Anyone who tries to predict oil usage in 50 years by assuming that there will not be drastic and dramatic changes in technology is totally nuts.
I seriously doubt that the government will allow every vehicle to carry a radioactive source among the general public.
Technology will improve, but not drastically change in 50 years. Nanotechnology has made drastic advances in that time, but when we are talking about applying force to move heavy objects, that realm moves much slower than that. Particularly when dealing with the general public’s use.
Until we have a better base for fuel/fertilizers/pharmaceuticals/plastics or something like them/road surfaces/and a host of other things, there will be demand for oil. If for nothing else than to grease the wheels, something which hasn't chnged since the wheel.
A tremendous amount of new technology is just old ideas in a new package.
A nuclear power plant is still boiling water to make steam to turn a wheel (a turbine, a more efficient wheel), to produce electricity.
Just a different 'fire' with more gadgetry, but the same basic concept.
Now, come up with something which will efficiently move along on its own most anywhere, with no moving parts, and you will be making progress, otherwise, you are just making the oxcart go in a different way.
That may happen in the next 50 years.
My grandfather saw from Orville and Wilbur Wright to the moon landings in his lifetime, and I have seen plenty of change since I was a child.
But remember, the more things change, the more they stay the same.
For now, we need today what we need today, and I and others will keep working to provide it.