Seattle’s North Link Light Rail, Originally Considered for Highway-Running, May Be Partially Tunneled

» Underground route may actually save money — but it raises possibility of altering the alignment.

Seattle’s light rail expansion program may be one of the most ambitious in the nation: Not only did the region open a 14-mile first segment last year, but it has a northern extension currently under construction and three further routes mostly funded and in advanced planning. Unlike most new light rail systems, Seattle’s is also being built to light metro standards, with capacity for four (long) cars at every station.

This huge investment does not correlate with a perfectly planned system, of course. One of its major flaws is its reliance on the Interstate 5 right-of-way north of Seattle for the $1.4 billion, 4.3-mile North Link section of the project. It’s a route alignment that will not only restrict commuter access to and from stations, but also result in lessened transit-oriented development because of the limited appeal of locating new construction directly adjacent to a major highway. But the decision to stretch the light rail project along the side of the road was routed in the presumption that using an existing transportation corridor would save on land acquisition costs and allow expensive tunneling to be avoided.

Another 8.2-mile extension of the line, planned for the stretch from Northgate to Lynnwood, is currently suggested also to follow I-5 closely.

A new report from the region’s Sound Transit agency, however, suggests that moving trains underground could actually save money compared to the originally planned elevated alignment for a part of the route between the planned Roosevelt and Northgate Stations situated north of the University of Washington and the city center. If the engineers are correct, conventional wisdom about the high costs of tunneling made need to be reversed. The strict adherence to existing road rights-of-way that typically constrain new transit projects may need to give way to a broader vision of how new transit capacity can be built.

Current plans would extend the light rail tunnel from the University of Washington to 75th Street Northeast, where trains would exit onto an elevated route towards Northgate. Sound Transit’s new report suggests that the agency could save five to ten million dollars by extending the tunnel ten blocks further north — about half a mile — with the added benefit of reducing neighborhood environmental effects. Though the costs of tunneling have been reduced in recent years thanks to new boring methods, they’re rarely as cheap as transit built on the surface; this situation may prove to be an exception because it allows the light rail to avoid conflict with a number of road overpasses. And there are no planned stations along this stretch of track.

But the lesson is still worth considering more broadly speaking: As tunneling decreases in cost, the constraints that limit choices in rapid transit routes can be reduced and better alignments can be selected without damaging a project’s budget.

What the Seattle engineers are promoting now is merely a cost-saving solution to an expensive problem; it will keep the light rail in the I-5 right-of-way, just below it. Yet the realization that tunneling may actually be cheaper than building above ground raises questions about whether the city should continue to route the transit line along the Interstate alignment when other routes only accessible by underground tracks may be more appropriate for high-quality transit service.

It is probably too late to consider altering the alignment of North Link, since engineering is already underway, but it’s worth considering what could have been done differently in the stretch between Roosevelt and Northgate Stations had it been clear from the beginning that tunneling was a reasonable option.

The 2.3-mile distance between these two stations is a major concern; it puts a major population between the two out of easy walking distance to either station, reducing the appeal of the line and ridership prospects. The route along the highway left few desirable places for a third stop in between, but were trains routed under Roosevelt Way to the east of I-5 or under Aurora Avenue to the west of I-5, there are a number of areas that could support some increase in development coinciding with the arrival of rapid transit. Either of these routes would be a bit longer than the current alignment, but the added developmental activity resulting from their implementation could be enormous. Neither of these areas could have been served by ground-running trains because of limited street space.

Yet clear thinking about moving transit away from the highway right-of-way is only possible when it becomes obvious that the cost of inserting trains in tunnels is lower than other options. In Seattle, there is little chance that a wholehearted change in route is possible. But next time, either here or elsewhere, an underground route should not be dismissed as the “expensive,” and therefore infeasible, option.

Image above: Seattle Transit Tunnel, from Flickr user Jason Rodriguez