Idiots who slow down to gawk or stare at minor traffic accidents, pulled over drivers, or people with car trouble on the highway or roads. Rubbernecking causes traffic jams and more accidents due to drivers distracting themselves from the road.
The act of looking at something as you drive by. People do it when they come past an accident. Also tourists do it when they are observing the scenery when they are driving. It quite often causes traffic to slow down because the person looking generally slows down a bit to look out their window or windshield.
"That tourist in front of us is just rubber necking and causing this traffic jam."
"If that guy hadn't been rubber necking I wouldn't have been late for work."
when all of the sudden traffic gets really heavy and there is no reason for there to be so much traffic until you move up enough to see an accident. all the cars are slowing down and rubber necking to see the car accident which causes another accident and more rubbernecking and so on.