Distribution of census tract mean Road Terrain Ruggedness Index values

The Road Terrain Ruggedness Index (Road TRI) of grid cells was calculated using the method outlined in Riley et al. (1999) and only grid cells with a road passing through them as an input. This figure graphs the mean (average) Road TRI value, in meters, of grid cells along roads for the 72,765 land-based U.S. census tracts in 2010. The thresholds between Road Ruggedness Scale (RRS) categories were determined using the head/tail breaks method in the "ClassInt" package in R with a threshold of 0.39.