Whatever happened to Compressed natural gas? most homes in/close to urban areas have piped-gas. There were compressors available to to pump it into your vehicle . As the gas came from your normal meter. I assume the road tax would have to be uplifted to compensate for the lost fuel-duty revenue.
That same situation also arises with electric cars. LPG road-fuel is taxed and I've tried to compare it with ordinary bottled propane. I conclude the bottled-gas industry is charging the same as road-fuel and making a good profit margin!
Converting big petrol-cars to run on petrol/lpg was quite popular in the 1960's but price of gas rose and it's only a cost-effective conversion for high mileage-users.
I see a huge problem in building crash-worthy Hydrogen vehicles(OK, only a minor problem in Orkney! )
Carbon/kevlar tanks have revolutionised the medical oxygen business, but Hydrogen is a different beast altogether and despite Bill's reassurances, I think the average Joe Public will struggle to safely connect/disconnect the fill apparatus. -therefore, like LPG it will remain a niche market.
Unless battery-prices reduce dramatically, Electric will remain the province of the above -average -income sector. The man on the Clapham omnibus will not be able to afford the capital costs -even on HP, and the present subsidy -regime is unsustainable. I continue to run "bangers" at under 20p a mile, all-in.