For a start - you can't guarentee that joe public will react the same way to a cessna landing on the road infront/ontop of them - likely to cause chaos, and serious injury/death etc.
Also - PFLs are one thing - when the donkey stops for real, it may not be within gliding distance of a hard surface, and you can't push the glide. Also, the motorway/road may not be in the appropriate orientation for the prevaling wind on the day - landing into a field into the wind, or on a road (which might be busy or not quite as long and straight as you thought) with a tail wind? I know which I'd take!
Fields are pretty much availiable everywhere, so practicing into them is a good plan.