I believe that the bottom line is this. If you start doing practice PFL's on roads then you run a very major risk of causing traffic chaos or even a bad accident, even if you only go down to 300ft.
And don't forget that you might be invited for tea & biccies with the nice men from the CAA if you do PFLs down to 300ft near a motorway, as you'll be in breach of Rule 5 if a car comes along (which can often happen on roads...)
People have been known to put down on a freeway in the US but personally I wouldn't try it in the UK unless the motorway was closed or under construction (and then only the latter if there's a nice clear - and finished- stretch). Not very likely, so go for a decent field. Usually quite a few of them around, unless you're flying over water or mountainous terrain...
When I was training in Florida the instructor always told me to head for a road. They were invariably straight, long and empty. And plenty to choose from. In the UK there may be plenty to choose from but 99% of the time the other two factors don't apply....
