This is not my field but I remember there is something in the Road Traffic Act that prohibits you from causing an obstruction on the highway. Your local authority is probably the statutory highway authority and they should be able to guide you. They and the police also have powers to force you to carry out improvement.

If you go and look at other similar setups you may find that either there is no barrier (pay & display) or there is on site queuing. You are going to have to establish the traffic volumes, especially your peak volumes to allow you to design the scheme. You can do this by sitting there and logging them or you can use a datalogger.

If there are significant volumes or issues I would advise you to employ a consulting highway engineer to design the scheme.

We have barriers where I work (medium sized hospital in London) and they are sunken into the access road to allow a couple of vehicles in before they get to the barriers.
The biggest issue we have is pedestrians being hit by them. We now have lights on them and a sound warning when they are working. Make sure you have clearly defined alternative routes for people which are completely separate and away from the barriers.

Hope this helps

Risky