Low Cost Treatments for Horizontal Curve Safety

CHAPTER 5. RUMBLE STRIPS

DESCRIPTION

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:

• Milled rumble strips are formed with a machine that cuts a smooth groove in a roadway of either new or existing asphalt or cement concrete.
• Rolled rumble strips are pressed into freshly laid asphalt pavement by a roller with steel pipes welded to a drum.
• Formed rumble strips are added to fresh concrete shoulder with a corrugated form that is pressed onto the surface just after concrete placement and finishing operations.
• Raised rumble strips can be formed by applying asphalt material ( ¼ to ½ in high) as raised bars on the surface.

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

Application Guideline

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:

• Crash history indicates a large number of head-on or sideswipe crashes.
• Posted speed limit of 50 mi/h or greater.
• ADT threshold of at least 1500.
• Pavement width of at least 20 ft.
• Pavement type of asphalt in good condition with minimum depth of 2.5 in.
• CLRS are not recommended for:
• Bridge decks.
• In area of intersections with public roads or short distances between access points.
• Existing concrete pavements with an overlay depth less than 2.5 in.
• Roadways where residences are close to the highway—because of noise complaints.

Design

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.

Effectiveness

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 potential negative effects include:
• Disruption to motorcyclists and bicyclists.
• Roadside noise in residential areas.
• Pavement deterioration.

Cost

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

Application Guideline

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.

Design

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.

Effectiveness

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

Application Guideline

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:

• Documented crash problem where conventional treatments, discussed earlier, have failed to solve the safety problem.
• Road segment with distinct differences between the roadway speed limit and the curve advisory speed of at least 20 mi/h.
• Proximity of another geometric feature such as a side street with limited sight distance; a second, sharper curve a short distance beyond the first curve; or a downgrade leading into the curve.
• Long tangent section ends with a curve.

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:

• Noise complaints by nearby residents—avoid using them in residential areas.
• Motorist use opposing lanes to avoid rumble strips—a potential problem for local travelers that is reduced by using a discontinuous pattern design such as gaps in the bars or grooves across the pavement.
• Maintenance problems—grooved RRS is more durable, lasting longer than raised bars, especially in areas with snowplowing.
• Motorist worries—keep depth of groove or height of bar to ½ in or less and use a warning sign RUMBLE STRIPS AHEAD.
• Bicyclist and motorcyclist concerns—address by installing discontinuous pattern design—and warning sign RUMBLE STRIPS AHEAD.
• Overuse of rumble strips—leads to "crying wolf" attitude and therefore should be used sparingly.

Design

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.

Effectiveness

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:

• NCHRP Synthesis Report 339, Centerline Rumble Strips, Transportation Research Board, 2005.
• NCHRP Synthesis Report 191, Use of Rumble Strips to Enhance Safety, Transportation Research Board, 1993.
• Synthesis of Shoulder Rumble Strip Practices and Policies obtained from http://safety.fhwa.dot.gov/roadway_dept/pavement/rumble_strips/resources/synthesis/
• Technical Advisory T 5040.35 Roadway Shoulder Rumble Strips, Federal Highway Administration, Washington, DC, December 20, 2001.
• The Effectiveness and Use of Continuous Shoulder Rumble Strips. Perrillo, K. Federal Highway Administration, August 1998. See http://safety.fhwa.dot.gov/roadway_dept/research/conts_rumble/.