There's an interesting corollary to this in the school bus world. Beginning in 2004, the EPA started imposing emissions standards on diesel engines and the standards have become increasingly stringent over the years. The standards govern the allowed amounts of NOx (nitrous oxides) and particulate matter to be emitted, but the units measured are per-horsepower-miles, meaning that an engine with twice the horsepower is allowed to emit twice the NOx and twice the particulate matter amounts, which has led to bus engines that have much more power than their counterparts from twenty years ago did - despite this added power being largely unnecessary for hauling kids around at relatively low speeds.

And importantly, the EPA diesel engine standards do not in any way govern CO2 output, so today's school bus fleet is emitting far more of it than twenty years ago.