Merseyside police are pretty spot on at policing games in Liverpool. I don't have the same faith in other forces around the country.
Take into consideration, that both sets of fans have a minority of hot heads.
On derby day at Goodison, or Anfield, both Liverpool and Everton fans have their own pubs that they frequent, and for the most part, only a minority of away fans at either venue would come into contact with each other, as both sets of fans in the whole stay in their own pubs that they would use when they are playing at home - even when the fixture is an away ficture.
Ie: A liverpool fan that drinks in say, The Solly on a home game, would probably still drink there on an away game, and just walk down to Goodison. Same with the Everton fans who drink in say, the Winslow. They would just drink there on an away game, and just walk over the park etc.
Now can you imagine say 65-75,000 fans, all from Merseyside(well the vast majority anyway) all travelling the same rout, sharing the same transport, mixing in the same service stations, train stations, and sharing the same pubs, equally.
Then factor in that on the day, there will be a loser, who'll probably take umbridge with being goaded.
Now you will see why most of us don't want to meet Everton at Wembley, as the options for violence and general gobshitery, are just too numerous.
And thats not even touching on the transport logistics.
Twice the number of fans going from the same city on the same day, means that finding transport is twice as hard, and that susequently, the prices for said transport will also go up.
