Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (2008) / Chapter Skim
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Appendix B - Case Studies
Appendix B - Case Studies
Pages 68-94
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68 A P P E N D I X B Case Studies
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Whiskey Hill and Meridian roads are two-lane roadways. The west approach of Whiskey Hill Road features a horizontal curve, sight distance issues, and grade changes.
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• Based on crash data and public complaints, there was a need to reduce speeds in the northbound direction on Meridian Road to eliminate stop sign violations. 70 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections
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Therefore, no speed reduction treatments are needed on those approaches. B1.2 Treatment Screening Potential treatments are • Reduced lane width, • Visible shoulder treatments, • Speed tables, • Rumble strips, • Roadway environment, • Approach curvature, • Roundabouts, • Splitter islands, • Longitudinal pavement markings, • Transverse pavement markings, and • Dynamic warning sign.
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B1.2.2. Evaluate Potential Treatments After the research team identified the "fatal flaw" treatments that were not feasible to install at this intersection (reduced lane width, visible shoulder treatments, speed tables, rumble strips, roadway environment, approach curvature, roundabouts, splitter islands, and dynamic warning sign)
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Speed reduction was needed at the eastbound and northbound intersection approaches, beginning approximately 500 ft SSD from the intersection. B1.3 Selecting Treatments B1.3.1 Implementation -- Treatment Layout Based on the review of treatment information and the objectives for reducing speeds, the research team selected transverse pavement markings for this site.
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The markings have the potential to draw additional attention to those warning signs and encourage drivers to reduce their speeds as they approach the intersection. The installation points beyond the upstream treatments are located where the roadway environment changes, such as the approximate point of curvature (Whiskey Hill Road, mile post 1.47)
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Case Studies: Whiskey Hill Road/Meridian Road 75 Exhibit B1-4. Schematic of Whiskey Hill Road/Meridian Road intersection proposed treatment layouts.
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• The length of each peripheral bar depends on the existing lane width and the width of the wheel base for vehicles that commonly travel through the area. Exhibit B1-5 illustrates the proposed peripheral transverse pavement marking design for the Whiskey Hill Road/Meridian Road intersection.
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Exhibit B1-7. Eastbound on Whiskey Hill Road near mile post 1.41.
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B2.1.1.4 Speed Data The posted speed is 55 mph on SR 20 and Marysville Road. A speed study conducted on SR 20 revealed critical speeds of 66 mph.
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B2.2 TREATMENT SCREENING Potential treatments are • Reduced lane width, • Visible shoulder treatments, • Speed tables, • Rumble strips, • Roadway environment, • Approach curvature, • Roundabouts, • Splitter islands, • Longitudinal pavement markings, • Transverse pavement markings, and • Dynamic warning sign. B2.2.1 Fatal Flaws Due to issues with visibility and awareness of the upcoming intersection, Caltrans and the research team decided that a visual treatment may be appropriate for this intersection.
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As shown in Exhibits B2-1 through B2-3, visible shoulder treatments, roadway environment, and longitudinal pavement markings are also "fatal flaw" treatments due to the existing roadway conditions. Visible Shoulder Treatments -- Existing study roadways have minimal shoulders.
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This treatment was chosen for the Marysville Road southbound approach to supplement existing signing and Stop Ahead pavement markings. B2.2.2.2 Dynamic Warning Sign Past research also has shown this treatment to be effective at reducing speeds and making drivers more aware of an upcoming intersection or roadway features.
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(AASHTO, 2004) B2.3.1.2 SR 20 -- Dynamic Warning Sign Activated by Speed In the westbound direction on SR 20, a Marysville Road sign is approximately 865 feet from the intersection.
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Exhibit B2-4. Potential treatment layout for SR 20/Marysville Road.
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Potential dynamic warning sign design. They include five pavement markings placed in series.
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However, the agency plans to implement and monitor the treatments to observe their effectiveness at reducing speeds and increasing driver awareness at the intersection. Case Studies: SR 20/Marysville Road 85
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• This intersection had a significant crash history, including two fatalities. • WSDOT implemented reduced speed zones through this corridor due to drivers not complying with posted speed limits prior to this intersection.
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B3.2 Treatment Screening Potential treatments are • Reduced lane width, • Visible shoulder treatments, • Speed tables, • Rumble strips, • Roadway environment, • Approach curvature, • Roundabouts, • Splitter islands, • Longitudinal pavement markings, • Transverse pavement markings, and • Dynamic warning sign. B3.2.1 Fatal Flaws Reduced Lane Width -- The existing lane width was 12 ft with a minimal shoulder and guardrail.
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B3.2.2 Evaluate Potential Treatments After the research team identified the fatal flaw treatments that were not feasible to install at this intersection, three potential treatments remained: longitudinal pavement markings, transverse pavement markings, and a dynamic warning sign. At this time, the research team reviewed past research and evaluated each potential treatment to assess its effectiveness and determine which one was the most appropriate.
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• WSDOT was interested in designing a dynamic warning sign that combined a speed limit and Intersection Ahead sign with a dynamic message sign. How much speed reduction is desired?
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Exhibit B3-2. Preliminary treatment layout for SR 26/S.
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Exhibit B3-3. Dynamic warning sign design.
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Exhibit B3-5 illustrates the installed dynamic warning signs along eastbound and westbound SR 26. 92 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (Credit: WSDOT)
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• Macksburg Road forms an intersection in the middle of a curve on Canby-Marquam Highway. B4.1.1.4 Speed Data The posted speed is the "Basic Rule." Note: Although the county does not have speed data for this intersection, they believe that reducing vehicle speeds may decrease intersection crashes.
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94 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Exhibit B4-1. Aerial view of Macksburg road.
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