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