Frequently Asked Questions
Q: How are rumble strips installed?
Shoulder rumble strip milling operation in Iowa
A. There are four basic rumble strip designs or types: milled-in, rolled-in, formed, and raised. The current design used in Iowa is---milled-in. This design is made by cutting (or grinding) the pavement surface with a self-propelled milling machine with rotary-type cutting heads. The machine is programmed to cut the same shape, depth, width, and length into the pavement.
After the machine has cut the shape, the resulting debris is swept, collected and discarded. Because rumble strips are cut into the pavement, they are equally effective on new, existing or reconstructed pavement
As illustrated in the “Plan View” drawing above, milled-in shoulder rumble strips are cut 16 inches long and 7 inches wide, located 6 inches from the painted shoulder line, leaving approximately 7 inches to the edge of the 4-foot wide paved shoulder.
The “Profile View” drawing shows that the shoulder rumble strips are cut ½ to 5/8 inch deep, at a distance of 12 inches on center, with a 5-inch paved surface or gap between the grooves.
These dimensions contrast to the rolled rumble strips that have been used on Iowa roadways in the past, which are deeper and narrower.
The “Typical 4-foot Paved Shoulder Design” drawing illustrates that the shoulder rumble strips are installed in 48-foot lengths with 12-foot gaps. Rumble strips are not placed within the limits of structures, entrances, side roads, and entrance and exit ramps.
Eventually, shoulder rumble strips will be installed on all rural freeways and expressways, and two-lane roadways with an average daily traffic count of greater than 3,000 vehicles per day or where a safety engineering review warrants the installation.
Centerline rumble strips are placed between opposing lanes of traffic. The plan drawing above illustrates a highway project design that incorporates both centerline and shoulder rumble strips.
The Detail A drawing shows that the centerline rumble strips are milled at a length of 18 inches centered across the centerline of the roadway and are approximately 7 inches wide. The strips are milled ½ to 5/8 deep.
The centerline rumble strips are spaced on 1-foot centers continuous throughout the project. Centerline rumble strips do not have spacing gaps similar to the shoulder rumble strips. The only sections of mainline roadway without centerline rumble strips are at intersections.
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