Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (2008) / Chapter Skim
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From page 15... ...
Furthermore, there are relatively little data on the safety performance of potential treatments and, when information is provided, there is no direct correlation to the change in safety caused by reduced speeds. Quantifying the safety effects of speed reduction treatments at high-speed intersections will require conditions to be monitored for a number of years after the treatments are implemented.
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From page 16... ...
Appendix A provides a treatment implementation process framework to help the user assess an intersection and potential treatments. Appendix B provides several case studies that detail treatment selection and design.
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From page 17... ...
3.3.2.1 Identify Fatal Flaws Fatal-flaw screening may eliminate treatments based on cost, agency policies, or existing intersection characteristics. Examples of fatal-flaw screening include • Cost -- funding issues may prevent installing a dynamic warning sign or approach curvature.
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From page 18... ...
Exhibit 3-1 shows the key elements used in treatment design. 3.3.3.1 Target Approach Speed The target approach speed generally is selected so that drivers can operate safely and without an adverse affect on non-motorized users within the influence area.
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From page 19... ...
Depending on the type of treatment, it may be appropriate to carry the treatment through the intersection, or, as in the case of a dynamic warning sign, the treatment may be placed in one discrete location. In either case, the treatment should be perceived by motorists no later than the beginning of the transition area to ensure that adequate space and time are provided for the desired speed reduction.
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From page 20... ...
The first concept was to reduce the lane width, add rumble strips, and add pavement markings on the major road. The second concept was to install a mountable splitter island with stop signs on the minor road approaches.
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From page 21... ...
(a) Reduced lane width, added rumble strips, and added pavement markings on major road (Credit: FHWA, 2006)
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From page 22... ...
22 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Treatment Documented Uses Potential High-Speed Intersection Applications Dynamic Warning Signs Horizontal curves Work zones Rural, unsignalized intersections Use caution in high driver-workload contexts Transverse Pavement Markings Traffic circle and stop-controlled intersection approaches Horizontal curves, bridges, freeway off-ramps Work zones History of high-speed crashes Transverse Rumble Strips Approaches to intersections Approaches to toll plazas Use caution in noise-sensitive contexts and with motorcycle traffic Longitudinal Rumble Strips Rural highways Consider impacts to bicycle traffic Wider Longitudinal Pavement Markings Crash history involving curves, hills, roadway cross section Work zones Run-off-road crashes Use caution as increased visibility may lead to increased speeds Roundabouts Rural highways Transition areas Rural Highways 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 Plateaus installed on facilities with 35 to 50 mph speeds in New Zealand Stop-controlled approaches Reduced Lane Width Work zones Low-speed urban and residential locations Rural two-lane highways Uncontrolled approaches Visible Shoulder Treatments Rural two-lane highways Wide shoulders Run-off-road crashes Roadside Design Features (Including Gateways and Landscaping) Transition areas Adjacent to roadway or intersection beyond clear zone and line of sight Transition areas Consider sight distance at driveways Note: Shaded rows indicate treatments tested through NCHRP Project 3-74.
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From page 23... ...
No $$ No Visible Shoulder Treatments Alert drivers to upcoming intersection Improve intersection visibility No data Yes $ No Roadside Design Features (Including Gateways and Landscaping) Reinforce transitioning roadway environment Improve intersection visibility Reduce comfortable approach speed Yes*
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From page 24... ...
uncontrolled approaches 1.3 mph mean speed reduction at the perception-response time location Transverse Rumble Strips Others Some stop-controlled approaches Toll plazas, horizontal curves, and work zones More gradual deceleration Increased speed variation Not successful in significantly reducing approach speeds High-speed intersections No data No data Longitudinal Rumble Strips Others Shoulder or center of expressways, parkways, and rural highways No data High-speed intersections No data No data Wider Longitudinal Pavement Markings Others Work zones Vertical and horizontal curves Access-controlled highways Toll roads No data High-speed intersections Roundabouts Others Many applications on high-speed and low- speed facilities Roundabout type and design vary based on facility speed, access, volumes, vehicle composition, and other factors Geometry yields 13 to 17 mph 85 th - percentile entry speeds Note: Shaded rows indicate treatments tested through NCHRP Project 3-74. Exhibit 3-6.
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From page 25... ...
Selecting an Appropriate Treatment 25 Treatment Documented Safety Effects Key Considerations No data Dynamic Warning Signs Reduction in rollover crashes on freeway ramps Power supply Urban vs. rural applications Target speed Coordinate with other treatments closer to intersection No data Transverse Pavement Markings 50-65% crash reduction at roundabouts and high crash locations Driver familiarity Human factors considerations No data Transverse Rumble Strips Marginal safety benefits at best Noise impacts to adjacent land uses May make bicycling difficult Vibrations can startle drivers and bicyclists, causing quick and unsafe maneuvers Maintenance No data Longitudinal Rumble Strips Reductions in injury, sideswipe, and opposite direction crashes 46% reduction in shoulder encroachment Truck traffic, if narrowing lane width Vibrations can startle drivers and bicyclists, causing quick and unsafe maneuvers Maintenance No data Wider Longitudinal Pavement Markings No data Improved visibility, especially with older drivers Roundabouts All roundabout conversions: approximate 35% reduction in total crashes and 77% reduction in injury crashes Rural two-way stop conversions: approximate 72% reduction in total crashes and 87% reduction in injury crashes Operational performance Right-of-way needs Pedestrian and bicycle safety Gateway treatment Note: Shaded rows indicate treatments tested through NCHRP Project 3-74.
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From page 26... ...
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 warning signs, transverse pavement markings, roundabouts, approach curvature, and rumble strips.
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From page 27... ...
In most cases, speed reduction is desired not only at the intersection proper, but on the approach as well. Collecting speed data at several points on an intersection approach will give the best picture of drivers' deceleration curves (a speed profile)
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From page 28... ...
The effectiveness of a particular treatment will vary considerably depending on the conditions of the specific application. The potential treatments have a wide range of applicability, cost, secondary impacts, implementation considerations, and potential effectiveness in reducing speeds and increasing safety.
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