Check out page 49. The results, except for loose gravel, are pretty compelling, IMHO.
http://www.nhtsa.dot.gov/staticfiles/DOT/NHTSA/NRD/Multimedia/PDFs/VRTC/ca/capubs/NHTSAabsT4FinalRpt.pdf
If we are talking a 10 to 20% difference for a loaded vehicle, the difference is huge. Can a person...