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HomeMy WebLinkAboutRenton PW-Transportation Response 12-19-17Development: Tiffany Park RE: Retaining Wall E-mail response from Barghausen: Ann, after reviewing this with our team, I don’t believe that guardrails at the ends of the walls are warranted.  According to the AASHTO green book Clear Zones are not applicable to Local Urban Streets (Section 5.3.4) which is exactly what this road is.  The AASHTO manual does provide for Lateral Offsets on urban streets of 1.5’ from face of curb, which we certainly comply with here.  I would propose that we install Object Markers on each end to delineate the ends of the wall to drivers and make the ends more conspicuous.  See detail below. I believe that we can lower the top of wall to the elevation of the sidewalk which will satisfy the functionality concern of the barrier.  I believe the remaining comments are minor and can be quickly addressed with a revised plan.  Can we get concurrence on what I have stated above that these changes will result in the issuance of an approved wall permit.  Thank you. Barry Response from City/PW/Transportation: Refer to AASHTO A Policy on Geometric Design of Highways and Streets (Green Book), 6th Edition, 2011, Chapter 5 – Local Roads and Streets, Section 5.3.4 Roadside Design: Clear Zone Clear zones are not applicable to local urban streets. Lateral Offsets Lateral offset is defined in Section 4.6.2. Further discussion and suggested guidance on the application of lateral offsets is provided in the AASHTO Roadside Design Guide (9). On all streets a minimum lateral offset of 0.5 m (1.5 ft) should be provided between the curb face and obstructions such as utility poles, lighting poles, and fire hydrants. In areas of dense pedestrian traffic, the construction of vertical curbing (typically 150 to 225 mm (6 to 9 in.) high) aids in delineating areas with high-volume pedestrian traffic. Trees are acceptable along local streets where speeds are 60 km/h (40 mph) or less, where curbs are present, and where adequate sight distance is available from intersection streets and driveways. Guardrail is not used extensively on local streets except where there is a significant risk to motorists and pedestrians, such as along sections with steep foreslopes and at approaches to overcrossing structures. On facilities without a curb and with a shoulder width less than 1.2 m (4 ft), a minimum lateral offset of 1.2 m (4 ft) from the edge of the traveled way should be provided. Observations from this section in AASHTO. The project is located on a local street. The area is designated as urban. Therefore, AASHTO suggests that a full clear zone is not applicable. Refer to items 2 and 4 below. It appears that AASHTO defines urban conditions to be where a street project is constructed in a setting with established property boundaries, buildings and other permanent appurtenances where right-of-way acquisition would be cost prohibitive. This is not really applicable in this instance given this is a new development located on undeveloped property. Additionally, the lateral offset is not necessarily a set 1.5 ft as indicated in the response. There are two permanent hazards involved: 1) a drop off of approximately 20 ft; and 2) a permanent concrete barricade. Note that AASHTO lists various obstructions but not these hazards. It goes on to suggest the application of guardrail where there is a significant risk to motorists and pedestrians. These hazards would appear to represent a significant risk to motorists. Refer to AASHTO A Policy on Geometric Design of Highways and Streets (Green Book), 6th Edition, 2011,, Chapter 4 – Cross-Section Elements, Section 4.6.2 Lateral Offset: In an urban environment, right-of-way is often extremely limited and in many cases it is not practical to establish a full-width clear zone using the guidance in the AASHTO Roadside Design Guide (13). These urban environments are characterized by sidewalks beginning at the face of the curb, enclosed drainage, numerous fixed objects (e.g. signs, utility poles, luminaire supports, fire hydrants, sidewalk furniture, etc.), and frequent traffic stops. These environments typically have lower operating speeds and on-street parking may be provided. In these environments, a lateral offset to vertical obstructions (signs, utility poles, luminaire supports, fire hydrants, etc., including breakaway devices) is needed to accommodate motorists operating on the roadway and parked vehicles. This lateral offset to obstructions helps to: Avoid adverse impacts on vehicle lane position and encroachments into opposing or adjacent lanes; Improve driveway and horizontal sight distances; Reduce the travel lane encroachments from occasional parked and disabled vehicles; Improve travel lane capacity; and Minimize contact between obstructions and vehicle mirrors, car doors, and trucks that overhang the edge when turning. Further discussion and suggested guidance on the application of lateral offsets is provided in the Roadside Design Guide (13). When curb is present, the lateral offset is measured from the face of curb. The Roadside Design Guide provides a discussion of lateral offsets where curbs are present. Traffic barriers should be located in accordance with the Roadside Design Guide, which may recommend that the barrier should be placed in front of or at the face of the curb. On curbed facilities located in transition areas between rural and urban settings, there may be opportunity to provide greater lateral offset in the placement of fixed objects. These facilities are generally characterized by higher operating speeds and may have sidewalks separated from the curb by a buffer strip. On facilities without a curb and where shoulders are present, the Roadside Design Guide provides suggested guidance concerning the provision of lateral offsets. Observations from this section in AASHTO. Note that this section does not discuss lateral offsets for hazards such as significant drop-offs or concrete barricades. Refer to AASHTO Roadside Design Guide, 4th Edition, 2011, Chapter 3 – Roadside Topography and Drainage Features, 3.1 The Clear-Zone Concept, page 3-2, second paragraph: The 1977 AASHTO Guide for Selecting, Locating, and Designing Traffic Barriers (1) modified the earlier clear-zone concept by introducing variable clear-zone distances based on traffic volumes, speeds, and roadside geometry. Table 3-1 can be used to determine the suggested clear-zone distance for selected traffic volumes and speeds. However, Table 3-1 provides only a general approximation of the needed clear-zone distance. These data area based on limited empirical data that were extrapolated to provide information for a wide range of conditions. The designer should keep in mind site-specific conditions, design speeds, rural versus urban locations, and practicality. The distances obtained from Table 3-1 should suggest only the approximate center of a range to be considered and not a precise distance to be held as absolute. For roadways with low traffic volumes, it may not be practical to apply even the minimum values found in Table 3-1. Refer to Chapter 12 for additional considerations for low-volume roadways and Chapter 10 for additional guidance for urban applications. Observations from this section in AASHTO. Table 3-1 provides suggested clear zone distances based on design speeds, volumes and roadside slopes. The main take away is that a designer is has some leeway to consider the site specific conditions. Refer to AASHTO Roadside Design Guide, 4th Edition, 2011, Chapter 10 – Roadside Safety in Urban or Restricted Environments, 10.0 Overview, page 10-2, third paragraph: Section 2.1.2 mentions that the highway designer has a significant degree of control over roadside geometry and appurtenances. That statement is more applicable for rural conditions and especially so for new rural highways. In urban or restricted conditions, the roadside environment (e.g., houses, businesses, trees, utility poles, signals, walkways) is already established and less flexible. Consequently, the designer has the challenge of providing roadside safety given the many preexisting constraints at hand. Observations from this section in AASHTO. The highlighted statement assumes a project is being constructed within an existing corridor with established features. It does not appear to apply to a new development, even though located within an urban setting. Refer to AASHTO Roadside Design Guide, 4th Edition, 2011, Chapter 10 – Roadside Safety in Urban or Restricted Environments, 10.1 Evaluation of Critical Urban Roadside Locations, first paragraph: Although the clear roadside concept is still the goal of the designer, many compromises are likely in urban or restricted environment areas. One misconception is that a curb with a 0.5 m (1.5 ft) lateral offset behind it satisfies the clear roadside concept. Realistically, curbs have limited redirectional capabilities and these occur only at low speeds, approximately 40 km/h (25 mph) or lower. Consequently, fixed objects located adjacent to the travel lane, even in the presence of curbs, pose a potential hazard. Achieving the clear zone distances suggested in Chapter 3 may be unlikely in an urban setting. As a result, a secondary goal for roadside design in an urban setting is to identify critical urban roadside locations – those that are more prone to crashes – and give these locations priority attention for roadside safety improvements. One way to achieve this improved safety is to establish specific lateral offsets for unique urban locations where the roadside is not shielded by features such as on-street parking. Observations from this section in AASHTO. A lateral offset of 1.5 ft does not necessarily satisfy design requirements for roadside safety in this case. Refer to AASHTO Roadside Design Guide, 4th Edition, 2011, Chapter 10 – Roadside Safety in Urban or Restricted Environments, 10.1.3.1 Obstacles in Close Proximity to Curb Face of Lane Edge: Historically, the lateral distance value (referred to as an operational offset) of 0.5 m (1.5 ft) has been considered a minimum lateral distance for placing the edge of objects from the curb face. This minimum lateral offset was never intended to represent an acceptable safety design criteria, though sometimes it has been misinterpreted as such. In a constrained urban environment, there is still a need to position rigid objects as far away from the active traveled way as possible. Research (5) has shown that in an urban environment, approximately 80 percent of roadside crashes involved an object with a lateral offset from the curb face equal or less than 1.2 m (4 ft) and more than 90 percent of urban roadside crashes have a lateral offset less than or equal to 1.8 m (6 ft). Objects located on the outside of curves also are hit more frequently than at other locations. It seems prudent, therefore, to achieve larger lateral offsets at these curve locations. As Figure 10-1 illustrates, a recommended goal is to achieve at least a 1.8 m (6 ft) lateral offset from the face of the curb at these outside-of-curve locations while maintain at least a 1.2 m (4 ft) lateral offset elsewhere. For urban locations without a vertical curb, lateral offsets of 3.6 m (12 ft) on the outside of horizontal curves and 2.4 m (8 ft) at tangent locations are reasonable goals when the clear zone widths suggested in Chapter 3 cannot be achieved. In addition to creating a wider lateral offset on the outside of horizontal curves, attention should be paid to sight distances at the inside of sharp horizontal curves to assure that it is not obstructed by roadside objects. As indicated in Figure 10-1, a driver’s line of sight that is suitable to provide the required stopping sight distance should be maintained. Observations from this section in AASHTO. A minimum lateral offset of 1.5 ft does not represent an acceptable safety design criterial according to AASHTO. For urban setting AASHTO recommends a minimum lateral offset of 6-ft for the outside of a curve. On the inside of curve, the lateral offset depends on stopping sight distance. Refer to AASHTO Roadside Design Guide, 4th Edition, 2011, Chapter 5 – Roadside Barriers, 5.2 Barrier Recommendations: Barrier recommendations are based on the premise that a traffic barrier should generally be installed if it reduces the severity of potential crashes. It is important to note that the probability or frequency of run-off-the-road crashes is not directly related to the severity of potential crashes. The mere installation of barriers could lead to higher incident rates due to the proximity of the barriers to the traveled way. Typically, barrier recommendations have been based on a subjective analysis of certain roadside elements or conditions. A barrier is considered if the consequences of a vehicle striking a fixed object or running off the road are believed to be more serious than hitting a traffic barriers. While this approach can be used, often there are instances where it is not immediately obvious whether the barrier of the unshielded conditions presents the greater risk. Furthermore, the subjective method does not directly consider either the probability of a crash occurring or the costs associated with shielded and unshielded conditions. Barrier installation criteria may also be established by using a benefit-cost analysis (B/C) whereby factors, such as design speed and traffic volume, can be evaluated in relation to barrier need. Costs associated with the barrier, such as installation costs, maintenance costs, and crash costs, are compared to similar costs without barriers. This procedure is typically used to evaluate three options: (1) remove or reduce the area of concern so that it no longer requires shielding, (2) install an appropriate barrier, or (3) leave the area of concern unshielded. The third option would normally be cost-effective only on facilities with low volume, low speed, or both, or where engineering studies show the probability of crashes is low. The Roadside Safety Analysis Program (RSAP) is an analysis procedure that can be used to compare several alternative safety treatments and provide guidance to the designer in selecting an appropriate design. Observations from this section in AASHTO. Due to the potential catastrophic result of an errant vehicle leaving the street and falling off of the retaining wall, the decision was made to install a concrete traffic barricade on top of the retaining wall. The lateral offset from the face of curb to the back face of retaining wall is approximately 7.5-ft. This is more than the lateral offset distance of 6-ft discussed in item 6, above. Conclusions: The engineer of record is responsible for the design of the appropriate roadside safety elements for this project. A guardrail protecting the ends of the concrete traffic barricade would protect errant vehicles from two hazards: (1) ends of the concrete traffic barricades; (2) drop off at the face of rockeries at the ends of the retaining wall. If the engineer of record is comfortable with a lateral offset of approximately 7.5-ft to both hazards, and believes that the site conditions, ADT and speed, justify no additional roadside safety features, the engineer of record should provide a written statement stating this and site the applicable design guide sections as justification. This statement shall be stamped and signed by the engineer of record and will be entered in the permanent records for this retaining wall.