Peter Chant wrote: "I don't understand Priuses, as a technology demonstrator they are interesting but from what I have seen they are not as efficient as a diesel."
I drive turbodiesels all the time in Europe.
Power is not their strong point. As a breed, they are way less responsive than the Prius. Their driving characteristics are not well-matched to American tastes, at least for passenger cars. Truckers love them, of course, but then again they don't mind constantly sawing their way through 13-speed transmissions to keep their engines in a driveable power-band.
Add to that: costlier fuel (although that wasn't always the case), less-convenient fuel availability, poorer cold-running performance, narrower power-band, more noise, more vibration, more weight and intractable emissions issues... and it's easy to see why diesels have fallen out of favor in recent years in the U.S., especially in areas with tough pollution standards.
Add to that GM's disastrous late-Seventies adventure in marketing passenger-car diesels, and there is outright antipathy towards this type of engine here among many folks with memories of those days.
Diesel engines do tend to be reliable and last a long time, though, since diesel fuel is a lubricant as opposed to gasoline being a solvent, and the engines must be hunky-built to withstand the high compression ratios necessary for sparkless ignition. However, the rest of the car might be another story. I've had a Renault turbodiesel clutch linkage disintegrate on me, and last month a brand-new current-model 2006 VW Passat's emergency brake jammed, nearly necessitating a tow. That Passat , incidentally, also had the single worst turbo-lag I've ever endured, which was a real disappointment as the last turbodiesel Passat I'd driven, in 2005, was pretty good in that respect. In any case, while diesel engines can be long-lived, you tend to see fewer really high-mileage cars in Europe than in the U.S.. The rest of the car falls apart just as much, or it fails to meet stringent annual inspections.
As to the Prius being a "technology demonstrator," I suspect you are uninformed. A common misconception about the Prius is that it is "complicated," perhaps a view you've inadvertently bought-into. I believe it stems from the inability of most folks (and certainly most idiot journalists) to wrap their minds around how planetary gearsets work, even though they're at the heart of virtually every automatic transmission and differential manufactured in the past century. Toyota's brilliant new use for that simple and proven assembly means: no transmission (not even a CVT, contrary to much inaccurate reportage), no clutches or torque converter, no synchros, no hydraulics, no linkages, no throw-out bearing, no shifter, no starter, no alternator, no alternator belt or idler, no power steering pump, no pressurized-hydraulics plumbing. It's all replaced by the two brushless permanent-magnet motors and the planetary thing. And a honkin' battery, of course. (All of which is warranted for 100,000 miles, more in some states.) I'd personally say it all makes the Prius quite a bit simpler than the typical car. And they're holding up well, according to Consumer Reports and others. Forbes just last week ranked the Prius as one of the ten most reliable cars sold in the U.S.
"Technology Demonstrator"? All technologies should demonstrate so well, be so simple, and operate so efficiently and reliably with so few compromises.