I do this only very occasionally when an opportunity arises and I can't pass it up. I don't worry about getting hit by another vehicle as you are moving about the same speed as traffic. I worry about hitting a pothole or junk in the road that I can't see due to the vehicle in front of me. Also, keep a good eye on traffic ahead if you can, and leave yourself some room to the side in case the car/truck slams on their brakes. I suppose another car or truck could pull out of a side street and not see you behind whatever you are drafting, but you would likely be close enough to the vehicle in front of you for this to be a non issue.

I did some real motorpacing behind a motorcycle with a roller bar long long ago as a junior. It is really fun and a great workout. I'd recommend it to anyone that has the opportunity.