Methodology and Interpretation

Road density (km road/km2)
The density of roads is calculated by summing the length of roads and dividing by the area of the reporting unit. Values are reported as km of all road types (i.e., freeways, highways, surface streets, rural routes, etc.) per km2. High total road densities are generally well correlated with high human population and urban development. Roads increase the impermeability of land surfaces, may increase the amount of runoff to streams and lakes, and potentially increase the transport of road salts or other chemicals from paved surfaces (e.g., trace metals and hydrocarbons). Roads also fragment habitat and may act as barriers to animals (e.g., amphibians or large mammals).

Quantile: Each class contains an approximately equal number (count) of features. A quantile classification is well-suited to linearly distributed data. Because features are grouped by the number within each class, the resulting map can be misleading, in that similar features can be separated into adjacent classes, or features with widely different values can be lumped into the same class. This distortion can be minimized by increasing the number of classes.

Metric input GIS data:

• United States Hydrologic Units (8-digit HUCs) - Metadata

• United States Coastal Change Analysis Program (CCAP) - Metadata

• United States Census 2000 (c2k) - Metadata

• United States Roads (Wessex / GDT) - Metadata