M.J. Bradley & Associates authored a report for The ABA Foundation, Natural Resources Defense Council, Reason Foundation, and Taxpayers for America that is the first cost and environmental efficiency study to compare alternative transportation options to the federal Essential Air Service Program. The report finds that it is possible to protect and expand rural mobility while reducing the financial burden on taxpayers and decreasing fuel consumption and emissions by using bus service to link rural airports to major hub airports.
A summary of the report can be read here. The full report is available here, along with the Appendix.