Phill Have you considered a gate or gates? I made a set of simple softwood field gates a couple of years ago which cost very little, look fine and are a definite barrier. One of them is a 3ft pedestrian gate and the other 8ft wide.
Alternatively a drop down or hinged metal barrier might be expensive to buy but not too difficult to make perhaps even in timber.
Another idea is one length of alloy scaffold pipe just dropped loose on to a post or bracket at each side, they really are quite light and easily lifted off and laid to the side when you need access. You could paint or wrap yellow tape around to make it highly visible.
Maybe if you post a photo of your driveway some of the good people on here might come up with clever ideas.
Bob