BioEd Online

A new type of road map, which highlights roadless areas by showing them as mountain peaks, could prove a valuable aid to conservationists — or even to hikers searching for remote corners of the globe.

Researchers can calculate a useful metric called 'roadless volume' from the peaks, which reflects the extent to which a given landscape is carved up by tarmac roads. It's novel in that it measures "the holes in the road network instead of the road network itself," says Raymond Watts of the US Geological Survey in Fort Collins, Colorado, lead author of the study, which is published in this week's Science¹.

Although the myriad effects of roads on the environment are still being determined, the results of numerous studies indicate that their impact is substantial and invariably negative. Animals such as amphibians, for example, can't cross dry, hot tarmac, so roads chop their habitat into ever-smaller fragments. Roads also transport chemical contaminants and foreign species into ecosystems.

For it to usefully inform policy decisions, however, conservationists need a 'road metric' that properly reflects these damaging effects.

Short cut

There are many problems with commonly used road metrics, says Watts. For example, most metrics don't tell you whether a road cuts straight through a particular area or goes around it: in the latter case its harmful effects may be much less.

Using digital maps, Watts and his team divided up the United States into about 10 billion squares, each measuring 30 metres across. For the corners of each square, they worked out the distance to the nearest road. This was then mapped as an 'elevation'. The higher the 'peak' in the computerized terrain, the greater the distance between that point and the nearest road.

If a new road cuts straight through the middle of an area of parkland it dramatically alters the peak on such a map. But if a new road runs close to an old one, a much smaller piece of the computer-generated hill is shaved off.

The volume beneath the terrain equals the 'roadless volume'. By dividing the average measure of this for each county by population, Watts and his colleagues generated a map of roadless volume per capita — a picture of user pressure on roads throughout the United States. The county with the worst record (the lowest per-capita roadless volume) was found to be Kings County in New York, with about 4,000 cubic metres per person.

Traffic report

Kent Cavender-Bares of the H. John Heinz III Center for Science, Economics and the Environment in Washington DC, who works on developing indicators to reflect the state of the environment, says Watts and his colleagues have come up with a "very elegant metric".

But, adds Cavender-Bares, it doesn't capture all the important information. He would like to see the team expand the method to take into account, for example, different traffic volumes on different types of road.

Whether or not the roadless volume metric proves useful globally will depend to a large extent on the availability and quality of road maps. "Any analysis like this one will only be as good as the underlying database," says Cavender-Bares. "I'd love to see this work stimulate a concerted effort to produce a world database on roads."