Sometimes they are. They also sometimes place smashed/mangled cars from real accidents on the side of the road with a message warning people to slow down.
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they are not real footage, they are exact re-enactments...some of the euro ones are as bad...
http://www.youtube.com/watch?v=a8QxZJZfU5Q
I've been told that the RTA (our traffic authority) gets the people who have actually lost family to be in the adds (they are pretty happy to help usually)