seattle: rail-bus tradeoffs on the 520 bridge

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

and possibly also, depending on how access to the growing South Lake Union employment area develops ...

• 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.

Comments

[Cascadian]

Overlake to downtown Seattle via 520 wouldn't really be that much faster than on I-90. I work in Overlake and on the occasions I drive to Seattle after work I usually take 90 even though 520 is a shorter distance, because of traffic. Rail along either corridor will be close enough in travel time that I don't see the need for two corridors. For South Lake Union, it's close enough to downtown (and connected by streetcar already) that there's no real advantage there either.

The theoretical advantage is for trips north of the Ship Canal across the lake. University is the obvious Seattle destination, but also Wallingford, Fremont, and Ballard further to the west (not shown on your map). But connecting directly to Kirkland would potentially be a better idea, using a third transit-only bridge or some kind of suspended underwater tunnel. 520 can remain the bus (and eventually BRT) corridor. Some of the guys at Seattle Transit Blog have mentioned the idea of this third bridge concept. But we're talking way down the road. A west side light rail line (Ballard-downtown-West Seattle) and the Seattle side of the Ballard to University line would both take priority. On the Eastside, a north-south line connecting Kirkland to Bellevue and continuing to Renton, as well as an extension to Issaquah, are the priorities before any second lake crossing.

[Adam Parast]

Most important people besides the mayor agree. In my mind the major sticking points are:

- Increasing service levels to account for the loss of the Montlake Flyer stop
- Use toll revenue to fund this
- Implement high quality BRT (I have my doubts the high quality part of it will happen)
- Ensuring that transit only lanes connection SR-520 to UW
- Ensure the HOV lanes meet strict performance criteria per WSDOT (45 mph at least 90% of rush hour, *not* over a 24 hour period)

[Cap'n Transit]

What about connections?

[Ben Allen]

I'm from Seattle itself, so my perspective on things Eastside might be a bit skewed, but to my eye the Sound Transit route from Overlake-and-hopefully-eventually-Redmond to downtown Seattle isn't quite as circuitous as it might look on a map. One real complicating factor (and here someone actually from the Eastside might have to chime in) is that the 520 is at least in my experience the most painfully overcrowded road in the metro area, at all times of day, and for any trip anywhere (except to the UW or north Seattle) I wouldn't blame drivers for heading down and taking I-90 across instead, despite the added distance.

[CroMagnon]

I believe circuitous routes can work if the service is very fast through the diversion. It would probably have to be on exclusive ROW there.

[EngineerScotty]

Many Bellevue NIMBYs, concerned about hoods riding Link to their pristine neighborhoods (or ideologically opposed to the project) have been calling for a 520 routing of East Link. Some view this proposal as little more than a way to kill ANY LRT across the Lake.

[Jim]

Keep in mind also that trying to tie into the north south tunnel that is currently being built will be very difficult because of restraints on capacity. That tunnel is going to be used to capacity for people coming from the north. It will fill up fast. Trying to add trains coming from the east will be more than a challenge. It would likely need a new set of tunnels to get downtown.

[Jim]

I do love the idea of a dedicated busway when we get the additional lanes running in the 520 corridor.

[M1EK]

Problem with this type of analysis is that it presumes that all, or even just most, of the potential light rail ridership will also ride buses in busways.

Unlike your very cogent arguments about the lack of mobility in shared-lane streetcar versus city bus, I find your implicit contention completely uncompelling given actual results on the ground in modern US cities (i.e. ones like Seattle, not New York).

Yes, some commuters take the bus - but you're cutting your potential market by perhaps half, or even worse.

The worst mistake most transit planners make (and many laypeople as well) is assuming that providing a bus for people to ride is the same thing as getting a bunch of people to ride the bus.

[Matthew Miller]

No hope of using transfers to avoid the need for the 'train interchange"? For example: A Kirkland to Bellevue line crossing a U of W to Overlake line.

[Jeffrey Bridgman]

If they do chose light rail and building all the necessary connections is too costly, I hope they at least chose to make the appropriate investment in rapid buses connecting to the line...

Also, are there examples of such a transit corridor being shared between multiple modes? Could light rail and buses run on the same lanes across the bridge?

[Jeffrey Bridgman]

If they do opt for the light rail option, but decide it isn't feasible to built all the connections necessary, I hope they will at least invest in rapid bus service with good connections to the line. For example, a light rail line running east towards Redmond could be intersected by several rapid bus routes running north to south, paralleling I-405. Some sort of segregated ROW + frequent service would make the transfer penalty minimal.

Also, are there examples of multiple modes sharing transit lanes? Could the transit lanes on the 520 bridge be used for both bus and rail?

[Mike]

[Reposting, as this was lost]

Since the city has resisted a direct transit solution for commuters between the high density Overlake/Microsoft work area and the high density residential area around Capitol Hill, the 520 is under unnecessary pressure at peak times.

The bus connections off from Montlake at the western end of the 520 are completely inadequate for fanning out to Capitol Hill, the U district etc. I've spent long hours shivering here, waiting for services that simply failed to arrive. At some point you give up and go back to using a private vehicle.

[Nathanael]

"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."

It will, because what other route will they take? Ever since the ferries ended, they either have to divert to the south (the route of the light rail) or to the north (the route of SR 520). For downtown Seattle, the routes are practically equivalent in speed.

For the U of Washington and various points north, probably most Bellevue & Overlake riders will not take the rail "detour". But is that really a strong origin-destination pair?

If it is, rail over 520 makes sense. If it isn't, who cares?

[Nathanael]

"A busway solution will almost certainly require less infrastructure for the same mobility. "

Correction: for worse mobility. "Less infrastructure" is a synonym for "crawling slowly through city streets and running into stop lights, followed by stopping at unprotected street corners for non-level boarding".

If you don't think you need the better mobility of rail, then fine, save the money. If you do need it, don't screw around with buses -- you're just throwing away the 5-15% of your riders who will not ride buses period (that's the rail bias factor).