The U.K. Highways Agency plans to build a series of new "bat bridges" to help endangered bats fly across the busy A11 highway near Norfolk in eastern England without being killed by cars and trucks, but even though measures to protect bats from projects like this are required by law some politicians are balking at the cost.

Five previous bat bridge projects built by the agency over the past few years carried a price tag of nearly $800,000. Depending on the final project plans, the six new bridges along the A11—which will be built as part of a broader highway improvement construction plan—would cost a total of between $180,000 and $240,000. Last month House of Lords Peer Baron Marlesford called the bridges a ''waste of taxpayers' money" although the project has already been approved.

The bridges are actually wire and mesh structures that stretch across a road to guide bats "at the most important bat 'commuting routes' that cross the road in order to reduce the risk of bats being killed or injured by a vehicle," according to a prepared statement from a Highways Agency spokesperson. Bats can sense these wires when they echolocate, so they fly above the bridges, keeping them out of the way of traffic.

A statement released by the Bat Conservation Trust discusses why roads are dangerous to bats: "The construction of a road can sever the bats' natural flight path, isolating them from important areas, as they avoid lit areas or open spaces. In some cases, bats fly dangerously low and can be hit by traffic. Mitigation for the impacts of roads is therefore an essential part of helping to ensure the survival of our bat species." The Highways Agency says there are no official counts of bats dying because of traffic, since any carcasses would be too small, too easily carried away from the scene by the vehicles that kill them, or too quickly predated on to leave a record.

All 16 bat species in the UK are protected under the 1981 Wildlife and Countryside Act, which requires that any projects that would disturb the bats or their roosting habitats have a minimal impact. Six endangered bat species can be found in the Norfolk area: the Barbastelle (Barbastella barbastellus), common Pipistrelle (Pipistrellus pipistrellus), Leisler's (Nyctalus leisleri), brown long-eared (Plecotus auritus), Daubenton's (Myotis daubentonii), serotine (Eptesicus serotinus), and Soprano Pipstrelle (Pipistrellus pygmaeus). None of the six species is especially rare but all face shrinking populations and are protected at various levels across the European Union.

"We are legally bound to protect endangered species such as bats, and introducing bat bridges is one way of reducing disruption to their environment and flight paths when a road improvement is carried out," said the Highways Agency spokesperson. Last month, the agency published a review of the risks highway projects pose to bats and the methods that could be used to protect them, including the bat bridges. The agency says that the idea for bat bridges in the UK came from an earlier review conducted in 2003 by London-based Catherine Bickmore Associates, which looked at ways to protect bats along trunk road networks in Wales. The consultancy did not respond to requests for more information for this article.

The Bat Conservation Trust is not sold on the idea of bat bridges. "Unfortunately," their statement read, "there is currently a lack of evidence into whether bat bridges of the type used in the A11 development are effective. In light of this, we would always recommend that mitigation provided for such schemes is designed to not only deliver a benefit for the bat species involved, but also delivers for other species impacted by such developments." The trust is calling for further monitoring of existing bat bridges to see if they save lives and says it is working with the UK-based Institute of Lighting Professionals to develop bat-friendly roadside light systems.

In addition to the bat bridges, the Highways Agency will take other steps to protect bats during construction along the A11, including installing bat boxes to serve as artificial nests as well as reconnecting woodland areas, another way to ensure that bats are flying high above the road. The agency told me they will also "clear trees in spring and autumn to coincide with the least vulnerable stages of bats' lifecycle, therefore minimizing disturbance."

Photo 1: Bat bridge on the A38 Dobwalls Bypass in the UK by Mike Calder via Wikipedia. Photo 2: Daubenton's bat (Myotis daubentonii) by Gilles San Martin via Wikipedia. Used under Creative Commons license