A shoulder rumble strip is a longitudinal safety feature installed on a paved roadway shoulder near the outside edge of the travel lane. It is made of a series of milled or raised elements intended to alert inattentive drivers (through vibration and sound) that their vehicles have left the travel lane. An edge line rumble strip is a special type of shoulder rumble strip placed directly at the edge of the travel lane with the edge line pavement marking placed through the line of rumble strips. This placement is sometimes referred to as an edge line rumble stripe.
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A center line rumble strip is a longitudinal safety feature installed at or near the center line of a paved roadway. It is made of a series of milled or raised elements intended to alert inattentive drivers (through vibration and sound) that their vehicles have left the travel lane. In most cases, the center line pavement marking is placed over the rumble strip, which is sometimes referred to as a center line rumble stripe. Center line rumble strips can also be installed outside of center line pavement markings.
Reaction times vary and the recovery area along corridors also varies, so there is likely to be some benefit with edge line or shoulder rumbles even with limited recovery areas.
In most cases there are still incremental benefits. Although several states do not install rumbles unless they have a minimum amount of additional shoulder or lateral clearance to fixed objects, these practices are generally in response to issues other than crash reductions (i.e., bicyclist use, noise impacts). Recovery areas include not just the paved shoulder but also any unpaved shoulder and recoverable slopes, including those that may be at the bottom of a non-recoverable but traversable slope.
Profiled pavement markings (e.g., raised thermoplastic, very thick epoxy or tape) are not meant to alert a distracted or drowsy driver like a rumble strip. They are meant to improve visibility under wet nighttime conditions. However, there are some raised rumble strip designs that insert a "cookie" within the striping. These have not been studied to the extent of milled rumble strips, but will clearly be less effective due to their limited dimensions.
For drift-off-road crashes, studies of milled freeway shoulder rumble strips in Michigan and New York documented drift-off-road crash reductions of 38 and 79 percent.
Reductions were also shown on other types of roadways, but the estimates are not as statistically reliable.
Rumble strips are installed as a low-cost safety countermeasure that can reduce single-vehicle run-off-road (SVROR) and cross-center line crashes. Rumble strips, whether center line, edge line, or shoulder, alert inattentive or drowsy motorists whose vehicles have drifted out of their travel lane. As motor vehicle tires pass over the rumble strips, the drifting motorist receives auditory and tactile warnings to correct their path of steering.
For centerline rumble strips, agencies need to specify the measuring point where the depth is to be full depth depending on the position of the milled section with the crown in the pavement. The depth at the edges (B) will be less than at the center (A). Agencies may also want to specify that the rumble strip be symmetrical to the crown. This is primarily an issue with older equipment where the milling head is on the side rather than in the center of the vehicle.
The tolerance should take into account the desired depth and the acceptable deviations from that depth. As an example, a specification such as, "a minimum 1 ⁄ 2 -inch depth to a maximum of 5 ⁄ 8 -inch depth" is more appropriate than specifying " 1 ⁄ 2 -inch deep cut with a 1 ⁄ 8 -inch tolerance," because a 1 ⁄ 8 -inch tolerance represents 25% of the milled depth, resulting in rumble strips that may not perform adequately.
Yes, it is important to include tolerances in construction specifications to account for variations in how the equipment performs as well as the pavement conditions. If the specification is not stated properly, then the inspector has no basis for evaluating the installation to determine acceptance of the rumble strips.
Some vary the rumble strip depth by making it slightly shallower. For example, Pennsylvania DOT varies the depth of rumble strips in passing zone from a typical depth of 1 ⁄ 2 " to 3 ⁄ 8 ".
Many agencies continue center line rumble strips in passing zones and no passing zones (such as Missouri, Mississippi, and Texas), arguing that a typical drift-off head-on crash is as likely (or even more likely) to occur in a straight tangent passing zone than in a no passing zone.
Several resources are available that share the latest research, practices, and techniques used by transportation agencies. For information on rumble strips and their effectiveness, reference NCHRP Report 641: Guidance for the Design and Application of Shoulder and Centerline Rumble Strips .
The input requested from these groups should not be an all-or-nothing acceptance, but criteria for decision making.
Based on rumble strip adoption and widespread use by several State DOTs, the following actions could help to achieve expanded rumble strip adoption:
Since specific circumstances dictate the method and costs, it is difficult to provide a price range. For example, in Arkansas, when a portion of the center line rumble strips had to be filled in due to noise concerns, crews hand placed a patching type material in the rumbles (somewhat similar to pothole repairs). The materials were inexpensive, but it was a labor-intensive process that would probably only be cost-effective for removing small sections of rumbles. In another example, Michigan milled out 10 miles of rumbles on both shoulders and inlaid fresh asphalt. The cost was $13,000 per shoulder-mile.
It is often difficult to compare unit prices for these very different methods as some may include only the cost for milling the rumbles, while others include the cost of lane closures or other incidentals.
Mobilization, temporary traffic control, temporary pavement markings, and milled material disposal are sometimes the driving cost factors for rumble strip installation. Costs for rumbles installed as a stand-alone project may vary significantly compared to installation as part of a larger reconstruction, resurfacing, restriping, or crack sealing project based on a number of factors, including traffic control, scheduling of work, mobilization, type of work to be performed, etc.
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Unit prices have been estimated to range between $0.10 and $1.20 per linear foot (about $500 to $6000 per mile). In general, contracts where rumble strip installation is the primary item are lower in unit cost than other contracts.
When rumble strips were in the early implementation phase, many construction methods were tried and evaluated for cost, productivity, effectiveness, and quality. Rolled-in rumbles, rumble strips constructed as part of the conventional paving process (modifications to the compaction roller), were tried and found lacking in all factors listed above. Milled rumble strips have become the standard construction practice due to their flexibility to construct various rumble strip shapes and spacing within acceptable tolerances and cost effective operations. The equipment used for milling rumble strips continues to evolve.
Agencies vary in their requirements and may desire to coordinate with their pavement/materials/maintenance engineer. All pavements (hot mix asphalt (HMA), ultra-thin HMA, chip seal, micro-surfacing, etc.) need to have a minimum thickness of "bound" pavement material before rumble strips are installed. "Bound" pavement materials are a combination of all layers of the materials listed above, but it does not include cold in place (CIP) recycling. In general, the thickness of the most recent overlay needs to exceed the depth of the rumble strip so that the previous layers of pavement are not exposed, allowing moisture to infiltrate and compromise their integrity. For example:
Rumble strips are not normally applied to bridge decks due to the presence of rebar.
Several agencies have experienced delamination of the chip seal (pavement surface treatment constructed by spraying liquid asphalt on the pavement surface followed by an application of small size aggregate) near the area of the rumble strip when milling rumble strips into chip seal surfaces. Many agencies either mill the rumble strip before applying the chip seal or place the chip seal over an existing rumble strip.
Michigan DOT has found that milling rumble strips to 5/8" depth prior to applying the chip seal provides good quality rumble strip and often a second chip seal over these rumble strips has adequate alerting noise and vibration without the need for the rumble strips to be re-milled.
In Idaho, standard practice is to install rumble strips before applying a chip seal. While they have installed rumble strips after applying the chip seal, they found that the milling process causes chips to unravel.
In Washington, standard practice is to install rumble strips before applying a chip seal due to experiences with delamination when the chip seal is placed prior to milling in the rumbles.
Chip seal on top of an existing rumble strip has been shown to retain the basic shape of the rumble, although losing some cross-section. However, stones from the chip seal enhance the noise and vibratory properties of the rumble.
Recent experience in Michigan has found that preventative maintenance treatments such as ultra-thin hot mix asphalt and micro-surface can be compatible with rumble strips. Micro-surface and ultra-thin hot-mix asphalt overlays fill in existing lines of rumble strips, but a fresh line of rumble strips can be cut into the overlay at the same location without significant delaminating caused by the underlying filled-in rumbles.
All pavements (hot mix asphalt (HMA), ultra-thin HMA, chip seal, micro-surfacing, etc.) need to have a minimum thickness of "bound" pavement material before rumble strips are installed. "Bound" pavement materials are a combination of all layers of the materials listed above but does not include cold in place (CIP) recycling.
Micro-surfacing fills in existing lines of rumble strips, but a fresh line of rumble strips can be cut into the overlay at the same location without significant delaminating caused by the underlying filled-in rumbles.
Michigan DOT requires that pavement be cleaned and existing rumble strips filled prior to applying the top course of micro-surface, either by a leveling course or a separate pass.
Some agencies have expressed concerns that center line rumble strips installed across longitudinal joints would contribute to early joint or pavement deterioration. For the most part, these concerns have been shown to be unfounded. When installing rumble strips on existing pavement, the pavement should be rated as fair or better to be able to accept the milling process without raveling or deteriorating the pavement. For example, Missouri DOT has not experienced issues milling through the joint as long as the pavement is in good condition. Michigan DOT applies center line rumble strips across the joint line.
In other cases State DOTs have shifted the joint to be off-center so the center line rumble strip and centerline marking can be placed at the center of the roadway but not on the joint.
In 2001, Colorado DOT performed an evaluation of center line rumble strips. After a period of five years, it was determined that the rumble strips did not have any significant detrimental effect on pavement life.
In another example, Wayne State University studied rumble strips in Michigan and found that rumble strips were not connected to short-term pavement deterioration.
Some agencies re-install the rumbles immediately, as part of the resurfacing project. Others choose to re-install during area-wide rumble strip installation projects, which are scheduled at intervals (such as annually), and replace rumbles on all recently paved projects.
Surface preparation when paving over existing rumble strips also varies. Some agencies have found that simply overlaying the rumble strip works adequately, while others mill out the rumbles and either inlay or overlay. Many agencies mill the pavement before placing the new asphalt overlay – the rumble strip is removed by this standard milling process, thus the situation of treating existing rumbles is not experienced in many new asphalt overlays.
Recent experience in Michigan has found that shoulder preventative maintenance treatments such as chip seal, ultra-thin hot mix asphalt, and micro-surface, can be compatible with rumble strips, but the preferred techniques vary slightly. Chip seal on top of an existing rumble strip has been shown to retain the basic shape of the rumble, although losing some cross-section. However, stones from the chip seal enhance the noise and vibratory properties of the rumble. Micro-surface and ultra-thin hot-mix asphalt overlays fill in existing lines of rumble strips, but a fresh line of rumble strips can be cut into the overlay at the same location without significant delaminating caused by the underlying filled-in rumbles.
It is not advisable to mill in rumbles after chip sealing as delamination of the chip seal in the area surrounding the rumble has been experienced in many instances.
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