Philadelphia's new disaster plan website - http://readyphiladelphia.org leaves bicyclists in a gray area, there are no designated routes for bicycles, although it can be safely assumed that bicyclists will be using the pedestrian routes, which ironically includes the Ben Franklin Bridge.
Problems may arise on streets with bike lanes, it is likely that without any guidance emergency personnel will simply open up the bike lanes and any available parking area or shoulder to automobile traffic.
City officials need to look at making emergency bicycle/pedestrian provisions on the Betsy Ross, Walt Whitman and Tacony Palmyra Bridges. While it is less likely that there will be a mass pedestrian exodus A La the Brooklyn Bridge on 9/11, it's conceivable that masses of pedestrians could be stranded at the foot of these bridges as this account of the Verrazano Narrows Bridge during the 2003 Great NE Blackout shows.
"Finally, during the 2003 Blackout, thousands of SI (Staten Island) residents were able to walk out of Manhattan on the bridges, trying to walk home, only to be stopped dead at the VNB. There was no path, there was not enough buses and the bridge police were not prepared to open a lane for people to walk across. NYC had an emergency evacuation crises here on a smaller scale than New Orleans, and like NO, failed to meet emergency needs."
Also note that the Schuylkill River Trail is not included in the evacuation plans, unlike Washington DC which lists multi use paths as evacuation routes in its emergency evacuation brochure.
Lessons from Katrina and Rita is an excellent post analysis of the consequences of not using transit and non motorized modes during the 2005 Hurricane season in Lousiana and Texas. Published by Todd Litman of the Victoria Policy Transportation Institute.