About one-fifth of fuel-efficient U.K. carsʼ energy-saving benefits are eroded because motorists end up driving them more, a study finds.

It is the first estimation of the so-called rebound effect, a term used to describe the increased consumption of cheaper energy services, for car travel in the region.

Energy experts at the University of Sussex used data from 1970 to 2011 to find a long-term rebound effect among British drivers of about 20%.

The study, published in the journal Energy Economics, says nearly all previous studies have focused on U.S. motorists and so provide only limited insights for U.K. policymakers due to the differences in population densities and car use between the two countries.

Lee Stapleton, a research fellow in the universityʼs Center on Innovation and Energy Demand, says fuel-efficiency improvements should lead to less fuel consumption.

“But since improved fuel efficiency makes driving cheaper, some of the potential fuel savings are taken back through increased driving,” he says in a statement.

“We call this the rebound effect and it is well-documented in other sectors. For instance, we know that insulation of housing encourages people to enjoy warmer homes, rather than taking all the benefits in the form of lower (heating) bills.”

Until now, Stapleton says, the size of the rebound effect on U.K. motoring was not known.

Part of the reason reliable calculations have proved difficult is due to disagreements on the best way to measure the rebound effect.

In this latest study researchers found rebound estimates could be obtained by measuring the combined response to both improvements in fuel efficiency and reductions in fuel prices, as both make driving cheaper. But ideally, the researchers say, only the first of these would be used.

In an attempt to make their estimate as accurate as possible, the Sussex researchers tested more than 100 different models, each focusing on different variables. Their estimates of the rebound effect ranged from 9% to 36%. The average was 20%.