Traffic clearing the car park is going to be an issue, I think. Just how an annoying an issue remains to be seen. I don't think there's a stadium in the world where you can expect to drive off site without expecting some kind of delay!
I do think there's a problem that's going to make things a bit worse, though. The stadium event management plan said a new footpath would be constructed on the eastern side of the A16 - currently this is a grass verge and is impassable. Last time I was at the site, this footpath had not been constructed. The upshot of that is that lots of fans leaving the stadium will be using the pedestrian crossing to get to the footpath on the other side of the road. This crossing going red is going to cause traffic to back up onto the roundabout and clog it up. I actually saw this happen a year or so ago when a fault with the lights meant they were turning red without any pedestrians pressing the button - while I was sipping my coffee I could see there was room for perhaps three or four cars on the road before they'd start fouling up the roundabout, which was happening even under normal traffic conditions. I don't think you need to be a traffic expert like our own Dr Fox to appreciate that if traffic can't move freely through the roundabout, it's going to cause queues back into the car park.
The latest car park management plan I can find says larger fixtures may be subject to a phased release, so you would have to wait for a steward's say so before you're allowed to leave.
Shuttle bus - this was also stated to be happening in the event management plan, but there's been no news of it happening yet:
"On match days dedicated shuttle buses would be introduced for journeys between the town centre and the stadium with services commencing say 75 minutes before kick-off, with return services operating for a similar period after the match. The shuttle services would operate on a flexible basis with buses leaving as soon as they are full."
The plan suggested there would be the option to add shuttle bus passes to a season ticket package. I'm not sure, then, if we should read anything into the fact that this option wasn't offered this week?
(The event management plan I have is now seven years old - there could well be an updated version lurking out there!)