@acsimpson - I think @jonty's suggestion was more about implementing a time delay than allowing particular vehicles based on a list of number plates. A bit like average speed cameras. Would that still require a change of law?
I suggested something similar to Midlothian Council when they last consulted on the A701. My suggestion was to put a "bus gate" at either end of Straiton. You'd be allowed to drive through one bus gate without any problem, but you wouldn't then be allowed to drive out the other end unless you waited (say) 30 minutes. That would maintain access to the retail park from both ends, but would force anyone driving straight through to take a different route.
I hope that made sense. There's probably a far easier solution using a simple bus gate in the right place, though.