When you're driving on the expressway and all of a sudden you come to a screetching halt because of traffic congestion that appears to come out of nowhere (no accident, no bottlenecking, no construction). So you then cruise at about 5 mph for a short while wondering what the hell the deal is. Then all of a sudden, the road clears up and you find yourself cruising back again at a normal speed.
I was drivin' on the Parkway goin' 75 mph then found myself slammin' on the brakes because of some friggin' unidentified traffic jam. A minute later the roadway cleared as if nothing happened.