General rule as I remember it from hauling lumber (in Michigan and California)
If your load extends beyond the back of your vehicle more than 2-4 feet you need to tie on a red flag. That's it.

The load issue doesn't seem to be addressed.
Consider my overweight Great Auk, 75 pounds. That's what a child weighs. A Metro that carries only two people should be able to easily carry this weight. This weight however is on the roof--there's a lot more leverage than if it was sitting in the passenger seat.

Stiffer shocks or springs help. This is what I had to do with my 87 Toyota Pick up truck. Test drove like a passenger car, put one hammer and a bag of nails in the bed and the back end dropped and slewed about (I exaggerate a bit but you get the idea). Adding some off-road helper springs and stiffer shocks steadied it right up.

What kind of car are we talking about?

: Unless local laws prevent it, no reason at all you can't transport
: one on the roof of the car. Have even seen a 4 man rowing shell
: on the roof a VW, now that looked really strange :)

: Bill H.