Exit 11 is an exit, just north of Portland, off the I-295 that connects to an interstate spur which leads onto the I-95, the Maine Turnpike.
David Drayton uses Exit 11 to get into Portland while trying to get clear of the Mist. In the film, the gantry marking Exit 11 is partially collapsed, presumably due to either a passing creature, or tremors like the ones the Grocery Store experienced at the Mist's arrival.