[Wardad]

Originally posted by geoschmo:
...It's quite reasonable actually. By concentrating the burning of fossil fuels to large electrical and H2 generation facilites you can retain stricter controls over the level of emmisions even if you don't reduce the overall use of fossil fuels...

Well said...
I'd like to add that the weight of the emission equipment is important in a moving vehicle. Moving this weight around adds the the combustion vehicles inefficency and results in more fuel burned.
A fixed location for emissions controls makes the weight a mute consideration. It also opens an opportunity for better controls.