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Treatment Descriptions 39 R&T, 2007) Milled continuous shoulder rumble strips are easy to install on existing and new pavement, maintain the integrity of the pavement structure, and produce more noise and vibra- tion than other types of rumble strips. (FHWA Safety, 2007) 4.4.4 Speed Effects At the high-speed intersection approaches tested through NCHRP Project 3-74, rumble strips produced statistically significant speed reductions at the perceptionresponse time data collec- tion location, where a mean speed reduction of 1.3 mph (standard error of 0.5 mph) was observed. At each site, this data collection point was beyond the rumble strips, and about 250 ft upstream of the intersection. Overall, no statistically significant speed reduction was observed at the data collection point where the rumble strips were installed or at the accident avoidance data collection point (roughly 100 ft upstream of the intersections). Transverse rumble strips have been used with mixed results. On stop-controlled approaches, transverse rumble strips have repeatedly resulted in more gradual deceleration by drivers and increased the percentage of drivers making a full stop at the stop sign by about 30%. (Kermit and Hein, 1962; Owens, 1967; Zaidel et al., 1986; Harder et al., 2001) However, they have also increased speed variance. (Morgan and McAuliffe, 1997) The research conducted by Owens found an increase in speed variance on the intersection approach, indicating that some drivers slowed down more than others. University of Minnesota research involving a driving simulator concluded that drivers started to slow down and finish braking at the same time with and without rumble strips, but braking occurred earlier with rumble strips. It was also found that drivers brake more and brake earlier with full-width rumble strips than with wheel-track rumble strips. (Harder et al., 2001) Miles et al. (2005) found that transverse rumble strips produced mostly small changes in traf- fic operations at both horizontal curve and stop-controlled intersection sites. This research con- cluded that the treatment was not successful in significantly reducing approach speeds to an intersection. 4.4.5 Safety Effects NCHRP Synthesis 191 (Harwood, 1993) summarized 10 before-and-after studies that investi- gated the safety effectiveness of transverse rumble strips. The synthesis concluded that transverse rumble strips may effectively reduce crashes, although more rigorous evaluation is needed to bet- ter understand the safety effects. (Harwood, 1993) However, a 2006 Texas Transportation Insti- tute (TTI) study found that transverse rumble strips produced marginal safety benefits at best, only slightly reducing erratic lane movements. The study did note a 46% reduction in shoulder encroachments. (Miles et al., 2005) 4.5 Longitudinal Rumble Strips 4.5.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. Longitudinal rumble strips provide audible and tactile warning to reduce comfortable speed and to minimize off-the-road or crossover crashes. These treatments can be applied to rural, undivided highways. Design variations are paved, rolled, or milled; raised or depressed; painted or unpainted; and cluster spacing. Noise, as well as adverse effects for motorcycles and bicycles, should be considered.
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40 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections 4.5.2 Applicability and Considerations Like transverse rumble strips, longitudinal rumble strips are raised or grooved patterns installed on the roadway travel lane or shoulder pavements to warn drivers of an upcoming need to act. Longitudinal rumble strips are placed parallel to the direction of travel and may be located in the centerline or along the shoulder. Refer to Section 4.4 for more information on rumble strips. Longitudinal rumble strips are most commonly used to reduce head-on, sideswipe, and run-off-road crashes along roadway segments. The treatment may be a useful speed manage- ment treatment for high-speed intersections that exhibit these crash patterns. Bicycle and motorcycle impacts should be considered as the treatment for intersection applications is designed. Continuous shoulder rumble strips are the most common type of longitudinal rumble strip. These are placed on the roadway shoulder to help prevent drivers from running off the road and are generally used along expressways, interstates, parkways, or two-lane rural roadways. (FHWA Safety, 2007) Continuous shoulder rumble strips are typically installed with breaks or gaps only at exits and entrances to ramps and at street intersections or major driveways. This allows driv- ers and bicyclists to maneuver near the intersections and driveways without having to cross over the rumble strips. Centerline rumble strips are placed on either side of the centerline (see Exhibit 4-8), located either along the width of the centerline pavement markings (see Exhibit 4-9), or extend slightly into the travel lane (see Exhibit 4-10). Centerline rumble strips are generally installed to reduce head-on and sideswipe crashes along undivided roadways. Their primary function is to use tac- tile and auditory stimulation to alert inattentive or drowsy drivers that their vehicles are encroaching on the opposing lane. Centerline rumble strips may also discourage drivers from cutting across the inside of horizontal curves. (Credit: MnDOT, 2004) Exhibit 4-8. Centerline rumble strips.
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Treatment Descriptions 41 (Credit: Torbic et al., 2004) Exhibit 4-9. Centerline rumble strips on the centerline pavement markings. (Credit: Torbic et al., 2004) Exhibit 4-10. Centerline rumble strips extending into the travel lane.