I wasn't at today's event so cannot comment on the arrangements, but the most recent airshow I attended was at Biggin Hill which had a large crowd and no traffic issues.
The traffic was moved via a feeder lane onto the airfield estate where a long winding route ensured that there was no tailback onto the main road. The cars were parked very efficiently by marshals onto several acres of land. Once parked up the visitors could collect their belongings and go through the entrance to the show where tickets were checked. I suspect the problems at today's event might have been due to tickets being checked on entry which would inevitably lead to congestion on the main road.