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Selecting an Appropriate Treatment 21 (Credit: FHWA, 2006) (a) Reduced lane width, added rumble strips, and added pavement markings on major road (Credit: FHWA, 2006) (b) Installed mountable splitter island with stop signs on minor road approaches Exhibit 3-4. FHWA low-cost treatment concepts. several years of data are needed to determine the potential effects the treatments had on road- way safety. (FHWA, 2006) 3.5 Treatment Considerations This section provides information to aid in screening treatments and to identify one or more treatments that may be most appropriate for a particular condition. Exhibits 3-5 and 3-6 pro- vide a summary of the considerations for each treatment. Treatments for which specific testing has been conducted are shown in the shaded sections. As described in the following sections, Exhibit 3-5 summarizes typical uses of the treatments, indicates how they may be applied at high-speed intersections, indicates whether research was found to document their effectiveness, provides order-of-magnitude cost relationships, and indicates the treatments' experimental status with respect to the MUTCD. Exhibit 3-6 provides the documented treatment applications, their effectiveness, and noteworthy considerations. Section 4 provides supplemental information, diagrams, and photos for each treatment. The information provided in Section 4 represents the best available data based on the extensive lit- erature review and field testing conducted under NCHRP Project 3-74.
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22 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Treatment Documented Uses Potential High-Speed Intersection Applications Dynamic Warning Horizontal curves Rural, unsignalized intersections Signs Work zones Use caution in high driver-workload contexts Transverse Pavement Traffic circle and stop-controlled intersection History of high-speed crashes Markings approaches Horizontal curves, bridges, freeway off-ramps Work zones Transverse Rumble Approaches to intersections Use caution in noise-sensitive contexts and with Strips Approaches to toll plazas motorcycle traffic Longitudinal Rumble Rural highways Consider impacts to bicycle traffic Strips Wider Longitudinal Crash history involving curves, hills, roadway cross Run-off-road crashes Pavement Markings section Use caution as increased visibility may lead to Work zones increased speeds Roundabouts Rural highways Rural Highways Transition areas Gateways Use caution with steep grades, unusual geometry, constrained right-of-way Approach Curvature Roundabout approaches Stop- and yield-controlled approaches Use caution with run-off-road crash history, grades Splitter Islands Roundabout and stop-controlled approaches Minor approaches of T-intersections and two-way, stop-controlled intersections Speed Tables/ Plateau Local and collector streets Stop-controlled approaches Plateaus installed on facilities with 35 to 50 mph speeds in New Zealand Reduced Lane Width Work zones Uncontrolled approaches Low-speed urban and residential locations Rural two-lane highways Visible Shoulder Rural two-lane highways Wide shoulders Treatments Run-off-road crashes Roadside Design Transition areas Transition areas Features (Including Adjacent to roadway or intersection beyond clear Consider sight distance at driveways Gateways and zone and line of sight Landscaping) Note: Shaded rows indicate treatments tested through NCHRP Project 3-74. Exhibit 3-5. Treatment application summaries matrix.
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Selecting an Appropriate Treatment 23 Treatment Function Documented Documented Cost Experimental Speed Safety Status Reduction Improvement Dynamic Warning Encourage deceleration Yes Yes $$ No Signs Reduce comfortable approach speed Transverse Pavement Improve intersection visibility Yes Yes $ Yes Markings Alert drivers to upcoming intersection Transverse Rumble Encourage deceleration Yes No $$ No Strips Reduce comfortable approach speed Longitudinal Rumble Reduce comfortable approach speed No data Yes* $$ No Strips Wider Longitudinal Increase visibility and demark the No data No data $ No Pavement Markings intersection influence area Roundabouts Reduce speed through intersection Yes Yes $$$ No Reduce conflicts Approach Curvature Reduce comfortable approach speed Yes* Yes $$$ Yes Splitter Islands Improve intersection visibility No data Yes $$ Yes Alert drivers to upcoming intersection Speed Tables/Plateau Encourage deceleration Yes* Yes* $$ No Reduce comfortable approach speed Reduced Lane Width Reduce comfortable approach speed Yes* No $$ No Visible Shoulder Alert drivers to upcoming intersection No data Yes $ No Treatments Improve intersection visibility Roadside Design Reinforce transitioning roadway Yes* Yes* Varies No Features (Including environment Gateways and Improve intersection visibility Landscaping) Reduce comfortable approach speed Notes: Shaded rows indicate treatments tested through NCHRP Project 3-74. * Indicates data for low-speed intersections and/or roadway segments only. Exhibit 3-5. (Continued).
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24 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Treatment Documented Applications Documented Speed Effects Dynamic Warning High-speed 2 test sites (WA, TX) uncontrolled Mean speeds reduced up to 2.8 mph Signs intersections approaches Initial reduction of 3 to 4 mph at two TX sites, one site had sustainable results Others Rural interstate work zones (SD, VA) 1.4 to 4 mph mean speed reduction in Freeway ramps (MD, VA) work zones School zones (TX) Reduction in the number of speeding vehicles in work zones: 2025% for cars, Curve approaches 40% for trucks Transverse Pavement High-speed 5 test sites (OR) Up to 0.9 mph mean speed reduction Markings intersections Stop-controlled and uncontrolled approaches Others Horizontal curves 20-30% speed reduction on segments Roundabout approaches Peripheral lines almost as effective as full Work zones transverse lines Highway off-ramps Bridges Transverse Rumble High-speed 3 test sites (TX) uncontrolled approaches 1.3 mph mean speed reduction at the Strips intersections perception-response time location Others Some stop-controlled approaches More gradual deceleration Toll plazas, horizontal curves, and work Increased speed variation zones Not successful in significantly reducing approach speeds Longitudinal Rumble High-speed No data No data Strips intersections Others Shoulder or center of expressways, No data parkways, and rural highways Wider Longitudinal High-speed No data No data Pavement Markings intersections Others Work zones No data Vertical and horizontal curves Access-controlled highways Toll roads Roundabouts High-speed Many applications on high-speed and low- Geometry yields 13 to 17 mph 85 th - intersections speed facilities percentile entry speeds Others Roundabout type and design vary based on facility speed, access, volumes, vehicle composition, and other factors Note: Shaded rows indicate treatments tested through NCHRP Project 3-74. Exhibit 3-6. Treatment effectiveness summaries.
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Selecting an Appropriate Treatment 25 Treatment Documented Safety Effects Key Considerations Dynamic Warning No data Power supply Signs Urban vs. rural applications Reduction in rollover crashes on freeway ramps Target speed Coordinate with other treatments closer to intersection Transverse Pavement No data Driver familiarity Markings Human factors considerations 50-65% crash reduction at roundabouts and high crash locations Transverse Rumble No data Noise impacts to adjacent land uses Strips May make bicycling difficult Marginal safety benefits at best Vibrations can startle drivers and bicyclists, causing quick and unsafe maneuvers Maintenance Longitudinal Rumble No data Truck traffic, if narrowing lane width Strips Vibrations can startle drivers and bicyclists, causing Reductions in injury, sideswipe, and opposite quick and unsafe maneuvers direction crashes Maintenance 46% reduction in shoulder encroachment Wider Longitudinal No data Improved visibility, especially with older drivers Pavement Markings No data Roundabouts All roundabout conversions: approximate 35% Operational performance reduction in total crashes and 77% reduction in Right-of-way needs injury crashes Pedestrian and bicycle safety Rural two-way stop conversions: approximate 72% reduction in total crashes and 87% reduction in Gateway treatment injury crashes Note: Shaded rows indicate treatments tested through NCHRP Project 3-74. Exhibit 3-6. (Continued).
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26 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections 3.5.1 Typical Uses Many of the treatments mentioned in Section 4 have not been documented as having been used at high-speed intersections; however, they may be effective for those applications. The typical uses listed for each treatment are provided to indicate applications for which the treatment is known to have been used. The documented effects are primarily based on these known applications. The lists of typical uses are helpful when considering driver expectations associated with a treatment. 3.5.2 High-Speed Intersection Applications The high-speed intersection applications provided in Section 4 describe high-speed intersec- tion conditions for which a treatment may be well suited. This section also describes conditions for which the treatment is not well suited. For example, dynamic warning signs may be appro- priate on approaches to rural unsignalized intersections, but may not be appropriate in contexts where there is already a high driver workload or in a complex visual environment where the dynamic warning signs may prove ineffective. 3.5.3 Function The function(s) of the treatments describe the role that each treatment plays in the intersec- tion environment to produce lower speeds. The treatments should be screened to ensure that those selected have functions that are suited to the specific speed problem(s) identified in the intersection assessment. For example, if speed adaptation is identified as the root of a speeding problem, it is likely that a treatment that reinforces an impression of changes in the character of the roadway environment sufficiently in advance of the target speed location will be most suc- cessful in reducing speeds. Similarly, if driver attentiveness is identified as the likely root of the problem, appropriate treatments might be those that alert drivers to the upcoming intersection. 3.5.4 Documented Effects There is limited documented research that identifies how much speed reduction (if any) may be expected from a given treatment. Most available documentation relates to segment speed reduction applications rather than intersection speed reduction. However, these data are included because it is logical to assume that, in many cases, similar effects may be achieved on high-speed intersection approaches. Data that specifically address speed reduction effectiveness on high-speed intersection approaches are available for the following treatments: dynamic warn- ing signs, transverse pavement markings, roundabouts, approach curvature, and rumble strips. Because the extent of these data is quite limited, the speed reduction actually realized will likely vary with each application. The information available related to each specific treatment's effec- tiveness is provided as part of the detailed treatment descriptions. 3.5.5 Cost Cost is often a factor that limits treatment implementation. In general, treatments that do not involve changes to the paved roadway section have the lowest costs, while treatments that mod- ify or reconstruct the roadway section are the most costly. Secondary costs such as establishing a power source in a rural location could also increase implementation costs. 3.5.6 Experimental Status Identifying whether a treatment would be considered experimental--whether or not it is included in the MUTCD--helps the user determine if additional authorization may be needed before an experimental treatment can be installed.