"Once it gets started it is self perpetuating. You can't go through faster than the cars ahead of you. The
accident scene is generally accompanied by the flashing lights of a police or sheriff's car, enough in itself
to cause the majority of drivers to slow down. Then all it takes is people exercising extra caution in an
area liable to have restricted traffic flow anyway. And even if there is epicaricacy in evidence, it need
only be by an extremely small minority to affect everybody's speed."
Quite. The Traffic Check Wallahs on my local radio station frequently make judgmental remarks about "onlooker delays" causing significant slowing of traffic. Apparently we are supposed to forge ahead, with callous disregard for the car in front of us that has just come to a halt, possibly because the driver didn't want to hit police officers or emergency medical personnel.
