maybe if everyone commuted to trainyards ... and then hopped on freight trains ... those things can hold nearly infinite weight ... and they may not be too fast .... but I think you'd be able to cram about 150 to 200 people per car ... and the average freight train is like .. 100 to 150 cars long .. thats between 15000 and 30000 people per freight train. I know if it was an area like Chicago, where railways are all over the place ... getting hundreds upon hundreds of freight trains out of the city, as well as loading and unloading them, would not be a problem, 300 trains would get between 4.5 and 9 million people out ... any international airports could probably handle another 50000 or so people ... I Figure freight ships are about 1000 feet long by 100 feet wide, thats around 100,000 square feet of surface area ... now lots of it would be filled with obsticles, but there are also multiple decks on some ships, 10,000 to 20,000 people should be able to fit aboard a freight ship without too much discomfort, assuming that there are between 50 and 100 ships available ... thats between 500,000 and 2,000,000 people safe out at sea.
On the expressways ... if every lane is used for outgoing traffic, the roads should be able to handle about 100 cars per minute assuming no gridlock ... if there are 12 total lanes of expressway ... thats about 144,000 people over a two hour period. Assuming that there are hundreds of state, county, and local roads, another couple hundred thousand should be able to escape city limmits ...
Anyone that has boats would be able to take their boat out of the city ... imagine every boat from every marina in the city filled to capacity ... that would probably get at least another 100,000 people out to safety ...
It's definatly theoretically possible to evacuate a major city in a few hours ... assuming all lagre scale methods of transportation were used and combined, however, as many others have pointed out, due to human nature, it's not realisticly possible. (just like how comunism may sound good on paper, but fails miserably in real life)