I’d imagine a fair portion of the people who read this blog already know some or all of these reasons that Link is going over I-90 before it goes over SR-520, but I thought I’d enumerate them for easy linking and just to fill in any holes.
I-90 offers a direct connection to the downtown Seattle transit tunnel. If you looked at my earlier tour of Central Link construction, I had a google maps link to the south transit tunnel entrance – you can see there the two tracks we’ve built, plus the space to either side where feeder tracks join with the I-90 center roadway. This kind of a connection offers us the opportunity to interline service – both trains going to the airport (or farther) and trains going to the eastside will come into downtown from the south and run on the same tracks in the tunnel.
It so happens that demand for the northern line (Northgate) is very close to the combined demand for an eastside line and a south line, so having East Link enter the tunnel from the south means that our commute patterns will much more efficiently use our infrastructure. This is also the big reason we didn’t pick buses for building from Seattle to Bellevue – they couldn’t efficiently interline with North Link to increase capacity there.
If we were to cross 520, we’d have two choices, both of them bad: One, we could build a surface level station to transfer at Husky Stadium, and force a transfer for commuters to already full trains coming in from Northgate. We’d create crush loaded trains. The other option would be to build a direct connection into the tunnel toward downtown – which would cost hundreds of millions on its own, potentially have large construction impacts on a residential area, and could be risky due to the depth. Such work would probably also delay University Link.
Even ignoring the capacity and technical issues in Seattle, the eastside would have a problem of its own. 520 is significantly north of downtown Bellevue, so trains would have to turn south first to serve the Bellevue downtown core, then north again to get to Redmond. When using I-90, we don’t have to go out of our way to serve south Bellevue, and the time between downtowns is lower.
Issaquah poses another problem with a 520 crossing.. We’re already planning to build to Redmond, but if we chose 520, later construction to Issaquah (part of the Sound Transit long range plan) would really necessitate an I-90 crossing anyway. With an initial I-90 crossing, it’s much simpler to continue east in or near the interstate right of way.
A 520 crossing would also impose any delays attached to construction of the new SR-520 bridge on Sound Transit’s schedule. The risk added by working with WSDOT on the project would likely also make Sound Transit less competitive for Federal Transit Administration grants.
All this, and I-90′s center roadway was built with conversion to high capacity transit in mind. I think it’s always been the clear choice, but hopefully this convinces more people who were worried about the decision!