By Kendal Butler
We’ve probably all experienced that unpleasant sound and vibration when we allow our tires to creep too far toward the edge of the roadway and we encounter a rumble strip.
Rumble strips have been used for over 20 years as important safety measures on road shoulders, recently on centerlines and as a caution methods in travel lanes.
How are they made?
There are typically three methods for adding rumble strips to asphalt roads:
• milled
• rolled
• raised.
The milled forms of rumble strips are made with a specialized milling machine, creating uniform and consistent grooves in asphalt pavement. Aside from the more consistent pattern, many believe milled rumble strips allow for better compaction and reduce tearing and raveling. Milled rumble strips are considered the most audible and provide the most vibration.
Rolled rumble strips are made of V-shaped or rounded grooves that are pressed into hot asphalt during construction. A roller with steel pipes affixed to the drum is often used to make the necessary depressions before the asphalt cools. The vibrations and sounds from rolled rumble strips are generally less noticeable than milled rumble strips.
Raised rumble strips are often visual as well as audible. They are made of round or rectangular markers that adhere directly to pavements. Raised buttons can be used as well. Yellow, white or fluorescent colors are used for these products.
Rumble strip usage
Most U.S. states have installed Shoulder Rumble Strips (SRS) on roads where speed limits are above 45 m.p.h. Often the rumble strip is placed within the white pavement marking resulting in Edge line Rumble Strips (ERS). Both SRS and ERS aim to prevent run-off-road vehicle crashes.
Increasingly, more states are installing Centerline Rumble Strips (CRS) on undivided roads. CRS are designed to prevent head-on collisions and opposite direction sideswipes.
“The Federal Highway Administration (FHWA) is seeing more states adopt centerline rumble strips to prevent crashes, in part because these types of strips are the most cost-effective measures available to address head-on crashes on two-lane roads.
“They are particularly effective in preventing accidents related to distracted driving because the strips provide warnings by sound and vibration and can reduce the severity of a crash by giving drivers time to slow down or steer away from hazards,” according to an official statement from the FHWA.
Safety
According to National Highway Cooperative Research Program (NCHRP) studies, milled shoulder and edge rumble strips reduce single-vehicle run-off-road injury crashes by as much as 24 percent on rural freeways and by as much as 46 percent on two-lane rural roads.
The NCHRP research shows even greater potential for CRS than other rumble strips. There is a 50 percent reduction of single-vehicle run-off-road injury crashes on rural highways and a whopping 91 percent reduction on urban two-lane roads.
Among the more obvious safety benefits, rumble strips can also serve as an effective mean for locating the proper travel lane when bad weather obscures pavement markings. The vibration can assist drivers from unintentionally leaving the roadway or in the case of a CRS – crossing into the path of oncoming traffic.
An unintended benefit of shoulder rumble strips in mountainous areas is that they can provide traction for some vehicles on extended slopes in snowy weather.
Maintaining rumble strips
Initial concerns about accelerated shoulder deterioration due to milled rumble strips have mostly been dismissed. However, it is still a common precautionary practice to place rumble strips a few inches from joints to reduce any chance of accelerated deterioration.
A measure to reduce possible deterioration in SRS from standing water in the strip is applying an asphalt fog seal. The seal can reduce oxidation and moisture penetration.
Pavement preservation measures are compatible with rumble strips. Applying a chip seal on an existing strip retains the shape of the rumble but can lose some of the cross section. Stones from the chip seal can increase the noise vibration of the rumble.
Micro-surface and ultra-thin asphalt overlays will fill in existing lines of rumble strips. A new line of rumple strips can be cut into the overlay in the same spot without significant delamination because of the underlying filled-in rumbles.
Raised rumble strips pose a significant problem for snowplows. The blades often scrape the markers off the road. For this reason, milled or rolled rumbles strips are strongly preferred in areas where snow removal happens often.
Overcoming objections
Some cyclists and neighboring homeowners aren’t crazy about rumble strips despite their many safety benefits for motorists.
Many bicyclists complain that SRS force them off the shoulder and make them ride in the travel lane. Riding on the rumble strips can cause cyclists to lose control of their bikes and fall.
Continuous rumble strips can make roadways unusable for many cyclists. Some cycling groups advocate not installing rumble strips on roads where cyclists are permitted or that road owners follow these design suggestions:
• install strips as close to the edge line as practical
• allow gaps in the strips for cyclists to merge, turn or pass other cyclists
• adjust dimensions (width, depth, spacing) to make strips more bicycle-tolerable
• widen shoulders to accommodate the rumble strip and room for cyclists.
Some states are coordinating with Bicycle and Pedestrian Coordinators when installing rumble strips. Other states are experimenting with replacing the typical rectangle-shaped rumble strips with football-shaped rumble strips.
The football-shaped strips reportedly allow for more controllability of the cycles and the ability to better traverse the strip. However, this new design is not considered to be as effective with protecting vehicle traffic as the traditional strips.
Adjacent noise levels increase on roadways with rumble strips have also caused concerns for nearby residents. Even if the noises are intermittent, they can cause objectionable disruptions to neighbors.
Some agencies ban the use of strips altogether in residential areas to avoid the noise complaints. Another alternative is to construct noise barriers to protect the neighbors from the sounds.
Conclusion
Using all manners of asphalt rumble strips has proven to be an effective countermeasure for preventing deadly roadway departure crashes. Inattentive or drowsy travelers are quickly alerted to straighten up and stay safe when they encounter that familiar noise and vibration.
Further info:
FHWA Technical Advisory T 5040.39: Shoulder and Edge Line Rumble Strips (2011)
FHWA Technical Advisory T 5040.40: Center Line Rumble Strips (2011)