oh, there was another reason, why:

Wobble of Millennium Bridge Solved

When crowds of people first crossed London 's Millennium Bridge, the structure swayed so violently it had to be closed.

Mathematician Steven Strogatz of Cornell University has now developed a mathematical description of the bridge's tendency to wobble as people walk across it.

It is based on the same 'chaos' mathematics used to understand synchronous behaviour in fireflies and brain cells. The equations should allow civil engineers to prevent the same thing happening to other bridges.

from here, but more here as well

Some pictures of the Alaska Way in Seattle, which came to my notice because Washington state is currently discussing (hey, even running a referendum) on whether to eliminate the gas tax which was voted earlier this year to, among other things, pay for the renovation (and the earthquake proofing) or replacement of that heavily trafficked bridge:

So with an old, earthquake threatened bridge, wobbly definitely applies, I suppose...

In the long run, we're all dead. John Maynard Keynes