In the longer term the price of fuel makes a big difference to the types of cars people buy, which is why cars in the USA have historically been much less efficient than those in Europe. And fuel price does affect demand- DfT^» (Department for Transport - about)'s traffic growth forecasts take the cost of driving into account and are higher than they would have been had fuel duty not been frozen.

Energy storage is clearly needed to cope with variable wind generation,  but it shouldn't be assumed that hydrogen is the best way to do this. Electrolysis isn't particularly efficient, nor are current fuel cells, so you only get back 30% to 40% of the original energy. This might still make sense if the energy would otherwise be wasted,  but there are more efficient energy storage methods that are also more easily scaled up than electrolysers. Liquified air has been mentioned before here, there are schemes to use heavy weights pulled up and down in old mine shafts, pumped hydraulic fluids, compressed air, flow batteries, to name a few. There is a risk that lots of money gets spent on hydrogen schemes that turn out to be uneconomic compared to other technologies and more sophisticated demand management.

These systems don't need to be continuous,  because the vehicles would be equipped with batteries, and gaps would be expected even on motorways. So there is no particular reason why major A roads would not be equipped.

Using hydrogen in an internal combustion engine is even less efficient than using a fuel cell. You would be lucky to get 20% of your energy back.

Hydrogen  may have a role, but its fans seldom mention that ~60% electrolysis efficiency x ~60% fuel cell efficiency means you are losing nearly two thirds of your energy.  There has to be a lot of "spare " renewable energy around to make that make sense. The technology should improve,  but is it really going to get up to say 80% for  both steps so that overall losses are lower than what you end up with?

I quite agree Tony. Similar questions can be asked about biodiesel,  for which a large part of the 'waste oil' used in the UK^▸ (United Kingdom) is imported from the far East. Apart from the absurdity of transporting it all that way using marine diesel, if it were truly a waste product why isn't it used in the countries of origin,  where there are also diesel  vehicles? There are concerns that waste oil is being diverted from uses such as animal feed, and so driving up demand for virgin palm oil.

The ethanol being added to petrol is produced from things like sugar beet and corn,  so displacing food crops somewhere else, and using lots of diesel and fertiliser. But it is a nice subsidised  crop for European farmers,  so there is a lot of political support.

Quite. The point that is being missed is that Britain looks good on percentage reductions,  because we started with a high baseline, with electricity mostly generated by coal, and lots of energy intensive industries, fed by coal. So it isn't surprising that we can achieve large percentage reductions by offshoring manufacturing and shifting electricity production to gas. For countries that have a larger contribution from nuclear and hydro, they were already starting from a lower baseline,  so further reductions are harder to  make. As you point out, emissions per capita are the better indicator of our actual impact.

Who has claimed that man-made emissions are the only cause?

And why do you think it is ok to make rather snide personal comments about one particular individual?

It certainly makes sense for biomethane to be used as a source of energy. But that is achieved simply by adding it to the mains gas system,  where it displaces natural gas that would otherwise be burned by domestic and commercial users. However,  it doesn't necessarily follow that it makes sense to use it as a vehicle fuel instead of diesel.  The  CNG^▸ (Compressed Natural Gas) engines are spark ignition,  so a lot less energy efficient than diesel engines. Figures I've seen for buses suggest about 30% more energy is used, or more. This means that carbon savings are only achieved if biomethane is used- they can actually increase for CNG. So it might make more sense to use the biomethane to reduce CO2 from existing gas consumers,  and find something more energy efficient for buses.

Using renewable electricity to produce hydrogen (at best 60% efficiency?)for any combustion process is appalling inefficient. Unfortunately lobbyists focus solely on the renewable origin of the energy for their individual scheme,  and ignore the overall energy efficiency. But this matters enormously as soon  as you think about scaling supply up production to make a significant contribution at a national level. Renewable electricity delivers far more heat to the home if provided as electricity,  especially if used in a heat pump, than if expensively converted to hydrogen and burned.

Or you could turn over a huge part of the world's agricultural land to growing biofuels, or grow it in the rainforest (sustainably,  of course...). The prospectus would then have pictures of sunflowers and green leaves on it.

There is a very vocal hydrogen lobby, but as you point out, producing hydrogen from electrolysis (55 to 60%?)  to burn in internal combustion engines (30% to 40%) is a very wasteful use of renewable energy. Fuel cells aren't fantastic in efficiency terms (~60%?), however electrical traction is much more efficient than internal combustion (>90%) and has the benefit that regenerative braking can be used, recovering around 25% of the energy in congested traffic. Nonetheless,  turning electricity into hydrogen and back gives you maybe a third of what you started with,  compared with ~80% if you put it in a battery.

Producing hydrogen from electrolysis and then putting it into the gas mains to heat homes would also be wasteful. 100 units of renewable electricity give you say ~50 units of gas heating, compared with 100 as electrical heating,  or potentially 300 units of heat from a heat pump.

Nb - there is quite a range in quoted efficiency of fuel cells and electrolysis,  and the values vary according to whether net or gross calorific value is used, but the figures I've seen for current systems look more like 60% than 80% (net CV). I'd be interested if anyone has any reliable figures- manufacturers are rather coy about it.

Talking of energy storage, the BBC^» (British Broadcasting Corporation - home page) has this article about using liquified air. Airbus liquified when there is surplus wind power, then used to drive a turbine when  demand peaks.

https://www.bbc.co.uk/news/business-54841528

It claims an efficiency of 60 to 70%, which is better than using electrolysis to produce hydrogen and then a fuel cell to get electricity back.

The CO2 figures also reflect the greater use of electricity for passenger use compared with freight, which is mostly diesel.

Some of the more unusual  methods  being proposed to store surplus renewable  energy should help in that respect,  for example as liquified air that would drive a turbine and heavy weights pulled up and down mine shafts  by motor/generators.  However other methods like hydrogen electrolysis and large  flow batteries will have the problem you describe.