The historical challenge with propane and Natural gas was that you had to get the tanks re-tested every five years.... at a cost of $3000 - $4000. This seems to negate the savings. Furthermore the LPG industry kept raising the safety/redunancy bar so it became harder and costlier to get the vehicle's propane system serviced (many tech got frustrated with the regs and let their LPG tickets lapse).
I am told that the new generation of CNG tanks won't need re-testing. But I will bet that CNG will start off as "worry-free" and every few years there will be additional stipulations and regulations... stuff that makes maintaining your vehicle a real pain in the neck.
BTW I understand the majority of Southern California's transit service is via CNG powered buses (initiated by California emissions regulations)
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