Centering Safety

Margaret Rys / March 12, 2008

In the U.S., crashes on rural roads account for 60% of all fatal ones. Approximately 90% of these fatal crashes occur on two-lane roads. Vehicles crossing the centerline of two-lane roads and either sideswiping or striking opposing vehicles head-on (sometimes referred to as cross-over crashes) account for 20% of all fatal crashes on these two-lane roads and result in approximately 4,500 annual fatalities. Clearly there is a need for an effective method of keeping the occasionally inattentive drivers in their lanes.

Middle ground

Centerline rumble strips (CLRS) are similar to shoulder rumble strips (SRS) in that they are cut indentations used to make drivers aware of their position on the roadway. The difference, however, is the placement on the roadway. As the name suggests, centerline rumble strips are placed along the center of the roadway, usually on two-lane, two-way rural roads. The primary purpose of CLRS is to warn drivers whose vehicles are crossing centerlines of two-lane, two-way roadways to avoid potential crashes with opposing traffic. Head-on and opposite-direction sideswipes are two types of crashes generally considered correctable by CLRS. Crashes that qualify as CLRS correctable are any cross-centerline (cross-over) crashes that begin with a vehicle encroaching on the opposing lane, excluding any crash that began by running off the road to the right and overcorrecting and any crash that began by a vehicle going out of control due to water, ice, snow, etc., prior to crossing the centerline.

Putting on miles

The use of CLRS has been growing since around 1999. Early surveys in 2000 indicated that 20 states and one Canadian province with CLRS appeared to have experimental sections of a few miles, and the maximum reported treatment was 15 miles. In a 2004 survey, 22 states and two Canadian provinces reported they had CLRS, an increase of only two states and one Canadian province; however, the number of lane-miles of CLRS has greatly increased, with one state reporting 300 miles. In regard to where they were used, 14 respondents answered they were used continuously, four answered they were used only on no-passing sections, two said they were used only on curves and two answered they were used only on specific sections. It appears that at least the majority of the 22 states reporting their use believe they are an effective safety countermeasure to head-on and opposite-direction-sideswipe, cross-over crashes. There are indications in the survey that other states are waiting for more positive evidence before installing CLRS, which are clearly still in the experimental stage.

Although many states use CLRS, not all use the same design, pattern or placement.

Avoiding it head-on

The most compelling evidence supporting CLRS use is the reduced crashes and lives saved. Several states reported significant reduction in overall crashes, (opposite-direction-head-on and sideswipe crashes) and fatal crashes. A few states reported the data showed no significant decrease in crashes after the installation of CLRS. Most before-and-after studies are based on very few years of data.

Delaware first installed CLRS in 1994. The Delaware Department of Transportation (DelDOT) did a comparative study of average yearly accidents for the three years before installation and the eight years after installation of CLRS along Rte. 301. During the before period, six fatal accidents were reported, resulting in nine fatalities. DelDOT found the average yearly head-on collisions decreased by 95% after the installation of CLRS. Accidents caused by motorists crossing the centerline decreased by 60%. Most significantly, even with a 4% average yearly increase in traffic volumes, there were no fatal accidents reported during the eight-year period after installation of the CLRS.

Similar findings were seen in Colorado. The Colorado Department of Transportation (CDOT) completed a 44-month before-and-after study on a 17-mile section of State Highway 119 where CLRS were installed. CDOT found a 34% decrease in head-on collisions and a 36.5% decrease in sideswipe collisions while the average daily traffic increased 18%.

The California Department of Transportation tested the effects of CLRS in no-passing zones. A review of 36 months of before-and-after crash data found that crashes were reduced by 11% and fatalities were reduced by 77%.

A study for the Massachusetts Highway Department evaluating before-and-after-CLRS-installation crash data and the safety of the CLRS found there was no evidence from the before-and-after study that suggested the installation of CLRS significantly reduced crash rates. However, they did find that the CLRS were effective at gaining a driver’s attention during the simulator trials, and they stated that CLRS are an effective traffic-control device and safety countermeasure in areas with a history of cross-over-the-centerline fatal and severe-injury crashes.

The most reliable evidence of the value of CLRS is a study conducted by the Insurance Institute for Highway Safety. In this study, the researchers collected and analyzed all data in the U.S. they considered reliable, and using the Empirical Bayes method and data from seven states with 210 miles of CLRS concluded overall motor vehicle crashes at sites treated with centerline rumble strips were reduced 14%. Injury crashes were reduced by an estimated 15%. Frontal and opposing-direction-sideswipe crashes were reduced by an estimated 21%. Front and opposing-direction-sideswipe crashes involving injuries were reduced by an estimated 25%.

When the crashes were disaggregated into nighttime and daytime crashes, the percent reduction at night was greater than during the day—19% vs. 9%—but the difference was not statistically significant at the 5% level. Data on fatalities was insufficient to draw any conclusions.

Grooves aren’t all groovy

Although the overall conclusion from the available materials is that CLRS are a low-cost, effective countermeasure for mitigating opposite-direction crashes on two-lane roadways, there are some concerns. The top two reported in the survey were external noise and reduced visibility of the centerline striping material (generally paint). However, some respondents commented that the painted stripes over CLRS are more visible during rain. Pavement deterioration, ice buildup in the grooves, adverse impact on emergency vehicles and safety of bicyclists also were reported as concerns; however, these were isolated concerns with each being expressed by only one or two states. Another concern with the use of CLRS (and inside SRS on divided highways) is a driver’s ad hoc and a priori expectancies derived from previous experiences with SRS. Because of this expectancy, driver’s subconscious reaction to an unexpected encounter with SRS is to correct the trajectory of the vehicle by turning left, away from the SRS. Drivers who encounter a CLRS and who are unaware of their lane position may assume they are encountering a SRS and reactively turn left.

The main objective of the current research is to address some of the major concerns with the CLRS that could prevent their widespread use and safety benefits for Kansas motorists. One of the major concerns, and a possible drawback, to CLRS is unacceptable levels of noise to the roadside residents. Therefore, external noise toward roadsides and its effect on roadside residences will be investigated. If CLRS are used in no-passing zones only, it should not be a problem as theoretically cars should not be on them. In passing zones, however, that could be a problem.

The visibility of the centerline pavement markings is an important issue. Based on the results from the previous study, the Kansas State University team has found that it is a divided issue. Some respondents say visibility is lessened; others say it is enhanced. A laboratory and field study on the existing CLRS in Kansas and the neighboring states will be conducted to investigate that issue.

To determine the safety effectiveness of the CLRS the KSU team will compare the number of cross-over crashes, injuries and fatalities for two locations: U.S. 40 and U.S. 50.

About the Author

Rys is an associate professor in the Department of IMSE at Kansas State University.

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