FHWA Tech Advisory Lauds Milled Rumble Strips

In late December 2001, the Federal Highway Administration (FHWA) released a Technical Advisory (T 5040.35) that endorses shoulder rumble strips for driver safety from run-off-road (ROR) events, and lauds milled-in strips as the best option.

"In recent years several State transportation agencies and toll road authorities have installed and evaluated the effects of shoulder rumble strips," the FHWA said. "The results ... have consistently shown significant decreases in single-vehicle run-off-the-road crashes."

Rolled-in strips for freshly placed asphalt shoulders, or formed-in strips for concrete shoulders have been two early designs, FHWA said. But research by the Pennsylvania Turnpike Authority found strips milled into existing shoulders to be superior for a number of reasons.

In conclusion, the FHWA advises "Continuous, milled shoulder rumble strips should be installed on rural freeways and expressways on the National Highway System ... [w]hile they may be installed on a project-by-project basis, economies of scale and timely implementation of shoulder rumble strips make systemwide installation projects highly desirable."

The complete technical advisory may be downloaded at http://www.fhwa.dot.gov/legsregs/directives/techadvs/t504035.htm

Earlier Research Proves Value
The advisory is based on earlier research from FHWA which reaffirmed the cost-effectiveness and value of rumble strips on the shoulders of our nation’s highways.

And although the research used data only from rolled-in rumble strips -- that is, rumble strips formed during compaction of fresh hot mixed asphalt by a compactor with upraised ridges on its drum -- many of the conclusions intuitively are applicable to milled-in strips as well, such as those formed by the Wirtgen W 60 modified with the Rumbler rumble strip attachment.

Released in 1999, "Safety Evaluation of Rolled-In Continuous Shoulder Rumble Strips Installed on Freeways" said 1997 statistics from the Fatality Analysis Reporting System (FARS) show that 37,280 fatal crashes occurred, with 11,126 of these crashes being coded as single-vehicle run-off-the-road crashes.

"This significant safety problem is being addressed with continuous shoulder rumble strips (CSRS) and other safety treatments by many highway agencies," wrote author Michael S. Griffith at FHWA’s Turner Fairbank Highway Research Center. "The average safety effect of CSRS [continuous shoulder rumble strips] is estimated to be a reduction of single-vehicle run-off-the-road accidents by 18.3 percent," the new research reports. For injury crash data (omitting non-injury run-off-theroad accidents), the average safety effect of CSRS installed on rural freeways was a 13 percent reduction in such single-vehicle accidents.

Thus the expense of installing CSRS -- either milled or rolled -- can mean big savings to the motoring public. In the case of this research involving rolled-in or formed rumble strips, the savings were estimated by the research (milled-in strips cost somewhat more but also are functionally louder).

"It was estimated that approximately one single-vehicle run-off-the-road accident (at an average cost of $62,200) could be prevented every three years based on an investment of $217 to install rolled-in CSRS for one 1 km [0.62 mile]," Griffith writes. "Clearly this is a substantial return for a safety treatment that suggests widespread implementation."

The complete report is available at
http://safety.fhwa.dot.gov/programs/rumble.htm.

Milled-In Strips Preferred
While the paper cited above did not study milled-in rumble strips, in actuality, milled-in strips are preferred because of their louder noise, despite their slightly higher cost. "Milled rumble strips are the current favorite type of rumble strip among many states because they are easy to implement on new or existing asphalt and Portland cement concrete pavements and shoulders," the FHWA reports. "They have little or no effect on the integrity of the pavement structure; and they produce greater noise and vibration than rolled or formed rumble strips."

Milled rumble strips are generally installed with a longitudinal width of 180 mm (7 in.) and a transverse width of 400 mm (15.6 in.) while offset from the travel lane, typically at 300 to 400 mm (11.7 to 15.6 in.), FHWA said. "Tires passing over milled rumble strips drop roughly 1.3 mm [0.5 in.] into the groove, which causes tire noise and vehicle vibration," FHWA said. This compares to a tire drop in rolled rumble strips approximately 0.75 mm (0.3 in.).

"Field tests demonstrate that the noise and vibration from milled rumble strips are particularly effective in warning large trucks that leave the road," FHWA said. "One study determined the effect of the milled strip to be 12.6 times rougher and 3.4 times louder than that of the rolled rumble strip."

"Milled rumble strips are the preferred method for new or existing (retrofit) shoulders," reported FHWA’s Wyoming Division Office in 1998.

"Rolled rumble strips, while still in use, are not as effective as milled for new or existing shoulders."

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