Hondas, Nissans, Mazdas and Subarus all already drive themselves here in Japan. Government just finalising legal requirements and insurance requirements. Hydrogen is also going to be big here. All buses for 2020 Olympics will be hydrogen powered hybrids. Also they are utilising so much of the patches of grass next to railroads and putting solar panels in. Passing the other day there were kms of panels.
Hydrogen will lose the post-petrol war in the same way steam (and ironically electric) lost the war the first time around ~ 100 years ago
The biggest issue with hydrogen is it has a low energy density. It doesn't liquify under pressure, so to store a lot of it you need to hold it at a massive pressure. Higher the pressure the more steel you need in the tank, which means more weight, wiping out the gain from holding the extra fuel.
There's a few clever research concepts that could help. Bond hydrogen temporarily into a solid or absorb it into a nanoscale lattice. Either could do the job of the science works.
Then you get into production. Hydrogen currently is a byproduct of oil refining. Not the green fuel you'd be hoping for. To turn water into hydrogen at an industrial scale will require a lot of electricity. There are solar cells that create hydrogen from water rather than producing electricity, but they are a ways off being scaled up.
Hydrogen could theoretically replace petrol, but it's got a lot of catching up to do.