States in the 60’s Initially low tension; later higher tension
became more common Can take multiple hits before requiring
repair Many states have started using cable
barrier instead of concrete barrier Very few studies have examined the
combination of cable barrier with inside shoulder rumble strips
Study funded by FHWA as part of the DCMF contract
Objectives Develop crash modification factors
(CMFs) for combination of cable median barrier and inside shoulder rumble strips
Specific combination may vary depending on the state
Disaggregate analysis to determine if CMFs are associated with:• AADT• Median width• Expected crashes in the before period
Method Empirical Bayes before-after evaluation
Identify a reference group of similar sites (without a treatment) and develop safety performance functions (SPFs)
Estimate the expected number of crashes in the after period had the treatment not been implemented
Estimate CMFs along with the standard error of CMFs
Data Expressway, freeway, or interstate 4 states• Kentucky: 323 miles (R), 66 miles (T)• Illinois: 401 miles (R), 100 miles (T)• Wisconsin: 745 miles (R), 185 miles (T)• Missouri: 288 miles (T) (system-wide
implementation) Not exactly the same “treatment” in all
states Kentucky, Illinois, and Wisconsin• Before period: rumble strip without barrier• After period: rumble strip with barrier
Missouri• Before period: no rumble strip or barrier• After period: rumble strip with barrier
Total Injury and fatal
(KABC) Injury and fatal
(KAB) Cross median Injury and fatal cross
median (KABC) Median-related Area type (urban vs
rural) and crashes during winter (Dec, Jan, Feb)• All types mentioned
above
Definition of cross-median crashes• Some previous studies
did a manual review of crash reports
• We relied on coded crash types
o Cross median indicator (Missouri)
o Head ono Opposite direction
sideswipe• Multiple definitions
possible Definition of median
related
Crash Types
Data Issues in Kentucky Initial results in Kentucky showed large
increases (more than 20%) in injury and fatal Crashes• Conversation with Kentucky DOT and Kentucky
Transportation Center (University of Kentucky)o Detailed discussion over many weekso We found that ramp crashes in the more recent
years need to be excluded based on the last three digits of the route code
o Problems with the older data in Jefferson County (one of the largest counties in the state)
– Had to remove 58 sites (about 50 miles) from the analysis (out of a total of 116 miles)
KentuckyCrash Type EB Expected After Observed After CMF S.E. of CMF
Total 1151.3 1443 1.253 0.041
KABC 269.5 292 1.082 0.072
KAB 156.5 155 0.989 0.088
Cross median 36.3 19 0.520 0.125
Cross median (KABC)
10.0 4 0.395 0.199
Note: Cross median based on head-on and opposite direction sideswipe
IllinoisCrash Type EB Expected
AfterObserved
AfterCMF S.E. of CMF
Total 2168.1 2765 1.275 0.033
KABC 594.2 519 0.873 0.047
KAB 452.9 429 0.946 0.057
Cross median 23.3 12 0.512 0.152
Cross median (KABC)
16.2 8 0.490 0.177
Note: Cross median based on head-on and opposite direction sideswipe
WisconsinCrash Type EB Expected
AfterObserved
AfterCMF S.E. of CMF
Total 1346.6 1362 1.011 0.039
KABC 394.6 366 0.926 0.057
Cross median 19.9 17 0.855 0.210
Cross median (KABC)
8.8 5 0.565 0.253
Note: Cross median based on head-on and opposite direction sideswipe
MissouriCrash Type EB Expected
AfterObserved
AfterCMF S.E. of CMF
Total 1781.0 2221 1.247 0.034
KABC 589.1 439 0.745 0.040
KAB 171.0 134 0.783 0.073
Cross median 24.3 1 0.040 0.040
Cross median + head-on
41.9 5 0.119 0.053
Cross median + head-on (KABC)
30.6 3 0.098 0.057
Note: Cross median is a crash type in Missouri
Summary of Findings Cross median crashes decreased
significantly in all states except Wisconsin
Largest reduction in cross median crashes were in Missouri • Combined effect of rumble strips and barrier?• Different definition for cross median?
Total crashes increased significantly in all states (due to collisions with barriers) except Wisconsin
No significant change in injury crashes in Kentucky
Currently working on disaggregate analysis