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Figure 3-7. 2035 PM Peak Hour Congestion Levels

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Figure 3-8. Sidewalk Infrastructure

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Figure 3-9. Bicycle Infrastructure

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Other Infrastructure

Other infrastructure that act as a potential constraint for HCT planning, as it may need special technical and engineering requirements, are summarized in Figure 3-10. A number of facilities, including railroads, petroleum pipelines, power lines, major sewer lines, and highways, converge in the SODO and Renton areas. There are relatively few conflicts in West Seattle and Burien. Major barriers in the area that would require either large bridges or tunnels include the Duwamish Waterway, I-5, and I-405. Additionally, north of Sea-Tac International Airport, height and land-use restrictions within the Runway Protection Zone and Approach Area will require Federal Aviation Administration approval for any elevated or at-grade alignment near SR 518.

Given the land use densities in Downtown Seattle, there are numerous existing and planned major utility facilities in the area. Examples include the Great Northern Railroad Tunnel, the Downtown Seattle Transit Tunnel, the Elliott Bay Sewer Interceptor pipeline, the under construction SR 99 Deep Bored Tunnel, and numerous underground electrical and communications lines.

3.5 Natural Environment

This section summarizes the key features of the natural environment and Comprehensive Plan policies related to potential HCT corridors.

Topography and Slope Constraints

The corridor has varied topography with several steep areas. These slopes are barriers to surface-running rail given Sound Transit’s design guidelines that specify a maximum grade of 6 percent for segments under 2,500 feet and 4 percent for longer segments. While BRT is able to traverse steeper slopes, placement of transit stations in steep locations may pose design challenges that could increase the project costs. In general, steep slopes are located:

Along the shorelines of Elliott Bay and Puget Sound in Seattle and Burien

West of the Duwamish Waterway in Seattle and Burien

Between Burien and SeaTac, north of the Sea-Tac Airport runway

In Tukwila, roughly paralleling I-5 between SR 599 and S 178th Street

South and east of I-405 in Renton

Natural Environment – Highlights

Steep slopes are common throughout the corridor, particularly in West Seattle and between SeaTac and Tukwila

Most of the river valley bottoms (Duwamish, Green, Black, Cedar) are prone to liquefaction

Large portions of the corridor are in flood zones: most of West Seattle and along major rivers

Comprehensive Plans in the study area discourage development in critical areas, although development is allowed if there are no alternatives