A post on the Seattle blog Orphan Road recently suggested that driverless metro, which I've praised in Vancouver, might be the right technology for the Highway 520 bridge, the more northerly of the two bridges that connect Seattle to its vast eastern suburbs. It's an interesting type of problem that might interest readers in similar chokepoint-rich cities. I commented on the Orphan Road post on why driverless metro isn't appropriate there, but let's explore why any rail project will face challenges in this corridor. Seattle Mayor Mike McGinn has been promoting light rail on the bridge. (Seattle Transit Blog is covering in detail.) It's a live issue because a redesign of the bridge in being planned now.
Here's the basic geography. The orange line is a very very rough sketch of the currently planned Sound Transit light rail network, including the existing line from downtown south, and the North Link and East Link projects.
A basic fact of life about rail transit lines is that they're lines, well suited for connecting a series of destinations that are more or less in a straight line already. Reasonable straightness is a key feature of good rail transit because for these lines to be as strong as possible, they need to be the direct path between any two stations on the line. Light rail extending south and north out of Seattle is relatively straightforward because there is a string of destinations extending in these directions, and the resulting lines will pass that test.
By contrast, fitting rail transit to Seattle's eastern suburbs has always been difficult becuase the geography just doesn't work that way.
The light rail on the southerly bridge (Interstate 90) will work, if it does, only by linking up downtown Seattle to the major nodes of Bellevue and Overlake, and making the service attractive enough that people will ride it from Overlake to Seattle even though the line goes much further south than you would go if you were driving. In short, the line relies on the notion that light rail's intrinsic attractiveness will compensate for a somewhat circuitous route. We'll see if that really works.
On the other hand, for a trip between the University and Downtown Bellevue, the planned light rail, as sketched above, will serve both ends but probably won't be competitive, as the path it follows is more than twice as long as the path as the most direct path that a bus and car can take.
So should there be light rail on the northerly bridge (Highway 520) as well? This bridge is an even harder problem, because right at each end of the bridge, the major lines of demand immediately fan out in several directions. At the west end of the bridge, you have to turn north to go to the university, or south to go to downtown Seattle. Not far beyond the east end of the bridge, major paths of demand split three ways: south to Bellevue, east to Overlake/Microsoft, and north to Kirkland. As a result, there are several different high-demand transit links that all need to use the 520 bridge, including:
- Downtown Seattle to Kirkland
- University to Bellevue
- University to Overlake
- University to Kirkland
- South Lake Union to Overlake
- South Lake Union to Kirkland
What's more, while the currently planned line is probably tolerable for trips between Downtown Seattle and Overlake, these trips would certainly insist on going via the more direct 520 bridge if light rail were built there, so you'd have to accommodate them as well.
So here's the challenge for any Highway 520 light rail scheme: Light rail will only serve this market well only if it serves all these major branches. Even if we assume that the already-planned light rail gets built more or less as drawn above, so that it can be used to distribute some of the hypothetical trains on a 520 bridge, you would still need to add a branch north to Kirkland. You'd also be building a four-way light rail interchange east of the bridge (much like a freeway interchange, with direct links from every track to every other).
At the west end, you'd either need to build new light rail from the 520 bridge all the way to both the university and downtown Seattle, or else you'd have to design an interchange to feed bridge trains into the already-planned north-south line through the university. That's very tricky because the planned north-south line is underground but a line coming off the bridge would be on the surface, and you're in the middle of sensitive wetlands right there.
The Seattle region may choose to plan light rail on this grand scale, as Phoenix is thinking of doing. But such expensive infrastructure will probably only make sense if tremendous density is built around stations, such as we see around Skytrain in Vancouver. Unfortunately, suburbs at the immediate east end of the 520 bridge, Medina and Clyde Hill, are low-density affluent places that are likely to resist adding forests of towers around their stations, if they have stations at all.
As always, I emphasize that the Seattle region should build what it wants based on its own values, which includes tradeoffs between pure mobility and intrinsic values attached to rail. I'm not saying that there aren't environmental and urbanist reasons to prefer rail; clearly there are.
But if the region decides that cost-effective mobility -- getting people where they're going as fast as possible -- is an important objective, Highway 520 is a case where this goal is going to argue for a busway, because busway services can branch in several directions at each end of the bridge without needing dedicated infrastructure all the way to each destination. At the west end, for example, busway services heading for the University could branch off into university streets, whereas light rail would need either direct tracks into the heart of the university or some way to transition into the north-south underground rail line that will already be there. At the same time, busway services heading for downtown Seattle would be able to flow into downtown directly via appropriate bus lanes on existing highways.
No transit solution is going to be easy on the 520 bridge, but if you advocate light rail, it's important to be clear on how much light rail you'll need to build in order to have really served the market. A busway solution will almost certainly require less infrastructure for the same mobility. And given the pressure to spread scarce transit dollars over this enormous urban region, that's probably going to matter.