The ones on flat beds are normally for your tv programmes, where acting is involved, so you'll still get the road bumps and car movements, but they'll normally give it away by taking their eyes off the road for long enough to mow down a load of pedestrians on a zebra crossing.
Otherwise cars will be camera'd up at different positions to give loads of different shots, ie: Top Gear is a perfect example, and more than likely have a camera vehicle shadowing them.
The programmes you see about dangerous or learner drivers are certainly real, and if you see a camera car in front or behind them then avoid getting too close at all costs.
I'm a terrible passenger in a car these days, and i'm actually getting worse as time goes on, as i can't believe how blase people are when driving a car and to me don't seem to see even 50% of the dangers that i'm looking at.
I'd rather sit in the back and have the "out of sight, out of mind" attitude 'cos some drivers simply freak me out with their lack of awareness on the road.