It's worse than I thought.
"A report published in November 2016 by the U.S. Department of Transportation’s Federal Motor Carrier Safety Administration, which analyzed data from 2004 to 2013, found that fires in motor coaches (defined as buses designed for long-distance passenger transportation) occurred almost daily on average, while fires in school buses occurred more than daily for a combined average of over 550 each year."
Note that this does not include transit buses!
For example from a few weeks ago:
https://abc7news.com/traffic/all-lanes-on-i-580-in-oakland-open-after-greyhound-bus-fire/4809293/