Frequently Asked Questions on Rumble Strips
Q: Should there be gaps or breaks in our rumble strips?
A: Usually. Continuous shoulder and edge line rumbles often have bicycle gaps and are broken at intersections, interchanges, and sometimes across bridges. At interchanges, the rumbles frequently split and follow the right and left edges of the ramp. Similarly, for intersection turn lanes, the location of the rumble will continue at the same offset of the turn lane edge line, as they were on main line. For intersections without turn lanes, the breaks in the rumbles may include approaches if vehicles are likely to cross the edge line before reaching the intersection. Note that rumbles with bicycle gaps are still considered continuous rumble strips.
For center line rumble strips, gaps are typically provided only at intersections and their approaches. Some jurisdictions have chosen to discontinue center line rumbles in passing zones; however, studies indicate most head-on crashes result from a motorist making an unintentional maneuver, which is just as likely to occur in a passing zone as a no-passing zone. While there may be location specific reasons to discontinue the rumble strips, a policy of not installing in passing zones is for the most part unnecessary as research has shown that center line rumbles do not impede passing opportunities, even for motorcycles.
For both types of rumbles, agencies may consider discontinuing them across bridges where the depth of the rumble may diminish cover of steel reinforcement.
Q: How much does it cost to install rumble strips?
A: Unit prices have been estimated to range between ten and sixty cents per linear foot (about $500 to $3000 per mile). There are three key reasons for the large variations in the unit price: 1) longer projects typically have a lower unit price, 2) milling into concrete is significantly more expensive than asphalt because there is two to three times the wear on the cutting blade, and 3) costs for rumbles installed as a stand-alone project may vary significantly compared to installation as part of a larger reconstruction or resurfacing project based on a number of factors. 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.
Q: What is the cost to remove a rumble strip, should it be necessary?
A: Because 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 stripes 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. Although the process did not provide a completely smooth pavement, it reduced the noise to address the concerns. In another example, Michigan milled out ten miles of rumbles on both shoulders and inlaid fresh asphalt. The cost was $13,000 per shoulder-mile. Fortunately, this is a fairly rare occurrence, as the cost to remove can be substantially more than the cost to install rumble strips. The most common situation where removing rumble strips is likely to occur is during construction projects where the lane configuration changes during one of the project phases, with the result that the rumble strip may be in a wheel path. Even in this situation, project specific issues will dictate the method and costs.
Q: Is there any benefit to installing shoulder rumbles if there is no recovery area?
A: In most cases there are still incremental benefits. Although many states set policies not to install rumbles unless they have a minimum amount of additional shoulder or lateral clearance to fixed objects, these policies are generally in response to issues other than crash reductions. Recovery area includes 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. Reaction times vary and recovery area along corridors also varies. Unless the driver is going to go over a cliff or will hit a tree within a foot of the rumble strip, shoulder rumbles will most likely give the driver the opportunity to steer away from a more hazardous situation or brake so that the impact is less severe if they do hit a fixed object.
Q: How do you re-install rumbles after paving operations, especially thin overlays?
A: Agencies have taken various approaches to this. Some agencies re-install the rumbles immediately, as part of the resurfacing project, while others chose to re-install during area-wide milling projects, which are scheduled at intervals (such as annually) and replace rumbles on all recently paved projects. Additionally, there are various approaches to surface preparation when paving over existing rumble strips. Some agencies mill out the rumbles and either inlay and overlay or simply overlay. This level of quality control is particularly important where traffic is likely to cross the old rumble location frequently, such as a passing zone with center line rumble strips or a new lane in the location of an old shoulder with rumble strips. Other agencies have found that simply overlaying the rumble strip works adequately.
For thin 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.
Q: What are the appropriate dimensions for a rumble strip?
A: Appropriate measurements depend on the need and the facility. The most common dimensions (and those on which the crash reductions are based) are 7 X 16 X ½ inch with 12 inch center-to-center spacing. These dimensions (and even deeper average depths of 5/8 inches for shoulder rumble strips) are what the crash reductions (cited in the FHWA Technical Advisories and most studies) are based on. Several states vary one or more of these dimensions as their standard or for specific facility types. Another common design for center line rumbles is 7 X 12 X ½ inch. Adjusting the dimensions is particularly common on shoulder or edge line rumbles, due to narrow pavement widths or to provide additional shoulder width beyond the rumbles to accommodate cyclists. For several years it was thought that heavy trucks required the 16 inches or more, to ensure their large tires dropped into the rumble, but recent studies indicate the stimuli of rumbles inside the cab of commercial vehicles is often insignificant. Since this dimension has been shown to be less critical than once thought, many agencies have experimented with use on narrow pavements, so that 8-inch is not uncommon on certain road types with narrow pavements and 6 inch and 4 inch are even being used experimentally. Keep in mind that the effectiveness of the rumbles’ warning properties will be decreased by reducing the dimensions, but we don’t have specific information on the extent.
Q: Why are milled rumble strips preferred over rolled?
A: The two main reasons are that rolled-in rumbles typically have dimensions that do not provide the same alerting potential through vibration and noise, and they have to be placed on still-hot asphalt pavement, which makes it very inconvenient for safety improvement projects. Additionally, the rumbles typically take a larger portion of the shoulder width and the construction method does not allow for bicycle gaps. Construction difficulties have been reported with the installation of rolled-in rumble strips. The indentations may not reach the proper depth if the operation is performed when the temperature is too low and the asphalt may not stabilize if the asphalt temperature is too high, resulting in problems with both the depth and shape of the indentations. Insufficient asphalt compaction between the indentations may also result from the roller riding on steel pipes, which may lead to premature deterioration of the shoulder surface. Lastly, in warmer climates, rolled rumbles tend to slowly decrease in depth.
Q: When are transverse or in-lane rumble strips appropriate?
A: Transverse rumble strips are placed in the lane, generally 2-4 groups extending across the full lane width, to warn drivers of something unexpected. The most common use is in advance of a rural stop controlled intersection where drivers have been travelling unimpeded for some time. They are also sometimes used in work zones, in advance of tollbooths, and occasionally in advance of unexpected sharp curves.
Another design of longitudinal rumble strips that was explored for a very short time was in-lane rumble strips. The concept was to place the rumbles in the center of the lane, between the wheel paths, so that as a vehicle strayed from the center of the lane, they would encounter the rumble. While it did avoid placing the rumbles near a joint, it was problematic for motorcyclists and has not received any significant use.
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