Low Cost Treatments for Horizontal Curve Safety
CHAPTER 5. RUMBLE STRIPS
A rumble strip can be formed in the pavement surface by placing either grooves into the surface or strips of material above the surface according to a prescribed spacing pattern. A vehicle passing over the rumble strips produces noise and vibration and alerts the driver to a potentially hazardous situation. Agencies can install rumble strips on horizontal curves longitudinally with the centerline; with the edge line or on the shoulder; or transversally across the full lane in the advance of the curve.
Each of these three will alert drivers to different hazards and each achieves a different objective. Use the centerline rumble strip (CLRS) to alert drivers who drive into the opposing traffic lane and thereby avoid head-on or sideswipe-opposite direction crashes. Use the shoulder (or edge line) rumble strip (SRS) to alert motorists who drive onto the shoulder and beyond and thereby avoid run-off-road crashes. Use the roadway (transverse) rumble strip (RRS) to alert drivers approaching the curve of the potential need to reduce speed or at least be alert while driving through the curve. Because of the similarities in the applications, they are discussed together.
Illustrations of rumble strip for centerline (left), shoulder (middle), and across roadway (right).
There are four types of rumble strips:
The milled or rolled strips are typically used for the centerline and shoulder application. The milled type is preferred because it produces a higher noise level and vibration stimuli than do rolled rumble strips. The raised types may be used for the across the pavement roadway application. The noise and vibration effect is created by the tires bouncing over the raised bars.
CENTERLINE RUMBLE STRIP
Because motorists frequently cross over the centerline through curved sections, the centerline rumble strip (CLRS) is a candidate treatment for horizontal curve sections. However, the application of CLRS just along a curve section has not been identified as an actual practice. This is likely because the installation cost of would not justify their use for a relatively short section. Therefore, when used, agencies should install CLRS on a considerable section of roadway.
Several State highway departments have guidelines for applying CLRS, including:
There are variations in design patterns, but the most common types are milled, 12 in to 16 in long (perpendicular to the centerline), 7 in wide (along the centerline), ½ in deep. The two most common patterns are continuous rumble strips 12 in to 24 in apart, or alternating, with pairs of rumble strips 12 in or 24 in apart, with the pairs being 24 in or 48 in apart, respectively.
Special machinery is needed to install milled rumble strips.
Schematic for centerline rumble strips illustrates the applied pattern.
Based on a variety of information from States using CLRS, the positive effects far outweigh the potentially negative effects. The most significant positive effect is the reduction in overall, targeted (cross over), injury and/or fatal crashes reported by States that use them. For example, Delaware DOT reported a 90 percent reduction in the head-on collision rate after installing CLRS on a two-lane undivided rural highway with a high fatality rate. Also, studies show that motorists tend to position their vehicles farther away from the centerline, and that CLRS help drivers identify the centerline during adverse weather conditions, such as blowing snow.
The installation cost will vary based on a number of factors including the length of section, pavement type, pattern, and highway location. Those agencies installing them report costs of about $0.40 per linear ft; however, when applied just to a single curve, expect a higher cost.
SHOULDER RUMBLE STRIP
Rumble strips can be applied with the edge line or on the shoulder, if it is paved wide enough. There are no published guidelines for when to use this treatment, but it should be considered if there is a high number of run-off-road (ROR) crashes. As with the CLRS, the shoulder rumble strip (SRS) would likely not be used for a single curve because of high installation cost. The guidelines suggested for CLRS would be reasonable for SRS use as well.
The layout design for the shoulder rumble strip (SRS) varies among those States using them. The following table and figure show the layout of a typical milled type SRS.
Standard Dimensions of Milled and Rolled SRS.
Standard SRS measurements.
A concern for the SRS treatment is its effect on bicyclists. To reduce this potential negative effect, agencies can change the design to allow gaps in the pattern where bicyclist could travel to turn or change their position. The gap pattern adopted by Alaska is shown on the following page.
Gap pattern adopted by Alaska DOT.
The safety benefit of SRS is well established, at least for large volume, high speed roads. For example, the application of SRS on the New York State Thruway resulted in an 88 percent reduction on ROR crashes, with a 95 percent reduction in fatalities. Their safety effectiveness for lower volume roads might be less dramatic, but may still be cost effective for roadway sections with high ROR crash rates.
ROADWAY RUMBLE STRIP
Roadway (or Transverse) rumble strips (RRS) can be raised bars or grooves placed across the travel lane. For horizontal curve applications, RRSs are a warning device to supplement signing and alert drivers of the need to reduce speed or at least pay attention as they negotiate the curve.
Based on the available literature, the guidelines for their use on horizontal curves are:
For this RRS application a set of 5 grooves and painted lines were placed 200 ft before the curve.
Some of the concerns with the use of RRS include:
Generally, limit rumble strips to a maximum height or depth of ½ in to minimize the jarring action to vehicles. If thermoplastic materials are used to created raised bars for RRS, the material should be white.
Configure rumble strips so that most drivers are not tempted to go around the rumble strip by driving onto the shoulder or into an adjacent lane. Agencies can do this by extending the rumble strip over part of the shoulder as well as the full width of the traveled way or by using a discontinuous rumble strip design.
There is currently no conclusive evidence of roadway rumble strip effectiveness in reducing crashes at curves. They do tend to reduce speed, in most cases, but not to a practical level.
Agency Contacts and Further Information
More detailed information on rumble strips can be obtained from the following sources:
Roadside Design: Steel Strong Post W-beam. A guidance memo was issued on May 17, 2010 on the height of guardrail for new installations. Guidance regarding existing guardrail will be developed in the next several months, in consultation with AASHTO’s Technical Committee on Roadside Safety.