FWIW diesel-powered vehicles are much cleaner now than they were in the '80s. Diesel fuel is now sulfur-free, and since 2004 progressively stiffer EPA regulations have reduced the NOx and particulate matter output of diesel engines by orders of magnitude. Unfortunately, though, "cleaner" in this case does not include a reduction in greenhouse gases - in fact, school bus engines of today spit out more greenhouse gases per mile than did buses of the 1980s. This is because the EPA diesel regulations limit permitted emissions based on horsepower-miles, so an engine with twice the horsepower (like today's bus engines compared with older engines) is permitted to emit twice as much junk. And since modern bus engines have much more horsepower, they emit much more greenhouse gases.