In the scene where you see the Odoyle family driving down the road chanting Odoyle rules, they hit the banana peel that was thrown out earlier in the movie. If you look closely you can see when they go over the cliff there is only one person in the car.
I have to agree with you Kendyl! And if you're talking about paying stunt-men to be in that car, why don't you try to find a stunt man that will drive a car off of a 100 foot cliff. (It'll be tough:)
That clip was from an old sketch on Saturday Night Live, so I don't expect the Odoyle family to be in there. Remember, this movie wasn't up for any Academy Awards. It's just a goofy movie, and that's just an old clip of the car falling off a cliff that they used in a sketch that had a cat driving a car in it.
I'm pretty sure that was a clip from a skit on SNL called "Toonces The Driving Cat" and at the end Toonces always drove off a cliff. I think that's the whole point of the scene.