Introduction

Roads are conspicuous and pervasive components of most landscapes throughout the world, occurring in both highly modified urban areas and relatively pristine areas of wilderness. Roads the world over are remarkably similar in both form and function and are designed to transport people and goods between places as quickly and efficiently as possible. Road networks are typically designed and classified according to a hierarchy that at one end allows for high mobility and limited local access (arterial roads, variably termed highway, freeway or motorway) to roads with low mobility but high local access (local roads) at the other extreme (Forman et al., 2002).

Roads and traffic have ecological and biological effects on adjacent habitats and biota that can extend for hundreds or thousands of metres from the road itself (Trombulak and Frissell, 2000; Forman et al., 2002). Estimates for the United States of America and the Netherlands indicate that about one-fifth of the surface area of both these countries is directly affected ecologically by roads and traffic (Reijnen et al., 1995; Forman, 2000). Our understanding of these impacts has been well summarised previously (Forman et al., 2002), and includes the loss and fragmentation of habitat; input of pollutants (e.g. noise, chemicals and dust) into adjacent air, soil, vegetation and water; direct mortality; and the creation of barriers to wildlife movement.