We coded all eight input variables and add them up.
For the influence of railroads, we created 2 km buffer to identify all areas that are within 2 km of railroads. We gave influence score of 8 if within 2 km buffer and 0 otherwise. For Major roads we created, 2 km, 2-15 km and greater than 15 km buffers. We converted the buffer file into raster and gave influence score of 8 if within 2 km buffer, 4 (2-15 km buffer) and 0 beyond the 15km buffer.This is repeated for each of the input layers.
Then we created the composite Human Influence Index (HII) by adding up all the input variable rasters.
HII = roads score + railroads score + rivers score + land cover score ...
The output file HII is the composite influence raster.
Please see http://sedac.ciesin.columbia.edu/data/collection/wildareas-v2/methods for further details.