Engines are becoming less efficient.

Engines breathe better thanks to multi-valve technology and variable valve timing. This reduces pumping losses, enables fuel to burn more efficiently and thereby reduces pollution. At speed, internal mechanical frictional losses are relatively low compared with air drag. The upside of this is that modern streamlined cars travel at motorway speed with outstanding efficiency. Unfortunately it has become fashionable for roads to be designed to reduce traffic speed. This results in cars being driven in low gear and this increases mechanical frictional losses. The result? a considerable increase in carbon dioxide emissions.
Clearly this is not the way to comply with the Kyoto agreement. Roads will have to be redesigned to improve the efficiency of traffic using them. Perhaps a reversion to the transport philosophy used by the Romans 2,000 years ago?