Visual representation of airflow behind car

[landspeed]

This is a silly video I found on youtube:

http://www.youtube.com/watch?v=xFvx9dKE3tk

It shows a car with a blown head gasket (Nissan Maxima I think). The interesting
thing is you can see how the air is being dragged along behind it, almost
perfectly behind the trunk. This is despite the fact that the smoke is only
exiting from a small point on the exhaust.

This has given me an idea for aerodynamic testing - would it be possible to
get some kind of smoke generator, and mount it in different places while
driving along, and see how the air is flowing? (e.g. if you release the smoke
at the front of the car, you could see how the air flows over and behind
the car....)

Obviously you would need two cars, and it would be illegal, but so is engine
off coasting, and lots of other things that save on fuel!. You would have to
choose a day with no wind, and no cars in the local area.