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 highways. A 126 km long transportation network in a 201 km2 area of humid-tropical, mountainous, forested terrain in Puerto Rico was used in conjunction with a series of 20 buffer (disturbance) zones varying from 5 to 400 m in length, measured perpendicular to the highways. Average landslide frequency in the study area at distances greater than 85 m from roads was about six landslides per square kilometre. At distances of 85 m or less on either side of a highway, landslide frequency was about 30 landslides per square kilometre. On average, this elevated disturbance rate affected 330 m2 km−2 a−1 within the 170 m swath. The mass-wasting rate outside of the disturbance zone affected 40 m2 km−2 a−1. These results indicate that the rate of mass-wasting disturbance is increased from five to eight times in a 170 m wide swath along road corridors. The lateral extent of the environmental impact of roads in the study area is greater than is commonly perceived. The approach described herein demonstrates a simple method to assess the spatial association of mass-wasting with highways. © 1997 by John Wiley & Sons, Ltd.