Institute of the Environment

Intensity of human use (IHU) is a conceptual geographic characteristic that describes an area’s rank on the continuum from high use (e.g., urban area or active strip mine) to low use (roadless wilderness). Customary measures of IHU, such as human population density or road density, lose their utility at the low-use end of the spectrum — and it is here that human activities may have their greatest ecological effect on some ecological resources, such as wildlife habitat. Conceptually, we suggest that IHU is determined by four factors: IHU=P*D*A/C, where A is human accessibility, P is the population of potential visitors, D is attraction to a destination, and C is the dilution effect of alternate destinations. In our vehicle-centric culture, roads are essential determinants of human accessibility. Each time a road is built or opened, some area surrounding the opened road becomes more accessible, and each time a road is closed or reclaimed some area becomes less accessible. Our modeling efforts have focused on small enough areas that factors P, D, and C are essentially constant. Our geographic information system (GIS) model of A expresses inaccessibility (roughly the reciprocal of A) as minimum travel time T(x, y) from a paved road. The model depends on three digital geographic descriptors: elevation, land cover, and transportation. Calculations derive from estimates of vehicular speed on unpaved roads and walking speeds off-road. At present, our model ignores alternate off-road transportation modes, such as horse, motorized dirt bike, or all-terrain vehicle (ATV), although these can be easily incorporated under the basic model structure.