Sept. 22, 2004

For more information contact:
Tom Welch
515-239-1267

DOT to use milled shoulder rumble strips on two-lane highways

AMES, Iowa – Single vehicle, run-off-the-road crashes account for about one-third of fatal crashes in the U.S. each year.  In Iowa the statistics for 2003 were even more staggering where 43 percent of the state’s fatalities came in run-off-the-road crashes. One of the most effective measures that can be taken to prevent vehicles from running off the road is rumble strips on paved shoulders. Iowa Department of Transportation engineers feel so strongly about the safety benefits that milled in rumble strips are now standard on all new paved shoulder construction in rural areas. 

The DOT recently installed the first milled rumble strips on a two-lane roadway on U.S. 63 near New Hampton. Milled rumble strips are cut 16 inches long, seven inches wide and one-half inch deep into the four-foot wide paved shoulders. This contrasts to the rolled rumble strips that have been used on Iowa roadways in the past, which are deeper but much more narrow. (see attached graphic #1)  The milled strips can be cut into any existing paved shoulder in adequate condition, whereas rolled strips need to be installed at the time of paving.

The shape of the new rumble strip design provides superior performance. A vehicle’s tires drop slightly into the rumble strip, creating more noise and vibration, not only for cars but also for trucks and other vehicles with large tires.

Bicycle accommodation has been an issue in several states that have moved towards more widespread use of shoulder rumble strips. In an effort to garner input and advice from cyclists and address these issues up front, DOT officials worked with members of the biking community to come to a solution that includes a gap pattern for new rumble strips and wider six-foot shoulders on roadways where more bike traffic can be expected. Milled rumble strips will be placed in 48-foot sections, with a 12-foot break in between to allow cyclists to cross over. This pattern still provides 80 percent coverage of the shoulder with rumble strips. (see graphic #2)

Gaps give the bicyclists the flexibility they need to move between the shoulder and roadway. Traffic on the roadway might force a bicyclist to the shoulder, or debris on the shoulder might make the roadway a safer place to ride. In addition to the gaps in the rumble strip pattern, installing wider shoulders will also be considered when bicycle traffic is expected to be heavy on certain roadways.  

“With our goal of safety always in mind, we’re very pleased that Iowa is taking the lead to work with interested groups to provide milled shoulder rumble strips while still making the roadways usable and safe for non-motorized vehicles,” said Tom Welch, DOT safety engineer.

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