Bearing in mind that, according to HT's answer, we wont' be seeing them anytime soon, I think the thing would be right if you could spawn x meters out of the end of the road. Wether that end be a town, a depot, an airfield OR the point where a well aimed bomb just fell. Road lines always between strategic points (i.e., non-linked roads not able to spawn any vehicle)
For instance, let's think of an airfield linked by 4 roads. It is not enough to bomb all roads to isolate the airfield. You will have to bomb them far enough to make the vehicle/supply convoys unable to spawn in the airfield's vincinity.
Roads would work as inst-o-matic transport system, given they are not cut in any point. You could spawn anywhere along an uncut road.
Railroads are different issue. You could think of it as automated supply lines. Rail yards would be the central depots, and trains would go regularly between strategic points, given railways are not cut.
You could supply strategic points either by train (automatic) or truck (manned) ....ehem <cough>...sorry, I think I am waaaay off track

As a resumče: I would LOVE this feature. <sigh>
Pepe