I remember seeing something on TV about a French company making compressed-air vehicles. The 120PSI from your bus's Bendix compressor won't help much, because these vehicles' air reservoirs need several thousand PSI. Just like hydrogen-fueled cars or even fuel-cell vehicles, these seem to be just another way to defer or relocate the pollution resulting from their power generation. Whether you burn fuel inside a vehicle's own engine, or burn it in a power station to produce the electricity needed to power the air compressor that fills the air tanks, or expend more energy producing hydrogen gas than the gas itself contains, you need to look at the big picture to understand the total energy consumption, not just what is used on the vehicle itself.
Now, if the electricity were produced from "non-polluting" renewable energy (such as solar, hydro, tidal, wind, geothermal, etc.), there may be some ethical justification for such vehicles. Remember that compressed air, like electricity stored in batteries (or, dare I mention it, hydrogen itself), is only an energy storage medium, not an energy source per se. Even renewable energy sources have their share of secondary pollution from factors such as the construction of their equipment. There is no truly non-polluting energy source known to man.
I'll be curious what the effective MPG of an air vehicle actually is, after one has factored in all the hidden costs of its raw energy input. Just FYI, a human being on a bicycle has an energy consumption equivalent to about 1,700 MPG, making it the most efficient way to get from A to B, at least purely from an energy point of view!
There ain't no free lunch (TANFL), ever.
John, just finishing eating his free lunch . . .