More than 15,000 tonnes of extra carbon emissions were recorded just one year after a stretch of smart motorway was opened on the M3 in Surrey. The sudden rise was two thirds higher than had been initially forecast by experts.
Highways England said the rise was linked to an increase in Heavy Goods Vehicles using the road under its new guise.
However, experts have warned emissions are increasing at a time when the focus should be on “dramatically reducing them”.
Chris Todd, director of Transport Action Network, warned smart motorways would “create new traffic”, leading to increases in pollution.
He said: “New roads create new traffic and this increases emissions at the very time we need to be drastically reducing them.
READ MORE: Drivers have ‘little or no confidence’ in smart motorways
It told Transport Network: “This study found that in the first year the number of road users was lower than expected, but there was a higher number of heavy goods vehicles, which is a factor in calculating carbon emissions.
“This evaluation provides an early indication of the scheme’s outcomes and we will evaluate the longer-term effect before concluding the impacts of the scheme.
“We take our environmental responsibilities seriously and are proud of our track record of delivering vital road upgrades in a way that respects the environment.”
The findings were made by Highways England as part of a review into the M3 smart motorway one year after its launch.
Despite the environmental concerns, the new road was generally praised by Highways England with the road achieving many of its objectives.
It said the new smart motorway had “helped ease congestion”, particularly traffic heading to London in the mornings.
The report said journey times “have improved across all time periods” but “particularly in the morning”.
Highways England said “journeys are smoother” and were “more reliable” which has instilled confidence in road users.
It found the number of personal injury collisions reported on the road has also decreased.
The report said: “The annual average number of collisions on this stretch of motorway decreased and was lower than our estimation of what we would expect without the scheme being built.
“During the first 12 months of the smart motorway being open there were 36 personal injury collisions compared to an estimated 57-107 collisions without it.
“When considered in light of changes in the number of road users, the annual average rate of personal injury collisions has also reduced.
“In the context of other findings in this report, these are positive early signs.
“Traffic levels are set to increase in later years, however, and so results at the follow-up evaluation will be essential to check if this trend continues.”