>>63931300
Most major cities are located near lakes and rivers with limited means of egress. Bridges particularly will jam almost instantly and highways themselves are pretty much just extremely long bridges. The thing about traffic jams is that everyone else with a brain is also trying to bypass them, bringing all other surrounding roads to a standstill. Unless you can get out several days ahead of the happening you're gonna get stuck, so you might as well hunker down instead and wait for it all to blow over.