The blogs are alive with news from the Bay Area that traffic got better following a fire that collapsed an interchange serving Interstate 880. Some are taking this as support for the argument that Seattle does not need another highway to replace the aging Alaskan Way Viaduct. Improved transit and a surface road will be sufficient to keep Seattle moving, they say. Take away roads and people use transit or give up "unnecessary trips." That may be wishful thinking. For starters, the eased commute along that corridor could reflect a number of conditions: the availability of other routes in the Bay Area, BART's decision to offer free service during the crisis, and people deciding to stay home or not move freight in the first week after the fire. Long term, commuter patterns may adjust and show that the I-80 to Interstate 880 interchange is less critical than assumed for the larger Bay Area traffic pattern. That certainly would be a victory for transit advocates. A similar claim was made here about closure of Seattle's bus tunnel under Third Avenue. Buses actually move through downtown faster than they did in the tunnel. But that happened because an entire downtown street was reserved for buses during peak hours. The zeal for improved transit and the war on the automobile may cause us to disregard an inescapable fact about Seattle's geography. Skinny in the middle, Seattle has just two major north-south corridors, Interstate 5 and Highway 99 (the Viaduct). No amount of transit, carpooling, telecommuting, etc., can change that. No one knows what's ahead for the Viaduct, which today carries 100,000 cars and trucks a day. The state and the city publicly pledged to end their war and work toward a compromise where none seemed possible before. Some are suggesting a compromise involving a four-lane "slowway" similar to Granville Avenue in Vancouver, B.C., that would carry far fewer cars. This is not intended as a comment on elevated or tunnel, just the notion that some are considering a scenario that would effectively diminish the only alternative to I-5, itself an aging elevated structure that got some seismic upgrades in recent years. It's not unimaginable that a truck carrying 8,600 gallons of gasoline could crash on I-5. Then what? The road system that serves our region is aleady choked, so stressed that a minor accident in one part of the city creates backups elsewhere. Yes, let's keep moving on making transit better, but the idea of reducing our city to one major corridor is a big risk that got lost in the elevated-versus-tunnel debate. Maybe that Bay Area fire will cast some light up here. Update: We got yet another reminder of the fragility of our transportation network. On Wednesday, water not fire caused a closure of Seattle's University Bridge, disrupting transit service and vehicle traffic along the Ship Canal. Quoting a report from the Seattle Times: More than 31,400 vehicles use that route every day, and the closure snarled the other bridges over the Ship Canal at the Montlake Cut, and in Fremont and Ballard. Update #2: The bridge reopened late afternoon Thursday, May 3, 2007.