The trucking industry looks set to enjoy three or four years of reasonable growth, according to Bruce Allen, professor emeritus of business economics and public policy at Wharton, and Dan Clark, president and general manager of transportation finance at GE Capital. But important challenges lie just ahead.Part 2 of this two-part podcast considers the looming transition in the fuel source from diesel to natural gas. This involves much higher costs for new equipment, but savings in fuel costs plus less environmental damage mean a relatively short payback period. By 2020 some 20% to 30% of the U.S. truck fleet may be running on some form of natural gas.