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58 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (Credit: Dan Burden) Exhibit 4-25. Example of a shoulder composition application. 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
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Treatment Descriptions 59 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) (Credit: FHWA, 2000a) Exhibit 4-26. Landscaped median treatment.
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60 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Exhibit 4-27. Gateway. 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.