Stuck in your car behind a wall of bicyclists on the last Friday of the month, you might be inclined to think, as one SF Weekly letter writer did, that "Critical Mass [seems] like little more than a monthly festival for hating cars and drivers that was deliberately intended to screw up traffic for the rest of us." But if you walk or ride over the bridge on 18th Street from Utah to San Bruno streets -- a catwalk from the Outer Mission to Potrero Hill -- you'll see the logic behind the Mass. The platform extends out over 101, a particularly stinky, congested segment of the Bayshore Freeway. Below you, cars honk, weave, spew exhaust, and generally remind us of the need for alternative transportation. A rush-hour, bird's-eye view of 101, the longest freeway in the state and the third largest in traffic volume, just might get you up on two wheels.