I was talking a bus to the airport. I asked the driver to take me one stop past the transit center. The driver dropped me past the tunnel. Since another bus wouldn't come for a long while, I walked northbound through the tunnel to get back to the airport terminal. There is only about a 2-3 ft space on the left side (on the right of the southbound lanes). There were signs saying no peds in the tunnel.

I can say I did it once. There is barely enough space for someone to walk on the left side. I don't plan to do it again.