Enough so you can gradually slow down until they start to move without stopping yourself. Watch how big rig drivers drive in traffic where it is almost required that you don't keep repeatedly stopping.

Big rigs definitely stop in these situations. No matter how much space they try to leave, other drivers speed around them to fill it and they have to stop anyway.

Sometimes they do, more often than not they aren't stopping, and somehow they seem to do better time than me on top of it.