Caravans and trailers are to be restricted to the inside lane on a section of the main holiday route into the South West.
There is concern crawling convoys could form
The government is to introduce the experimental restriction on a stretch of the M5 this summer.
The RAC says if the initiative creates smoother flowing traffic it will be a positive move forward.
But caravan site owners fear the rule will encourage holidaymakers to go the other parts of the country.
The AA says it has concerns about the way the scheme will be controlled.
The association says if lorries, trailers and caravans are forced into one lane, it could create queues 10 miles long, travelling at two miles an hour.
Some caravan owners are angry and say they are being singled out for special treatment.
Ian Gray, the proprietor of the Riverside Caravan Park in Plymouth, says modern cars and caravans rarely cause a problem on motorways.
He said: "Modern cars can pull vans at the speed of the motorway. It's only a problem occasionally on narrow country lanes.
"It's just one of the government's ideas, and they do get some funny ones.
"But if they go ahead and publicise that the restriction to caravans is only coming into the South West, it's bound to affect our trade.
"People may just decide to go to the New Forest, Scotland or elsewhere and that will be a bit devastating to us."
The Highways Agency says if the summer trial is deemed successful, it could be implemented for Bank Holidays and other weekends when heavy traffic is anticipated.