you know those diamond shaped signs people put in the back window of their cars?? they say things like 'baby on board' or 'princess on board' etc...
start off with a row of 3 cars at traffic lights, the first car is a citreon saxo with pink fluffy seat covers and theres a young girl at the wheel and as the camera goes back theres one of those signs with 'princess on board' on, the next car is a people carrier with a middle aged couple in the front and a child's seat with a kid in on the back seat and again as the camera goes back theres a sign with says 'child on board'. the third and last car is a crappy old brown mini with a scruffy man in, thick glasses, beard, parker coat, pack of haribo on the dash and when the camera goes back the punchline of the joke is the sign says 'pedo on board' !!