A14 toll road between Huntingdon and Cambridge outlined
A senior government minister has said he could not give "any guarantees" about the price of a road toll proposed for the A14 in the east of England.
The Highways Agency proposes cars pay up to £1.50 with lorries double that.
It is currently consulting on the plans for a stretch of the A14 through Cambridgeshire.
Roads minister Stephen Hammond said the charges had yet to be finalised and could change before the expected opening of the road by 2020.
"No one can give you any guarantees about six years hence," he said.
The Highways Agency is asking the public what they think about plans for the £1.5bn scheme between Cambridge and Huntingdon.
The plan put forward is for a new stretch of A14 between Ellington and Swavesey and a widening of the A1 between Brampton and Alconbury.
As a result, the bridge over the mainline railway, close to Huntingdon Station, will be demolished.
About 20% of the cost of the improvements - £300m - is expected to be raised by tolling a 12-mile (19km) stretch.
Ian Parker, from the Highways Agency, said: "The scheme is about keeping the right traffic on the right roads."
He said "strategic traffic" would be expected to use the toll road, with new "local access roads" built for traffic commuting in and out of Cambridge and Huntingdon.
Reaction has been mixed, with some drivers saying £1 or £1.50 was not much to pay if improvements were significant.
However, the Automobile Association (AA), said tolling the road when there were limited alternative free routes, was "a double whammy" and "a very unpleasant sting in the tail" for motorists already paying taxes.
Mr Hammond said he did not think a lack of alternative routes would be "an issue" in Cambridgeshire.
"This is a vital piece of infrastructure for HGVs," he said.
"It will reduce their journey time and overall will balance out any cost of the tolling.
"I accept that if you look at the alternative routes at the moment one of them might be through Huntingdon, one of them might be along the A428 around St Neots, and up the A1."
Linda Sewell, whose Brampton farm borders the A14, said she was concerned about the loss of valuable farmland.
"It's not the land they use to build the road, it's what they waste at the sides," she said.
"As we hear every day, our population's rising and we've got to be fed, yet they keep covering everything with concrete."
Asked whether the toll charge would be dropped once £300m had been raised, Mr Hammond said there was no way of telling at the moment when that might happen.
"There are always on-going maintenance costs and we also have not put in place what the level is to be, and who is to administer it," he said.
"I am not going to prejudice a decision about when it may stop."
Neil Greig, director of policy and research at the Institute of Advanced Motorists, welcomed the improvements to "one of the busiest and most congested roads in England".
But he added: "We have major concerns about the impact of tolling such a key east-west artery.
"With the current high cost of motoring a major concern for drivers, being asked to pay more for the promise of lower journey times and a better service are unlikely to be enough for tolling to be welcomed by many."