Serious worries were voiced by members of Preston Council at the city's planning committee on Monday.
Preston had been contacted for its opinion by Wyre Council, which will make the decision early next year.
But the city council, whose northern boundary borders Wyre has made its neighbour aware of strong objections.
Council leader Coun Ken Hudson led the attack on the Duchy/Royal Lancashire plans, pouring scorn on traffic management proposals, which, in a contingency situation would see traffic from Preston routed along the M55 to the Kirkham roundabout, Windy Harbour, Great Eccleston and St Michaels to the show site.
He described the traffic proposals as "an absolute joke" and predicted "the whole of north Preston will be jammed up."
Coun Hudson said it was crazy that the contingency route would go directly passed the Great Eccleston showground.
He added: "Who is going to go all that way round via Kirkham to get to Myerscough? They will put their sat navs on and go straight up the A6."
He also criticised the routing of traffic from east Lancashire via the rural roads between Longridge and Inglewhite, where he said there were many ultra-narrow roads.
He asked: "What if a tractor broke down on the road at Inglewhite? It would stop all the traffic and there would be a tailback as far as Blackburn" he said.
He also queried if Lancaashire police had agreed the traffic management plan.
He said the opening of the new Barton Grange garden centre nearby would add another one million visitors to the area.
Coun Alan Hackett said there was unfounded optimism over the traffic management plans.
He suggested there was an "it will be all right on the night attidude" before saying "It won't be!"
And he added: "We cannot have a National Exhibition Centre on the A6."
Coun Dave Hammond said the three days earmarked for the 2008 show would become two weeks as the showground would have to be set up and dismantled.
And he added that the 28 days extra for other shows in 2009 and 2010 would see activity through "half the summer" because of the time needed for setting up and dismantling.
He said: "I cannot see how the infrastructure of that area can cope."
Coun Alan Richardson said he had been involved in a traffic snarl-up when the show had been held at Ribchester, even though he was not going to the show.
He said: "I would hate that to happen in this area - and it will!"
Coun Neil Cartwright said there were too many flaws in the proposal.
Coun Bill Tyson said there were traffic problems all day in the villages north of Preston, and joked councillors needed helicopters to ferry them to meetings.
The city council planning committee voted unanimously to tell Wyre it objects to plans.
* Officials of Garstang and Great Eccleston Shows are still waiting for a response from the Queen and Prince Charles to letters sent to the Royals last week voicing concern at the impact of a move by the Royal Lancs to the Duchy estate at Myerscough.
* Several posters erected by commercial premises owners on the A6 who object to the Royal Lancs proposals, including one erected at Elite Motors, Garstang, have been stolen.