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HomeMy WebLinkAbout17254-CO-Renton-WSDOT Guardrail Requirements-2015-03-19-Memo Memo.docx MEMORANDUM DATE: March 19, 2015 TO: Clark H. Close Associate Planner City of Renton Community and Economic Development 1055 South Grady Way Renton, WA 98057 FROM: Bryan M. Schwartz, P.E. RE: WSDOT Guardrail Requirements Vuecrest II City of Renton Application File No. LUA15-000026, ECF, SHPL-A, MOD This memorandum has been prepared to explain our understanding of the WSDOT Design Manual regarding traffic barrier (guardrail) requirements for fill slopes. There are two pages attached to this memorandum from the WSDOT Design Manual. The first attached page (page 1600-5 from the WSDOT Design Manual) includes general guidance for determining when traffic barriers (guardrails) are required for Fill Slopes. As highlighted on page 1600-5: Fill slopes can present a risk to an errant vehicle with the degree of severity dependent upon the slope and height of the fill. Providing fill slopes that are 4H:1V or flatter can mitigate this condition. If flattening the slope is not feasible or cost-effective, the installation of a barrier might be appropriate. Exhibit 1600-6 represents a selection procedure used to determine whether a fill sideslope constitutes a condition for which a barrier is a cost-effective mitigation. The curves are based on the severity indexes and represent the points where total costs associated with a traffic barrier are equal to the predicted cost of collisions over the service life for selected slope heights without traffic barrier. If the ADT and height of fill intersect on the “Barrier Recommended” side of the embankment slope curve, then provide a barrier if flattening the slope is not feasible or cost- effective. Exhibit 1600-6 (Guidelines for Embankment Barrier) from the WSDOT Design Manual has been included with this memorandum as well. We have added some highlighting & text boxes to Exhibit 1600-6 to explain how we interpret this exhibit. A retaining wall is an embankment steeper than 1 1/2H:1V; therefore the top line is used on the graph. Per the Renton Municipal Code, the ADT for Residential Access Streets is 0-3000. As the key legend indicates, for conditions that occur on or above the embankment slope curve, a traffic barrier is not recommended. As a result, for an ADT of 3,000, a traffic barrier is not recommended for an embankment (retaining wall) height of less than or equal to 6-feet. Chapter 1600 Roadside Safety WSDOT Design Manual M 22.01.10 Page 1600-5 July 2013 1600.04 Mitigation Guidance There are three general categories of features to be mitigated: sideslopes, fixed objects, and water. This section provides guidance for determining when these objects present a significant risk to an errant motorist. For each case, the following conditions need added consideration: • Locations with high expected collision frequency. • Locations with pedestrian and bicycle usage. (See Chapters 1510, Pedestrian Design Considerations, 1515, Shared-Use Paths, and 1520, Roadway Bicycle Facilities.) • Playgrounds, monuments, and other locations with high social or economic value. • Redirectional land forms, also referred to as earth berms, were installed to mitigate objects located in depressed medians and at roadsides. They were constructed of materials that provided support for a traversing vehicle. With slopes in the range of 2H:1V to 3H:1V, they were intended to redirect errant vehicles. The use of redirectional land forms has been discontinued as a means for mitigating fixed objects. Where redirectional land forms currently exist as mitigation for a fixed object, provide designs where the feature they were intended to mitigate is removed, relocated, made crashworthy, or shielded with barrier. Landforms may be used to provide a smooth surface at the base of a rock cut slope. The use of a traffic barrier for features other than those described below requires justification. 1600.04(1) Side Slopes 1600.04(1)(a) Fill Slopes Fill slopes can present a risk to an errant vehicle with the degree of severity dependent upon the slope and height of the fill. Providing fill slopes that are 4H:1V or flatter can mitigate this condition. If flattening the slope is not feasible or cost-effective, the installation of a barrier might be appropriate. Exhibit 1600-6 represents a selection procedure used to determine whether a fill sideslope constitutes a condition for which a barrier is a cost-effective mitigation. The curves are based on the severity indexes and represent the points where total costs associated with a traffic barrier are equal to the predicted cost of collisions over the service life for selected slope heights without traffic barrier. If the ADT and height of fill intersect on the “Barrier Recommended” side of the embankment slope curve, then provide a barrier if flattening the slope is not feasible or cost-effective. Do not use Exhibit 1600-6 for slope design. Design slopes consistent with guidance in Chapters 1130 and 1230, evaluating designs with clear, traversable slopes before pursuing a barrier option. Also, if Exhibit 1600-6 indicates that barrier is not recommended at an existing slope, that result is not justification for a deviation. For example, if the ADT is 4,000 and the embankment height is 10 feet, barrier might be cost-effective for a 2H:1V slope, but not for a 2.5H:1V slope. This process only addresses the potential risk of exposure to the slope. Obstacles on the slope can compound the condition. Where barrier is not cost-effective, use the recovery area formula to evaluate fixed objects on critical fill slopes less than 10 feet high. 1600.04(1)(b) Cut Slopes A cut slope is usually less of a risk than a traffic barrier. The exception is a rock cut with a rough face that might cause vehicle snagging rather than providing relatively smooth redirection. Roadside Safety Chapter 1600 Page 1600-20 WSDOT Design Manual M 22.01.10 July 2013 Below curve: barrier recommended on new installations Note: Routes with ADTs under 400 may be evaluated on a case-by-case basis. Guidelines for Embankment Barrier Exhibit 1600-6 Per Renton Municipal Code,the ADT for Residential Access Streets is 0-3,000 Embankment / Wall heights less thanor equal to 6-feet do not require aguardrail. (retaining wall)