Philadelphia is blessed by several feet of sub-street layers (stone fill, belgian block, concrete backfill, and terrible asphalt), embedded rail, pipes, and utilities that are all owned and managed by different local, state, and private entities. Oh and fairly wide temperature swings throughout the year, generous precipitation, salt, and let's not forget the drivers themselves. It's a miracle the roads are in as good a shape as they are - but it does have a traffic calming effect :)