The bat bridges are only regularly used by 11 to 17 animals
Two bridges built to help bats' flight paths as part of a £54m bypass in Cornwall are costing up to £27,000 per animal, highways chiefs have admitted.
The two so-called "bat bridges" at Dobwalls, costing £300,000, were built so the animals could find roosts after hedges they had followed were removed.
The Highways Agency said they were used by between 11 and 17 animals each day.
It added that it was legally bound to protect endangered species and it was monitoring the bridges for five years.
The bridges, made out of steel wire and netting stretched in a V-shape, were built as part of the bypass project to stop the bats from becoming confused as a result of the hedges' removal.
Bats send out sound and move around following the echo sent back from structures on the ground.
The removal of features which the signals bounce off along routes they frequently use can confuse them.
The agency carried out a survey before work started on the road in 2008 which showed there were about 40 bats in the area.
But a survey earlier this year showed that 11 to 17 bats used the bridge each day, the agency said.
Eleven bats using the bridges equals costs of about £27,300 per mammal.
The Highways Agency said that although the total sounded like a lot of money per animal, it was a success in that the bridges successful in their purpose.
The agency added: "We are legally bound to protect endangered species such as bats.
"The project involves a five-year monitoring programme and the hope is that more bats will use the crossing in future years."