Basically, transportation in the USA meets so many objectives or our society that it will be difficult to address the overall efficiciency without coming right up against social, economic and political problems.
We could make more efficient personal vehicles, but that might result in a rebound effect of more driving.
Just an example of what a sticky problem this can be. A hard lesson we learned from Hurricane Katrina is that people were not willing to leave their pets behind. One aspect of personal vehicles is the ability to take your dog or cat.
There are many other aspects of this problem, but you shouldn't assume that there will be an easy solution.