Realistically I don't think theres much they can do, whilst the barriers don't help they are not the root cause of all problems, When there are 1000's of cars all leaving at the the same time there will always be congestion, I don't think you can say 4 barriers cause it because there are ultimately only 2 lanes out of the park.
Often after football I wait at least an hour to get out of the car park and thats without barriers!