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36 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections At the Newbridge Roundabout in Scotland, transverse pavement markings reduced the number of reported crashes from 14 in the year prior to installation to 2 in the 16 months after installation. (Katz et al., 2003) A study conducted on US Highway 60 in Meade County, Kentucky, found crashes were reduced after transverse pavement markings were installed. During the previous six years, an average of eight crashes occurred at this location each year, and speed was identified as a con- tributing factor in 75% of those crashes. During the year after installation, three crashes were reported, with one attributed to high speed. (Agent, 1980) Data for additional years after treat- ment installation were not included in the study. In another study, transverse lines were applied at 42 approaches to roundabouts. Each approach had a minimum of 3.2 km (2 mi) of uninterrupted road to allow drivers to adapt to the high-speed environment. During the two-year period after the transverse lines were installed on the approaches, speeds were reduced by 57%. The before-and-after studies conducted at all 42 approaches also indicated that the number of crashes decreased from 96 to 47. A follow-up study conducted at seven of the sites four years after installation indicated that the treatment continued to be effective with speed-related crashes showing a significant decline. (Human Factors North, Inc., 2002) 4.4 Transverse Rumble Strips 4.4.1 Overview Three NCHRP Project 3-74 test sites in Texas provided documented applications for the high- speed intersection treatments discussed in this section. Rumble strips provide audible and tac- tile warning to encourage deceleration and reduce comfortable speed. These treatments can be applied to stop-controlled approaches, toll plazas, horizontal curves, and work zones. Design variations include paved, rolled, or milled; raised or depressed; painted or unpainted; and clus- ter spacing. Noise, as well as adverse effects for motorcycles and bicycles, should be considered. 4.4.2 Applicability and Considerations Rumble strips are raised or grooved patterns installed on the roadway travel lane or shoulder pavements. The texture of rumble strips is different from pavement and produces both an audi- ble warning and physical vibration when vehicle tires pass over them. (FHWA R&T, 2007) Rum- ble strips can be installed to warn drivers of an upcoming need to act, such as the need to stop at a traffic signal, slow down at an intersection, change lanes in a work zone, or steer back into the travelway. Their purpose is to provide motorists with an audible and tactile warning that their vehicles are approaching a decision point of critical importance to safety. Although rumble strips warn drivers that some action may be necessary, they do not identify what action is appropriate. The driver must use visual cues to decide what type of action is appro- priate. Thus, rumble strips serve only to supplement, or call attention to, information that reaches the driver visually. In many cases, the objective of a transverse rumble strip is to call attention to a specific traffic control device, such as a Stop Ahead sign. Transverse rumble strips are placed perpendicular to the direction of travel, and are designed to reduce the deceleration rate and the potential for sudden braking, skidding, and loss of control. The most common use of rumble strips is on intersection approaches controlled by a stop sign, but they have also been used on approaches to signalized intersections, especially for isolated signals on high-speed roadways where drivers may not expect the presence of a signal. Transverse rumble

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Treatment Descriptions 37 strips are generally installed on approaches to intersections of expressways, rural highways, and park- ways. Transverse rumble strips have also been placed prior to toll plazas, horizontal curves, and work zones. (FHWA R&T, 2007) The noise impacts of transverse rumble strips may make them a poor choice for locations where pedestrians or others will spend time adjacent to the treatment. Transverse rumble strips have the potential to adversely affect bicycles and motorcycles due to the vibrations generated, startle drivers as they cross over the rumble strips, and cause drivers to maneuver quickly to avoid the in-lane rumble strips. Rumble strip design, as shown in Exhibits 4-5, 4-6, and 4-7, varies by state and depends on the type of facility to which the treatment is being applied. 4.4.3 Treatment Layout/Design Rumble strips should be placed so that either the upcoming decision point, or a sign that iden- tifies the action that may be required, is clearly visible as the driver passes over the rumble strip. (Credit: TTI, 2006) Exhibit 4-5. Transverse rumble strips in wheel path. Exhibit 4-6. Transverse rumble strips.

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38 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (Credit: Corkle et al., 2001) Exhibit 4-7. Transverse rumble strips across the entire travel lane. Rumble strip locations should be selected to provide adequate advance warning time for drivers to take the potentially required action. Values for deceleration in the "Green Book" (AASHTO, 2004) provide a starting point for locating the strips. Transverse rumble strips can be installed to cover either the entire width of the travel lane, as shown previously in Exhibit 4-7, or just the width of a vehicle's wheel path, as shown previously in Exhibit 4-5, enabling drivers familiar with the area to straddle the rumble strips to avoid driv- ing over them. (Corkle et al., 2001) Transverse rumble strips may be installed in several small clusters on a high-speed intersec- tion approach. The number of strips, their spacing, and the distance from the intersection proper should be determined through a review of field conditions, driver sight lines and desired response, and local practice and judgment. Appropriate installation points may be selected to reinforce other new or existing treatments or features such as warning signs. The markings have potential to draw additional attention to those warning signs and to encourage drivers to reduce their speeds as they approach the inter- section. Other appropriate locations for installation may include the stopping sight distance for the approach speed, or at a point where the roadway segment environment changes, such as at a point of tangency or at a driveway in advance of the intersection. To accommodate bicycles on roadways with rumble strips, agencies should provide a smooth, clear, paved surface wide enough for a bicycle to travel comfortably to the right of the rumble strips. Some agencies have begun to redesign rumble strips to make them safer for bicycles. A skip pattern enhances rumble strip safety for bicycles, providing cyclists the opportunity to enter and exit the bike path without having to cross over the rumble strips. (Walls, 1999) There are a variety of rumble strips that vary in their installation methods, shape, size, and amount of noise and vibration produced. (FHWA Safety, 2007) Rolled rumble strips must be installed when constructed or reconstructed shoulder surfaces are compacted. Formed rumble strips are appropriate for Portland cement concrete shoulders and involve grooves or indenta- tions formed into the concrete surface during the finishing process. (Elefteriadou et al., 2001) Raised rumble strips are strips of material that adhere to new or existing surfaces. Raised and surface-mounted rumble strips can easily be removed by snowplows, causing the need for replace- ment. Thus, the use of raised rumble strips is usually restricted to warmer climates. (Morgan and McAuliffe, 1997) Some agencies paint over rumble strips to make them more visible. (FHWA