I live in California where the traffic is really bad sometimes. When I see a wreck slowing down the freeway, radio announcers blame 'rubberneckers'. But I don't think that's the problem at all.
Suppose you've got a freeway with three lanes in each direction. (Not uncommon here). Because of the wreck, one or two lanes are closed. Three lanes of 'rush hour' traffic have to funnel down to one lane, and that can back traffic up for a mile. I can't go any faster than the car in front of me, who can't go any faster because of the car in front of him, and so on, up to the wreck. When you finally get down to one lane, speed picks up again because there's no more merging.
Now, at this point, if people were 'rubbernecking', there'd be space between the cars. People would be going slower to look at the wreck. But they're not. They're still three or four feet behind the car in front. It's not 'rubbernecking' that's slowing down the freeway, it's one or two lanes being closed.
What does happen sometimes is that people coming the other way, at full speed, will look at the wreck and cause another accident on that side. That's rubbernecking, but that's not what slows down traffic behind the first wreck.