I tried this method and feel although you get an idea of what your data is showing, its not a true value. another way is to take those same polygon and construct a raster file (and you don't need to convert polygons to raster first). First thing you need to do is calculate the Euclidean Distance for your polygon layers (Each layer/shapefile will need its own euclidean distance calculated). This tool (ArcMap 10) can be found under spatial Analyst tools --> Distance --> Euclidean Distance. Once you have all the need layers you can use the raster calculator (Spatial Analyst tools --> Map Algebra --> Raster Calculate) to merge the rasters into one file. The raster will be auto set to 'black to white' color ramp but one can change to "Red to green' or any other chosen ramp. I provided images before and after.
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