The trick to improving the efficiency of a car is to reduce its mass and make it more aerodynamic. VW has been working on a vehicle that was once called the 1 liter, but has modified into something that might attract normal buyers ... that is if it was affordable. The XL1 is more of a technology demonstrator that VW is using to learn and gain experience with. Lots of exotic design and materials. Wired has a test.
Edison2 is an attempt at something more practical than the XL1. The Very Light Car is built with inexpensive materials using onventional fabrication techniques.
Moving people towards seriously more efficient vehicles will probably be very difficult unless the costs are much lower than conventional vehicles - something that seems unlikely.