Is the Viaduct a corridor of importance to homeland security?

I made a passing reference in this post suggesting that there might be a strong Federal interest in Seattle's viaduct because "... SR 99 corridor has tactical defense implications." Of course I don't know for sure and that was just wild conjecture.

The Mayor of course is using parallel reasoning — Elliot Bay is a waterfront of national significance — in trying to find Federal money to rebuild the seawall. Could a similar reasoning be applied to the People's Waterfront Coalition's tear-it-down plan? i.e. that the Feds will kill the plan and insist on more than one limited-access corridor through Seattle's CBD as a matter of homeland security? I have no answer but just a question. But if SR 99 has any significance in terms of civil defense, I believe that that would just about kill the tear-it-down option.

For those who don't know, Seattle is a city of hills and water. The topography is very "broken" and there is little continuity in our street grid. We have Interstate 5 and State Route 99 running north/south through the heart of our city. Would it be unwise to remove 99? I haven't heard this issue of security addressed -- and btw security is not just terrorism but also the ability to evacuate (or provide help) to the west side of the city in case of natural disaster.