The crawl can be explained without invoking pleasure at all.

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.