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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 4 - Treatment Descriptions." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

4.1 Overview This section provides a summary of the findings reported in published literature on potential speed reduction treatments. Some of the reported treatments originate from applications for roadway segments; others are specific intersection applications. Once the intersection charac- teristics described previously in Section 3 have been assessed, the user can apply this informa- tion to help determine which treatment(s) may be appropriate at a specific location. This section contains an overview of each treatment, as well as more detailed descriptions, summaries of applicability and pertinent considerations such as maintenance, discussions of potential layouts and designs, and summaries of the documented effectiveness of each treatment in reducing speeds and improving safety. The treatments discussed are • Dynamic warning signs, • Transverse pavement markings, • Transverse rumble strips, • Longitudinal rumble strips, • Wider longitudinal pavement markings, • Roundabouts, • Approach curvature, • Splitter islands, • Speed tables and plateaus, • Reduced lane width, • Visible shoulder treatments, and • Roadside design features. In some cases, more than one treatment may be appropriate for a given intersection. To apply a treatment, users select a target speed and determine the location upstream of the intersection where sufficient distance is provided for drivers to react to potential con- flicts related to the intersection when traveling at the target speed. A transition area is needed upstream of this target speed location to allow drivers to decelerate from their segment speed. The transition length and target speed location help determine an appropriate treat- ment layout. The information provided in this section should be considered in tandem with appropriate local engineering practices and professional judgment related to the site-specific situation. 29 S E C T I O N 4 Treatment Descriptions

4.2 Dynamic Warning Signs 4.2.1 Overview Two NCHRP Project 3-74 test sites, one in Washington and one in Texas, provided docu- mented applications for the high-speed intersection treatments discussed in this section. Dynamic warning signs can alert drivers to the need to reduce their speed and encourage earlier deceleration. These treatments can be applied in rural, non-signalized intersections in advance of abnormal roadway features. Design variations include speed activation, message, image, size, and location. Driver workload and power supply issues should be considered. 4.2.2 Applicability and Considerations Dynamic warning signs are placed along the side of the roadway prior to a location that requires reduced speed. The signs are activated by vehicles that exceed a predetermined speed (typically in excess of the posted speed limit) or by potential vehicle conflicts at the intersection. This type of sign is not intended to enforce the speed limit; rather it is assumed that drivers will reduce their speeds once brought to their attention. (Maze et al., 2000) Although most com- monly applied in work zones, these signs have potential to be applied at intersections. Dynamic warning signs have been used at curve approaches, in work zones, and in other locations that require reduced speeds. Dynamic warning signs have also been used to warn drivers of conflicting cross-traffic at inter- sections. Called collision countermeasure systems (CCS) in this application, they are used to reduce side-impact crashes at rural non-signalized intersections. On low-volume rural highways with a history of high intersection crash rates, dynamic warning signs of this type are a cost- effective alternative to a conventional traffic signal. (Hanscom, 2001) Dynamic warning sign systems typically combine a radar device or pavement loop detectors with a variable message sign. The system measures the speed of the approaching vehicle or detects a potential vehicle conflict. The system provides a message to drivers who are traveling at exces- sive speeds or whose movements are in potential conflict with another vehicle. Examples of the types of messages displayed on the variable message sign include Slow Down, XX mph Curve Ahead, Your Speed XX mph or Traffic Ahead. (Torbic et al., 2004; Hanscom, 2001) Some systems are designed to provide messages to only a certain type of vehicle, such as trucks. In these cases, the dynamic warning systems involve weigh-in-motion devices, loop detectors, and height detec- tors. (Torbic et al., 2004) One of the challenges for implementing dynamic warning signs is to determine the maximum safe speed above which the sign will be activated. Vehicle loads, suspension, vehicle size, tires, and variable weather conditions can affect this speed. (Torbic et al., 2004) The maximum safe speeds selected by both Washington and Texas for the NCHRP Project 3-74 research test sites were higher than the posted speed limit. Dynamic warning signs can vary in complexity; therefore, some systems can be installed in a very short period of time, while others may take years to implement. The radar and video equipment, loop detectors, and weigh-in-motion detectors can be expected to encounter the same maintenance issues as when placed at conventional signalized intersections or other locations. When dynamic warning sign equipment and posts are placed outside of pedestrian and bicycle zones, they are not expected to impact multimodal users. Exhibits 4-1, 4-2, and 4-3 show three different types of dynamic warning signs. 30 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

Treatment Descriptions 31 (Credit: WSDOT) Exhibit 4-1. Dynamic warning sign. 4.2.3 Treatment Layout/Design Dynamic warning signs are placed in a location that provides adequate advance warning time for drivers to reduce their speed appropriately. The placement of a dynamic warning sign should be determined from estimated perception–reaction time, deceleration, and the stopping sight distance in advance of the intersection. Sign placement will reflect the desired target speed loca- tion and the driver deceleration rates from the operating speed of the upstream segment. Vertical and horizontal alignments must provide a clear sight line for the radar or video equip- ment. A power source is also necessary. (Torbic et al., 2004) Constraints such as steep slopes, bridges, and power supplies also should be considered to determine the appropriate location for the sign. If a radar unit is included as part of the dynamic sign treatment, it must be placed to detect drivers’ speeds in advance of the sign. 4.2.4 Speed Effects At each NCHRP Project 3-74 test site in Washington and Texas, dynamic warning signs were tested at two approaches. Dynamic warning signs reduced speeds significantly at three of these four high-speed intersection approaches. Speed data were collected at three locations on each intersection approach. A mean speed reduction of 1.7 mph was observed at the sign locations

32 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (Photo: TXDOT) Exhibit 4-2. Dynamic curve-warning sign. (Credit: PennDOT) Exhibit 4-3. Dynamic warning sign used as a collision avoidance system.

after a three-month acclimation period. At the perception–reaction data collection locations, mean speeds were reduced by 2.3 mph. At the accident avoidance data collection locations, mean speeds were reduced by 2.8 mph. During a study of warning signs installed at two high-speed intersection sites in Texas, Ullman and Rose (2004) recorded an initial reduction of 3 to 4 mph at both sites, with one site sustain- ing the lowered speeds after the novelty effect of the sign diminished. Studies examining the effectiveness of speed displays at rural interstate work zones in South Dakota and Virginia found that the speed monitoring displays reduced mean vehicle speeds by 1.4 to 4 mph within the work zones and reduced the number of vehicles speeding through the work zone by 20%-40%. Maze et al. (2000) concluded that sign placement can impact the effec- tiveness of this treatment. A study of three dynamic curve warning systems installed on ramps in Virginia and Maryland showed that the three systems significantly influenced truck speeds—drivers reduced their speeds if they had exceeded the maximum safe speed. (Torbic et al., 2004) In the field evaluation of the CCS installed in Prince William County, Virginia, it was observed that the CCS resulted in lower intersection approach speeds and longer projected times to colli- sion. (Hanscom, 2001) 4.2.5 Safety Effects No documentation of the safety benefits of dynamic warning systems at intersection approaches was found. However, the study of dynamic curve warning systems in Maryland and Virginia identified that prior to installation there had been 10 truck rollover crashes and after three years in operation, no rollover crashes had been reported. 4.3 Transverse Pavement Markings 4.3.1 Overview Five NCHRP Project 3-74 test sites in Oregon provided documented applications for the high-speed intersection treatments discussed in this section. Transverse pavement markings improve visibility and driver attention. These treatments can be applied to provide visual cue reinforcements to changing conditions and the need to reduce speed. Design variations include peripheral or full transverse lines. Long-term effectiveness and driver familiarity should be considered. 4.3.2 Applicability and Considerations As defined by the MUTCD, transverse pavement markings are “pavement markings that are generally placed perpendicular to and across the flow of traffic.” (FHWA, 2003, p. 1A-14) Peripheral transverse lines, as shown in Exhibit 4-4, involve bars only at the edge of a travel lane, instead of bars extending across the travel lane. Transverse chevrons are painted geometric arrows that converge to give the illusion of speed. (Griffin and Reinhardt, 1995) Transverse pave- ment markings are commonly used in speed management to reinforce the need to reduce speed or to warn drivers of an approaching condition that may require vehicular maneuvers. Common applications of transverse pavement marking locations include approaches to traffic circles and intersections, horizontal curves, construction areas, bridges, and freeway off-ramps. (Griffin and Reinhardt, 1995) Transverse pavement markings have also been placed at locations Treatment Descriptions 33

where excessive speed was a contributor to crashes and other speed reduction treatments had not been effective. (Agent, 1980) Peripheral transverse lines require less maintenance than transverse lines that extend across the travel lane because most drivers do not track over the markings. (Human Factors North, Inc., 2002) Transverse pavement markings have no known impact to multimodal users—it is not expected that multimodal users would be adversely affected by transverse pavement markings. 4.3.3 Treatment Layout/Design Transverse pavement markings should be placed in a location that provides adequate advance warning time for drivers to reduce their speeds appropriately. The placement of transverse pave- ment markings that is optimal to reduce speeds on intersection approaches has not been quan- tified. The “Green Book” (AASHTO, 2004) values for deceleration may provide a starting point in locating transverse pavement markings. Transverse pavement lines may be installed in several small clusters on a high-speed intersec- tion approach. The number of lines, their spacing, and the distance from the intersection proper should be determined through a combination of a review of field conditions, driver sight lines and desired response, and local practice and judgment. As guidance, the MUTCD suggests, “. . . because of the low approach angle at which pavement markings are viewed, transverse lines should be proportioned to provide visibility equal to that of longitudinal lines.” (FHWA, 2003, p.1A-14) The appropriate location and length of the lines depends on the speed at which the driver is traveling, the deceleration rate, and the driver’s perception–reaction time. (Agent, 1980) Appropriate installation points may be selected to reinforce other new or existing treatments or features on the intersection approach such as warning signs, shoulder markings, parking space markings, and pavement legends (SLOW DOWN, REDUCE SPEED, etc.), and at the intersec- tion proper (such as lane-use legends, stop lines, crosswalk lines, and others). The transverse pavement markings have the potential to draw additional attention to those signs and markings and to encourage drivers to reduce their speeds as they approach the intersection. Other appro- priate locations for treatment installation may include the stopping sight distance for the approach speed, or a point where the roadway environment changes, such as at the point of 34 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Exhibit 4-4. Peripheral transverse pavement markings.

tangency or at a driveway. Appendix B describes and illustrates the treatment layout for one of the NCHRP Project 3-74 research test sites. The spacing of the transverse bars and transverse chevrons may reflect the desired travel speeds of the vehicles on the roadway. Some applications of transverse bars have used a rate of advance- ment of two stripes per second. In some applications transverse bars are spaced progressively closer together at an increasing rate as the driver travels along the roadway. When applied in this way, the bars are referred to as optical speed bars. The intent is that the reduced spacing gives the driver the perception of acceleration, causing the driver to slow down; however, there are no data to support this claim. (Agent, 1980) 4.3.4 Speed Effects After a 90-day acclimation period, transverse pavement markings were found to reduce speed marginally at four high-speed intersection approaches tested through NCHRP Project 3-74. Overall, the markings reduced mean speeds by 0.6 mph (standard error of 0.3 mph). Addition- ally, at the perception–response time data collection locations, transverse pavement markings were found to reduce mean speeds by 0.9 mph (standard error of 0.4 mph). Studies of segment applications of transverse pavement markings have reported reduced mean and 85th-percentile speeds on the order of 20%-30%. (Katz et al., 2003; Griffin and Reinhardt, 1995) At the Newbridge Roundabout in Scotland, transverse pavement markings resulted in reduced mean and 85th-percentile speeds. The overall mean speed throughout the day was reduced by approximately 23%, and the overall 85th-percentile speed throughout the day was reduced by approximately 30%. (Katz et al., 2003) In a study conducted on US Highway 60 in Meade County, Kentucky, transverse bar pave- ment markings were placed prior to a sharp curve with a high crash rate. This study revealed that the treatment became less effective as a speed reduction technique as drivers became familiar with the treatment. Furthermore, the long-term effects during the nighttime were less than the long-term effects during the day. (Agent, 1980) Research by Godley et al. (2000) using a driving simulator found that transverse lines with both constant and reducing spacing (i.e., optical speed bars) lowered speeds. The research deter- mined that speed perception was not influenced by the decreased spacing of the lines. Addi- tionally, Godley found that the peripheral transverse lines induced speed reduction almost as effectively as full transverse lines. No studies have been found that evaluate the effectiveness of the transverse chevron markings with respect to speed. 4.3.5 Safety Effects No published data were found to address the effects of transverse pavement markings on safety at conventional intersections. Safety improvements associated with segment or roundabout applications of transverse pavement markings were reported by each of the studies referenced below. In 1993, a study in Osaka, Japan, reported that converging chevron pavement markings on the Yodogawa Bridge were more effective than conventional signing in helping to prevent crashes at a high-crash location. No crashes resulting in injuries occurred in the two years after the chevron markings were installed. In the past, the bridge had a history of crashes causing injuries and fatalities. (Griffin and Reinhardt, 1995) Treatment Descriptions 35

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 36 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

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. Treatment Descriptions 37 (Credit: TTI, 2006) Exhibit 4-5. Transverse rumble strips in wheel path. Exhibit 4-6. Transverse rumble strips.

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 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.

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 perception–response 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. Treatment Descriptions 39

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. 40 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (Credit: MnDOT, 2004) Exhibit 4-8. Centerline rumble strips.

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.

4.5.3 Treatment Layout/Design The placement and dimensions of longitudinal rumble strips on high-speed intersection approaches should be determined through a combined review of field conditions, driver sight lines and desired response, and local practice and judgment. The distance that the treatment extends from the intersection proper should be related to stopping sight distance and/or the distance required to achieve the desired deceleration at a com- fortable deceleration rate. The distance should also be selected to work in concert with other treatments or features, such as warning signs. The markings have the potential to draw additional attention to those warning signs and to encourage drivers to reduce their speeds as they approach the intersection. The dimensions of the rumble strips will vary by application. If the treatment is being used to nar- row the functional width of the approach lane, the treatment should extend beyond the existing striping. Consideration of truck traffic is likely to constrain the amount of narrowing that may occur. Treatment design considerations related to the types of rumble strips (raised, milled, etc.) were discussed previously in Section 4.4.3. 4.5.4 Speed Effects No documentation was found regarding the speed effects of longitudinal rumble strip appli- cations in segment or intersection locations. However, it is expected that longitudinal rumble strips have the greatest potential to reduce speeds when they are used to narrow the functional width of the roadway. FHWA is conducting research into these applications. However, this study applied multiple treatment types at a given location and, therefore, the direct effects of the rum- ble strips are not documented. At non-intersection locations on two-lane highways, one study found that the presence of cen- terline rumble strips had no effect for sites with 11- and 12-ft lanes. (Porter et al., 2004) 4.5.5 Safety Effects Safety improvements have been documented at locations where continuous or shoulder rum- ble strips were installed along roadway segments However, there has been no documentation to show the results of this specific application at intersection approaches. Shoulder rumble strips have proved to be a very inexpensive and effective treatment to reduce run-off-road crashes. Reports from Maine, New York, and California have reported run- off-road crash reductions of 20%-72% after continuous shoulder rumble strips were installed. (Corkle et al., 2001) Similarly, centerline rumble strips have been shown to improve safety along undivided highways. A study analyzing the safety effectiveness of centerline rumble strips along approximately 210 miles of treated roads in seven states found a 14% reduction for all injury crashes combined, as well as a 25% reduction for frontal and opposing-direction sideswipe injury crashes. (Persaud et al., 2004) 4.6 Wider Longitudinal Pavement Markings 4.6.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. Wider longitudinal pavement markings increase intersection visibility 42 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

and attract driver attention to the intersection ahead. These treatments can be applied to address the needs of older drivers and a lack of driver expectancy. Design variations include marking width, length, and reflectivity. A secondary effect of these treatments is that speed may increase with increased visibility. 4.6.2 Applicability and Considerations Many transportation departments increase the width of pavement markings to improve the visibility of centerline, lane line, and edge-line striping, which can provide added guidance to drivers from greater distances. (Gates and Hawkins, 2002) Although no documentation for wider longitudinal pavement marking applications at intersection approaches was found, this treatment may be an effective speed reduction treatment for some high-speed intersections because it may increase driver awareness of the presence of an intersection and help reinforce the need for drivers to operate differently at the intersection than in the roadway segment. The most common reasons that jurisdictions apply wider longitudinal pavement markings are to improve visibility, assist older drivers, and reduce crashes. Many transportation depart- ments have policies to apply wider longitudinal pavement markings to routes of a certain road- way classification—most commonly access-controlled highways. Some jurisdictions have policies that require using wider edge lines on all state routes while others install this treatment exclusively at hazardous locations that could benefit from greater driver visibility. (Gates and Hawkins, 2002) Wider longitudinal pavement markings (see Exhibit 4-11) assist peripheral vision by improv- ing the peripheral signal, thus decreasing driver workload and increasing driver comfort and performance. The largest visibility benefit is seen with older drivers, because visual and cogni- tive capabilities decline with age. (Gates and Hawkins, 2002) The higher cost of wider longitudinal pavement markings may be somewhat offset by greater durability. Pavement lighting systems are an alternative to wider longitudinal pavement markings that serve the same function, and may have similar effects. This treatment does require a power source that may require maintenance or replacement over time. 4.6.3 Treatment Layout/Design The placement and dimensions of wider longitudinal pavement markings on high-speed intersection approaches should be determined through a review of field conditions, driver sight lines and desired response, and local practice and judgment. Treatment Descriptions 43 Exhibit 4-11. Wider longitudinal pavement markings in France.

The distance that the treatment extends from the intersection proper should be related to stopping sight distance and/or distance to achieve the desired deceleration at a comfortable deceleration rate. The distance should also be selected to work in concert with other treatments or features such as in-pavement markings at pedestrian crosswalks, lane drops or adds, or warn- ing 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 intersection. The standard width for longitudinal pavement markings is four inches. Wider pavement markings generally range from 5 to 10 inches wide. 4.6.4 Speed Effects No specific information was found to describe the impacts of wider longitudinal pavement markings on speed at intersection approaches or roadway segments. The increased visibility and comfort associated with wider pavement markings could lead to increased speed in some appli- cations. Although this treatment may not directly affect reduced speeds, it may increase driver awareness of an impending intersection, thereby indirectly reducing speeds if drivers perceive a greater risk. FHWA is conducting demonstration projects in Alaska and Tennessee to evaluate the impacts and effectiveness of increasing the width of pavement marking edge lines from four inches to six inches; a report is due in June of 2009. 4.6.5 Safety Effects There is no conclusive data associated with the crash-reduction effects of wider longitudinal pavement markings. Cottrell (1988) conducted a before-and-after evaluation of wider pavement edge lines on rural two-lane highways in Virginia and found no evidence to indicate any safety benefit. Conclusive evidence suggests wider longitudinal pavement markings are easier for drivers to see, which can contribute to roadway safety. (Gates et al., 2002) The greatest improvement in visibility is achieved at night. 4.7 Roundabouts 4.7.1 Overview The high-speed intersection treatments discussed in this section are widely used throughout the United States, United Kingdom, France, and Australia. Roundabouts use intersection geometry to reduce speeds and conflict points. These treatments can be applied to a variety of applications. Design variations include roundabout type, size, number of lanes, and geometry. Secondary effects and considerations should include operations, right-of-way, access, horizontal and vertical geom- etry, and driver expectancy. 4.7.2 Applicability and Considerations A roundabout is a type of circular intersection with specific design and traffic control features to ensure that travel speeds on the circulatory roadway are typically less than 30 mph (50 km/h). These features include channelized approaches and geometric curvature. (FHWA, 2000b) The following qualities distinguish a roundabout from other circular intersections: 44 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

• Yield control is provided on all entries, • Right-of-way is given to circulating vehicles, • Pedestrians can cross only the roundabout approaches (behind the yield line), • Parking is not allowed within the circulatory roadway or at the entries, and • Vehicles circulate in a counter-clockwise direction. Roundabouts have been designed to improve some of the safety and operational deficiencies that occur in other types of circular intersections, such as traffic circles and rotaries. The specific design features of a roundabout can reduce speeds and the number and severity of collisions. Roundabouts are appropriate for locations with a high crash frequency or severity, intersec- tions where queues need to be minimized, intersections with irregular geometry, intersections that need to accommodate U-turns, and areas with a large amount of right-of-way available. Pedestrians are accommodated at a roundabout by crossings through splitter islands located around the perimeter of the roundabout. Roundabouts potentially are difficult for visually impaired pedestrians to navigate. NCHRP Project 3-78, Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities, provides additional information on the impacts roundabouts have on visually impaired pedestrians. At single-lane roundabouts (see Exhibit 4-12), bicyclists have the option to mix with traffic or use the pedestrian crossings. At multilane roundabouts, bicycle paths should be separate and designated from the circulatory roadway, for example, by providing a shared bicycle-pedestrian path. Landscaping placed on the center island of a roundabout may need to be maintained to keep the intersection aesthetically pleasing and to manage intersection sight distance. A maintenance program should be developed for the landscape design of a roundabout. (FHWA, 2000b) 4.7.3 Treatment Layout/Design Roundabouts can be adapted to a variety of user and vehicle types based on the environment, number of lanes, and space used by the intersection. Identifying the proper dimensions of the Treatment Descriptions 45 (Credit: Oregon DOT) Exhibit 4-12. Single-lane roundabout.

key design features—including an appropriate design vehicle—is critical to ensure proper oper- ations and safety for all users at roundabouts (FHWA, 2000b). To distinguish roundabouts from other types of circular intersections, key design features such as a central island, splitter islands, circulatory roadway, yield lines, pedestrian crossings and, in some cases, an apron, have been defined and are shown in Exhibit 4-13. A mountable apron may be designed around the perimeter of the central island to provide additional width required for tracking through a single-lane roundabout. Large vehicles may use the entire circulatory roadway at double-lane roundabouts to track and maneuver. (FHWA, 2000b) Single-lane, four-leg roundabouts have a typical daily service volume of approximately 20,000 entering vehicles per day. Two-lane, four-leg roundabouts have a typical daily service volume of approximately 40,000 entering vehicles per day. (FHWA, 2000b) 4.7.4 Speed Effects Roundabouts have the potential to lower speeds to allow drivers more time to react to poten- tial conflicts. Traffic measured at 43 locations revealed that the geometry yields 85th-percentile entry speeds between 13 and 17 mph. (Rodegerdts et al., 2007) 4.7.5 Safety Effects One of the most significant benefits of roundabout installation is the overall improvement to intersection safety. Crash reduction research for conversions of all types of intersections to roundabouts (55 sites studied) found a 35% reduction in all crashes and 76% reduction in injury crashes. Specifically for rural two-way stop-controlled intersections that have been converted to roundabouts (10 sites studied), research found a 72% reduction in all crashes and an 87% 46 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (Credit: FHWA, 2000b) Exhibit 4-13 . Key roundabout design features.

reduction in injury crashes. (Rodegerdts et al., 2007) The crash reduction is achieved because there are fewer conflict points at roundabouts than at conventional intersections and because speeds are reduced. (FHWA, 2000b) Single-lane and multilane roundabouts have different operational characteristics that affect their relative safety benefits. Although the number of crashes and the severity of injuries were reduced at both single-lane and multilane roundabouts, a smaller reduction in total crashes was observed at the multilane roundabouts. Both single-lane and multilane roundabouts exhibit sim- ilar reductions in injury crashes. (Rodegerdts et al., 2007) Roundabouts improve safety for bicyclists by slowing automobiles to speeds closer to bicycle speeds (thus reducing high-speed conflicts) and by simplifying turning movements for bicycles. (FHWA, 2000b) Crashes involving bicycles comprise approximately 1% of all reported crashes at roundabouts. (Rodegerdts et al., 2007) Roundabouts also decrease the risk of severe pedestrian collisions, due to reduced vehicle speeds and a reduced number of conflict points for pedestrians. A Dutch study conducted at 181 roundabouts found a 73% reduction in all pedestrian crashes and an 89% reduction in injury crashes for pedestrians. (FHWA, 2000b) Crashes involving pedestrians comprise approximately 1% of all reported crashes at roundabouts. (Rodegerdts et al., 2007) Bicycles do not experience the same safety benefit at roundabouts as vehicles and pedestrians. In some cases, roundabouts can be less safe for bicycles than conventional intersections, despite the speed reduction benefits. (FHWA, 2000b) 4.8 Approach Curvature 4.8.1 Overview No test sites provided documented applications for the high-speed intersection treatments dis- cussed in this section. The high-speed intersection treatments discussed in this section have been applied at roundabout approaches only. These treatment applications introduce horizontal curve geometry to induce lower speeds. Approach curvature can be applied to new roundabouts and may be especially effective on rural highways. Designs vary with curve geometry. Secondary effects and considerations should include right of way, grading, driver workload, and truck movements. 4.8.2 Applicability and Considerations Approach curvature is a geometric design treatment that can be used at high-speed intersection approaches to force a reduction in vehicle speed though the introduction of horizontal deflection. As shown in Exhibit 4-14, approach curvature consists of successive curves with progres- sively smaller radii. Research and applications of approach curvature have focused on Treatment Descriptions 47 (Credit: FHWA, 2000b) Exhibit 4-14. Approach curvature.

roundabouts. However, this geometric design treatment has potential to be applied to con- ventional intersections. Appropriately designed curve radii and length of curve can reduce vehicle speeds at the entry of a roundabout or conventional intersection, as well as reduce certain types of crashes. If successive curves are installed in high-speed environments, introducing other speed reduction treatments prior to the approach curvature may reduce the speed changes needed between successive curves. (Arndt, 2000) The use of approach curvature at downhill approaches is discouraged. Experience with approach curvature suggests that this geometric treatment should be used in conjunction with reduced speed limit signs or advisory speed signs. Sight distance should also be considered at the intersection approaches to ensure appropriate visibility of pedestrians and bicycles crossing the intersection approach or traveling through the intersection. Excessive sight distance should be discouraged to focus driver attention to the immediate roadway. Raised medians to reinforce speed reduction may become problematic for maintenance, espe- cially in areas subject to snow plowing. 4.8.3 Treatment Layout/Design The length and curve geometry should be determined from the upstream segment operating speed and the target speed at the intersection. Design control information from the “Green Book” (AASHTO, 2004), as well as published documentation from Australian applications at roundabouts (FHWA, 2000b), may provide design guidance. It may be appropriate to introduce the curved geometry concurrently with additional treatments such as warning signs. The dominant feature of approach curvature treatments is the radius of the curve. When designing successive curves, each curve must be visible before the next curve to allow drivers adequate time to adjust speeds. The length of the approach curvature affects speed reduction. Adequate curve length must be provided to discourage drivers from traveling into adjacent lanes; however, excessive curve lengths can result in an increase of single-vehicle crashes. (Arndt, 2000) Super-elevation and side friction also affect vehicles traveling through the cur- vature. To provide required super-elevation on each curve, short tangent sections can be designed between each successive curve. Similar to roundabout design, approach curvature should be designed with an appropriate “design vehicle” to ensure that all trucks, buses, and emergency vehicles may traverse the approach without encroaching on the shoulder or adjacent sidewalks. 4.8.4 Speed Effects Current information about approach curvature and how it impacts speed and safety relates exclusively to roundabouts. Although no published research or testing results have been found that describe the impacts on speed and safety at conventional intersection approaches, similar safety benefits might be realized by applying this treatment to conventional intersections. The geometric design of roundabouts shows that the curve radius directly impacts vehicle speeds. As the radius of the curve decreases, a larger reduction in vehicle speed is required to negotiate the curve. Exhibit 4-15 shows the speed–radius relationship for curves with +0.02 and −0.02 super-elevations. 48 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

4.8.5 Safety Effects Although the majority of research and testing includes approach curvature at roundabouts, similar safety benefits might be realized by applying this type of geometric treatment to conven- tional intersections. Approach curvature that includes successive curves reduces crashes at a roundabout and min- imizes single-vehicle crashes. A Queensland, Australia, study found that reducing the change in 85th-percentile speed on successive curves to 12 mph reduced single-vehicle and sideswipe crashes. Although decreasing the curve radius on an approach can generally minimize rear-end crashes, it may also increase the potential for single-vehicle crashes. (FHWA, 2000b) 4.9 Splitter Islands 4.9.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. However, the treatments in this section have been applied at round- about intersection approaches and conventional intersection approaches in New Zealand and France. Splitter islands slow, direct, and separate conflicting traffic. These treatments can be applied to stop- or yield-controlled approaches. Design variations include length, geometry, and landscaping. Secondary effects and considerations should recognize that splitter islands can pro- vide refuge for pedestrians crossing at the intersection. 4.9.2 Applicability and Considerations A splitter island is a raised or painted area on an intersection approach used to separate enter- ing and exiting traffic, to deflect and slow entering traffic, and to provide refuge for pedestrians crossing the road in two stages. (FHWA, 2000b) These islands can increase driver awareness by channeling the traffic on the minor approaches. In some cases, these treatments are called throat or fishtail islands; these are shown in Exhibit 4-16. The geometry of fishtail islands introduces deflection at the approach, thereby reducing vehicle speeds at the intersection. Treatment Descriptions 49 (Credit: FHWA, 2000b) Exhibit 4-15. Speed–radius relationship.

Although splitter islands have generally been used at roundabout approaches, this treatment has been applied to the minor approaches of T-intersections and two-way stop-controlled inter- sections in France and New Zealand. Exhibit 4-17 depicts a splitter island at a T-intersection in France. Similar to a splitter island at a roundabout, splitter islands at conventional intersections can benefit pedestrians crossing at the intersections. The island shortens the crossing distance, cre- ates a refuge area, and allows pedestrians to cross the approach in two stages, if necessary. FHWA has sponsored a study to determine the effectiveness of combining splitter islands on minor approaches with other types of speed reduction treatments, such as centerline rumble strips or pavement markings, on major approaches at two-way stop-controlled intersections on rural highways. 4.9.3 Treatment Layout/Design The length and geometry of splitter islands vary significantly. Splitter islands can either take the form of a cross-section change, including either visually or physically narrowing the travel way, or create horizontal deflection. In Exhibit 4-18, Splitter Island A creates a cross-section change without directly creating horizontal deflection. Splitter Island B conceptually creates hor- izontal deflection. Form A could present a visual cue to drivers to slow before the intersection. Form B could potentially reduce speed by creating a deflected path, similar to a roundabout through movement. 50 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Exhibit 4-16. Throat and fishtail islands. Exhibit 4-17. Splitter island at a T-intersection in France.

Exhibits 4-19 and 4-20 depict schematic layouts of signing and striping for teardrop splitter islands. The oval shape of the island aids in reducing vehicle approach speeds and increases inter- section visibility. This splitter island can be mountable to allow larger vehicles to maneuver through the inter- section. U.K. standards recommend that no obstacles, such as signs or signals, be placed on the splitter island; that the color of the median be uniform (no striping) but different than the pavement color to offer increased day and night visibility; and that the splitter islands be con- structed of a mineral treatment rather than turf or grassy compositions. (Ministry of Equip- ment, 1998) In U.S. applications, a sign is usually mounted on the splitter island to increase its visibility. 4.9.4 Speed Effects Much of the information about splitter islands and how they impact safety and speed relates specifically to roundabouts. There is little published research or test results to describe the impacts of approach splitter islands on speed or safety at conventional intersection approaches. Splitter islands create deflection at roundabouts, which reduces the speeds of vehicles travel- ing through the intersection. 4.9.5 Safety Effects The limited literature that is available on splitter islands shows that there is a safety benefit to applying this treatment to conventional intersections. However, there is no information to indi- cate to what extent the safety effects of splitter islands are due to speed reduction and to what extent they are due to the separation between traffic moving in opposite directions. A study of 134 intersections in New Zealand found that installing throat or fishtail islands resulted in crash reductions of 30%-60% for fatal, injury, and pedestrian crashes during both Treatment Descriptions 51 Exhibit 4-18. Splitter islands. Exhibit 4-19. Example layout of a splitter island at a T-intersection.

day and night. The majority of the reduction in injury crashes involved vehicles crossing paths at the intersection. Most of the treated intersections were four-legged intersections in urban areas on local roads. (LTSA, 2001; FHWA, 2006) Implementations in France and New Zealand resulted in a total crash reduction of 30% and a reduction of angle and crossing crashes by 30%. (FHWA, 2006) Although no evidence was found to support this claim, it is possible that installing these types of islands at intersections could increase collisions with obstructions. (LTSA, 2001) 4.10 Speed Tables and Plateaus 4.10.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. These applications create vertical deflection and alert drivers to the need to slow down. These treatments can be applied to stop-controlled approaches. Design variations include geometry and materials. Secondary effects and considerations should include snow removal and driver expectancy. 4.10.2 Applicability and Considerations Speed tables are speed humps with a flat section on top, allowing the entire wheelbase of a pas- senger car to rest on the raised, flat section. The flat section of the speed table can be aesthetically treated (see Exhibits 4-21 and 4-22) with decorative surface material or constructed with brick or other textured materials. Speed tables have gently sloped ramps on both ends, which allow slightly higher vehicle speeds and a smoother transition than speed humps. (ITE, 2007) A plateau is an alternative speed table design that has been used in the Netherlands and installed on facilities with speeds ranging from 60 to 80 km/h (35 to 50 mph). In applications of this treatment, plateaus are generally installed on all intersection approaches and the minor approaches are yield-controlled. Advance warning devices are also recommended on all approaches. (Schermers and van Vliet, 2001) Although speed tables are generally applied on low-speed facilities in the United States, they may have applications on approaches to high-speed intersections where low speeds are desired. 52 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Exhibit 4-20. Example layout of a splitter island at a four-legged intersection.

Speed tables can provide benefits for multimodal users when user needs are considered in design and application. Speed tables that are used at crosswalks can eliminate the need for curb ramps, increase pedestrian visibility, and provide more sidewalk area for multimodal users. Detectable warnings are recommended for speed tables used at crosswalks to accommodate visu- ally impaired users. (ITE, 2007) When designed with a gentle rise, speed tables have little impact on bicyclists. (Cochituate Rail Trail, 2007) Vertical deflection devices such as speed tables are sometimes prohibited on emergency response routes because of the friction they create. Snow and ice removal may require special attention when plowing roadways. Speed tables should be constructed downstream of storm sewer inlets and should have a tapered edge along the curb line to allow for drainage. (ITE, 1997) Additional or new street lighting may be necessary to improve the visibility of speed tables. 4.10.3 Treatment Layout/Design Speed tables and plateaus should be placed at a location where vehicles will not abruptly encounter them at a high speed. A report from the Institute of Transportation Engineers (ITE, 1993) suggests that the first speed table in a series be placed no more than 200 ft from a stop sign or horizontal curve, and not within 250 ft of a traffic signal. Both horizontal and vertical sight distance should be considered to determine the placement of a speed table. Many jurisdictions have standard placement guidelines for the location of speed tables. (Hallmark et al., 2002) Treatment Descriptions 53 (Credit: Fehr & Peers, 2007) Exhibit 4-21. Speed table combined with textured pavement in Naples, FL. (Credit: Fehr & Peers, 2007) Exhibit 4-22. Speed table combined with striping and colored concrete in Charlotte, NC.

The most distinguishing design features of a speed table are the flat section in the middle of the table, the ramps on each end, and the aesthetic treatments often applied to the top. The entire speed table is typically 22 ft long in the direction of travel, with a 10-ft flat section in the middle and 6-ft ramps on both ends. The vertical deflection of a speed table typically ranges from three to four inches, but has been designed for a height of six inches in some cases. The ramps on each side of the speed table typically have a parabolic or linear shape. (ITE, 2007) Plateaus should be located approximately 50 to 100 m (165 to 325 ft) from the intersection and designed for a speed of 40 km/h (25 mph). (Schermers and van Vliet, 2001) 4.10.4 Speed Effects Much of the information about the effectiveness of speed tables relates to roadway segments with operating speeds less than 45 mph. No information was found that describes the affect speed tables have on speeds at intersections on high-speed facilities. Most research has shown that speeds were reduced after speed tables were installed. A study in Gwinnett County, Georgia, in which 43 speed tables were installed, found that speeds were reduced by an average of 9 mph. (Bretherton, 2003) Another study reported an average 18% decrease in the 85th-percentile travel speeds for a sample of 58 test sites. (Fehr & Peers, 2007) However, in some cases speed tables may be too gentle to mitigate excessive speed. A test site in Fort Lauderdale, Florida, found no speed reduction. (ITE, 2007) When designed to the typical dimensions, speed tables have an 85th-percentile speed of 25 to 30 mph. (ITE, 1999) Although the speed reduction potential of speed tables is not well known, the ITE (1999) study noted above suggests that the provision of speed tables results in an 85th-percentile speed of 25 to 30 mph. Thus, if a speed table is used at an intersection, the speed reduction on the intersection approach may be from the 85th-percentile mid-block speed to 25 to 30 mph. It is expected that most intersections where speed tables have been used have moderate mid-block or approach speeds. Speed tables located at intersections and mid-block locations have been shown to reduce speeds and collisions as well as to lower traffic volumes. 4.10.5 Safety Effects Speed tables located at intersections and mid-block locations have been shown to reduce speeds and collisions as well as lower traffic volumes. Some studies have reported that collisions have been reduced by an average of 45% on streets treated with speed tables at particular test sites. (ITE, 2007; Fehr & Peers, 2007) Speed tables have potential to increase safety on the treated road by lowering volumes and reduc- ing the probability of a crash. The study conducted in Gwinnett County showed a 7% reduction in traffic volumes based on 24-hour traffic counts collected on 18 streets before and after speed tables were installed. (Bretherton, 2003) ITE reported a 12% reduction in traffic volumes on roadways with speed table applications, depending on the availability of alternative routes. (ITE, 2007) 4.11 Reduced Lane Width 4.11.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. Reduced lane widths heighten driver attention by narrowing the avail- able lane width. These treatments can be applied on intersection approaches. Design variations 54 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

can involve re-striping only or narrowing the paved section. Secondary effects and considera- tions should include using with caution when heavy truck traffic or multilane facilities are pres- ent and where curvilinear alignments exist. 4.11.2 Applicability and Considerations Reduced lane width could come in two basic forms: lanes narrowed with paint (see Exhibit 4-23) or a physically reduced roadbed width (see Exhibit 4-24). Lane narrowing benefits using paint may be negated by the relative open field of vision. Narrow roadbeds physically constrain the cross sec- tion but may have secondary impacts. Lane widths that are considered “reduced” tend to range from 9 to 12 ft. Lane-width reductions have been used in both temporary and permanent applications. Per- manent lane width reductions that reduce the overall pavement width are expected to have the greatest potential to reduce speeds because both the pavement width and the striping contribute to drivers’ perceptions of the roadway environment. Lane-width reductions have been docu- mented for work zone applications and low-speed urban and residential locations. Lane widths on multilane facilities, roadways with curvilinear alignments, and facilities with high heavy truck or transit vehicle use should be reduced with caution, particularly if the width of the paved section will be reduced. Reducing lane widths can negatively impact bicyclists if bicycle lanes or wide curb lanes are not maintained, but it can also improve bicyclist conditions if the additional space is used to provide or widen bicycle lanes. Reducing lane width can pro- vide positive effects such as space for other roadway features (i.e., medians and curbside park- ing), space for roadside features (i.e., sidewalks and clear zones), and reduced interference with existing roadside development. (Zegeer et al., 2002) Treatment Descriptions 55 Exhibit 4-23. Reduced lane width—painted. Exhibit 4-24. Reduced lane width—roadbed.

Reduced lane widths improve pedestrian conditions at intersections by reducing crossing distances and exposure time. Excessive crossing distances increase pedestrian exposure time, heighten the potential for vehicle–pedestrian conflicts, and may add to vehicle delay. At signal- ized intersection approaches, reducing lane widths and thereby the pedestrian crossing distance provides more flexibility for the intersection’s signal timing. 4.11.3 Treatment Layout/Design NCHRP Project 3-72, “Lane Widths, Channelized Right Turns, and Right-Turn Deceleration Lanes in Urban and Suburban Areas,” evaluated the operational, speed, and safety effects of reduced lane widths on roadway segments and intersections. The research findings indicate that reducing lane widths to less than 10 ft may not be advisable on four-lane undivided arterials or on the approaches to four-leg stop-controlled intersections. Additionally, it suggests that it may not be advisable to reduce lane widths to less than 9 ft on four-lane divided arterials. These find- ings are consistent with the “Green Book” (AASHTO, 2004), which recommends lane widths of 10 to 12 ft on urban and suburban arterials. Redesigned lanes can be reduced using concrete barriers (for short-term solutions), curbs, or standard striping. The effects of reducing the width of pavement on turning radii at inter- sections should be evaluated to ensure that adequate radii are available to accommodate the design vehicle. 4.11.4 Speed Effects Research related to the speed, safety, and operational effects of reduced lane widths provides inconsistent results, indicating that the relationships are complex and difficult to evaluate with- out considering other elements of the intersection or roadway environment. Research and analysis conducted for NCHRP Project 3-72, which included both high-speed and low-speed facilities, made the following conclusions regarding the speed effects of reduced lane widths: • Reduces mid-block speeds on four-lane arterials (average lane-width reduction of 2.7 ft asso- ciated with average speed reduction of 4 mph), • Reduces driver comfort on higher-speed facilities, • May decrease capacity due to reduced saturation flow rates, although the calculated reduc- tions were about half of the reductions suggested in the Highway Capacity Manual (HCM), and • May increase capacity at signalized intersections due to decreases in pedestrian crossing times. Research that evaluates how effectively reduced lane widths reduce speeds in work zones found that the treatment was most effective on two-lane rural roads and found that a 7% reduc- tion in speed was achieved by reducing lane widths to 11.5 and to 12.5 ft. (Benekohal et al., 1992) In low-speed environments, research has concluded that speed is not consistently related to lane width. (Gattis, 1999) It is also likely that wider lanes induce faster travel speeds that may increase crash risk and increase crash severity. Earlier editions of the HCM suggested that wider lanes on multilane highways also increase capacity and, therefore, reduce following distances. (TRB, 1985) How- ever, HCM editions since 1985 have indicated that wider lanes of up to 12 ft on multilane highways increase free-flow speeds but do not increase capacity and, therefore, do not reduce vehicle headways. (TRB, 1994; TRB, 2000) 56 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

4.11.5 Safety Effects The documented safety effects of reduced lane widths generally address segment applications, although some intersection approach data may be included. Despite inconsistent data, concern remains that narrower lanes may be accompanied by reduced safety. The two principal aspects to the potential link between lane width and safety are as follows: • Wider lanes increase the average separation between vehicles moving into adjacent lanes and may provide a wider buffer to accommodate small, random deviations from drivers’ intended paths; and • Wider lanes may provide more room for correction maneuvers by drivers in near-crash cir- cumstances. (Hauer, 2000) Research and analysis conducted for NCHRP Project 3-72 found no decrease in safety at mid- block locations or intersection approaches with narrow lanes except in the following limited cases: • Four-lane undivided arterials with lane widths less than 10 ft, • Four-lane divided arterials with lane widths less than 9 ft, and • Four-leg stop-controlled intersections with lane widths less than 10 ft. Research by Harwood et al. (2000) developed accident modification factors (AMFs) for crash types related to cross-section elements (including opposite direction, and single-vehicle run-off- road) for two-lane highways. The AMFs range from 1.05 to 1.5 for 9- to 11-ft lanes compared to 12-ft lanes, indicating that two-lane facilities with narrower lanes would be expected to experi- ence higher crash rates for certain crash types. (Harwood et al., 2000) The AMF of 1.50 for 9-ft lanes implies that a roadway with 9-ft lanes would be expected to experience 1.5 times as many crashes as a roadway with 12-ft lanes. However, some studies indicate that lanes wider than 12 ft may also be associated with higher crash frequencies. 4.12 Visible Shoulder Treatments 4.12.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. Visible shoulder treatments heighten driver attention by narrowing the visual width of the roadway. These treatments can be applied at transition areas, road- ways with existing shoulders, and roadways in rural areas with no sight line limitations. There are many design variations. Secondary effects and considerations should recognize that bicy- clists using the shoulder may be impacted by these treatments, depending on the materials used. 4.12.2 Applicability and Considerations Visible shoulder treatments are used to change the appearance of the paved area to either aes- thetically blend roadway facilities into the surrounding landscape or to create a contrast between the shoulder and travel lane to heighten drivers’ awareness of the roadway. Visible shoulder treatments can consist of shoulder pavement markings, pavement coloring, shoulder composition, and shoulder rumble strips. If introduced in advance of the intersection, these treatments may be used to alert drivers of the change from roadway segment to the intersection influence area. Exhibit 4-25 shows an example of a visible shoulder treatment using brick. Treatment Descriptions 57

The applicability of visible shoulder treatments depends on the presence or absence of existing shoulders and the ability to acquire right of way, if needed. Generally, visible shoulder treatments are most cost efficient for intersection approaches with existing shoulders. Shoulder coloring can have an effect on the thawing and freezing characteristics of the shoulder pavement. Black shoulders retain heat longer while light-colored shoulders have the potential to freeze more easily. Though not documented, colored treatments could fade or dete- riorate because of snow removal. (Straub et al., 1969) Consideration should be given to the effects of textured shoulder treatments on bicyclists. 4.12.3 Treatment Layout/Design Shoulder dimensions generally are presented in the roadway design typical sections. In cases where the shoulder exists or is planned, the special treatment area might be limited to the vicinity of the intersection influence area to help differentiate between the roadway segment and the impending intersection. 4.12.4 Speed Effects No documentation was found that describes the effect visible shoulder treatments have on speed or safety at high-speed intersection approaches. No documentation was found that describes multimodal impacts. 4.12.5 Safety Effects No documentation has been found that describes the safety benefits of visible shoulder treat- ment applications at high-speed intersection approaches. 4.13 Roadside Design Features 4.13.1 Overview No test sites provided documented applications for the high-speed intersection treatments dis- cussed in this section. Roadside design features reinforce the transitioning environment, draw 58 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (Credit: Dan Burden) Exhibit 4-25. Example of a shoulder composition application.

attention to multimodal users, and reduce comfortable approach speeds. These treatments have many applications. Design variations include roadside design features, gateways, and landscaping. Secondary effects and considerations should include horizontal and intersection sight distance. 4.13.2 Applicability and Considerations The roadway environment can influence drivers’ perceptions of the road and provide safety benefits when implemented appropriately. Landscaping, cross-sectional changes, and gateways are three characteristics of the roadway environment that can affect the speed and safety of a roadway while providing aesthetically pleasing surroundings. Landscaping is typically planted along the roadside, set back from the edge or within a center median (see Exhibit 4-26). Landscape improvements should not be planted in an area that obstructs signs, sight lines, or the visibility of motorists, bicyclists, or pedestrians. A roadway’s cross section includes travel lanes, shoulders, curbs, drainage channels, side slopes, and clear zones. Other cross-sectional elements include sidewalks, bike lanes, barriers, medians, and frontage roads. Specific design guidelines for each of these elements generally depend on surrounding environment, adjacent land uses, and roadway facility type. Gateways (see Exhibit 4-27) are prominent physical features that inform drivers they are tran- sitioning into a new roadway environment and can include landscaping, lighting, signage, or physical structures. Maintaining landscaping treatments can be costly and difficult. Maintenance tasks can be labor- and infrastructure intensive, and include mowing, pruning branches and shrubs, water- ing, and fertilizing. In addition, removing and replanting vegetation or trees can be a significant investment, especially during the first few years until the vegetation is established. Maintenance tasks require a minimum clear space of three ft from the edge of a travel lane to allow mainte- nance crews to work without being in danger of passing vehicles. (Mok et al., 2006) Treatment Descriptions 59 (Credit: FHWA, 2000a) Exhibit 4-26. Landscaped median treatment.

Gateways are typically installed as an entrance to a city, neighborhood, or downtown area. The intent of these treatments is to create a welcoming physical feature that informs motorists that they are about to enter an activity area where lower speeds are desirable. Gateways are also placed at the exits of these same areas, typically to inform drivers that they are leaving an activity area. 4.13.3 Treatment Layout/Design Drivers must have a clear sight distance to view other vehicles approaching or pulling out at intersections; landscape heights and locations should not impede visibility. Canada’s Ontario Ministry of Transportation conducted a study on transition zones near intersections; however, the results of this study are not yet available. 4.13.4 Speed Effects Much of the information about how the roadway environment impacts safety and speed relates to roadway segments. No information has been found to describe the speed reduction benefits associated with roadway environment changes at intersection approaches. Cross-sectional changes can influence the way a driver perceives the road and can affect driver behavior. A roadway with several lanes, wide shoulders, and clear zones gives the driver a feeling of openness, thus increasing the impression that they can drive fast. A narrow road with horizontal curves, steep slopes, or even a cliff on the side of the road, induces drivers to slow down. Changing the appearance of the road primarily impacts unfamiliar drivers. A study of 30 test sites in Canada reported 85th-percentile speeds of 30 mph on sites with side friction, such as parking, heavy pedes- trian and bicycle activity; whereas the 85th-percentile speed was measured to be 39 mph at sites with simple and open road situations. The posted speed limit at all of the sites was 30 mph. (Smiley, 1999) Trees and vegetation can help define the edge of the roadway and slow traffic. Shinar et al. (1974) conducted a study on two-lane rural highways that indicated that trees growing close to the edge of the road caused drivers to maintain lower speeds than on an open stretch of high- way. Drivers were requested to maintain a speed of 60 mph, yet, on the stretches of roadway with trees, drivers maintained a speed of 53 mph and on the open stretches of highway drivers main- tained 57 mph. (Human Factors North, Inc., 2002) 4.13.5 Safety Effects There is information quantifying how the roadway environment impacts safety, and much of the available information relates to roadway segments. 60 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Exhibit 4-27. Gateway.

Landscaping can be a significant safety hazard when placed within the clear zone or when it obstructs sight distance for drivers pulling out into the roadway. However, when implemented appropriately, it can provide a safety benefit. A before-and-after study conducted by TTI implemented landscape improvements to 10 Texas Department of Transportation projects. After the landscaping was in place for three years, the mean crash rate per 1 million vehicle miles traveled decreased approximately 20% while the pedestrian crash rate was reduced by 40%. (Mok et al., 2006) 4.14 Summary There are a variety of treatments and potential applications that should be customized for each unique intersection need. The information provided here should supplement and complement the user’s experience and professional judgment. Appendix A provides a treatment implemen- tation process flowchart. Appendix B provides scenario-based case studies to aid the user in applying the screening process. By understanding the fundamentals and considerations of speed and potential treatments and applications, the user can consider possible treatments. The case studies provide examples of actual projects where the process was applied and treatments were recommended. Treatment Descriptions 61

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TRB's National Cooperative Highway Research Program (NCHRP) Report 613: Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections explores the effectiveness of geometric design features as well as signage and pavement markings to reduce vehicle speeds at high-speed intersections. A final report documenting the entire research effort is available online as NCHRP Web-Only Document 124.

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