HOW WIDE IS A ROAD - THE ASSOCIATION OF ROADS AND MASS-WASTING IN A FORESTED MONTANE ENVIRONMENT

A spatial data base of 1609 landslides was analysed using a geographic information system to determine landslide frequency in relation to hi ghways. A 126 km long transportation network in a 201 km(2) area of hu mid-tropical, mountainous, forested terrain in Puerto Rico was used in conjunction with a series of 20 buffer (disturbance) zones varying fr om 5 to 400m in length, measured perpendicular to the highways. Averag e landslide frequency in the study area at distances greater than 85m from roads was about six landslides per square kilometre. At distances of 85m or less on either side of a highway, landslide frequency was a bout 30 landslides per square kilometre. On average, this elevated dis turbance rate affected 330 m(2) km(-2) a(-1) within the 170m swath. Th e mass-wasting rate outside of the disturbance zone affected 40 m(2) k m(-2) a(-1). These results indicate that the rate of mass-wasting dist urbance is increased from five to eight times in a 170m wide swath alo ng road corridors. The lateral extent of the environmental impact of r oads in the study area is greater than is commonly perceived. The appr oach described herein demonstrates a simple method to assess the spati al association of mass-wasting with highways. (C) 1997 by John Wiley & Sons, Ltd.