Probably the single biggest thing that impairs our ability to have energy efficiency in transportation is that the vast majority of the cars on the road at any given moment are far, far, far too much car for what they're carrying. On a typical day driving down the road, if you look into the cars you're passing or being passed by, almost every one of them will have a single person and very little cargo. A car designed for that situation could get 75mpg without any new technology, but no one would buy it.
Consider whatever luggage you have, if you have any. Most people who have luggage have more than one suitcase: one smaller one of a size suitable for a weekend trip, a larger one for a week-long trip, maybe a backpack for overnights, and they can probably use several bags for even longer trips. But cars are too expensive to have several of them for different uses. So even if 99% of the time there's just one person and little or no cargo in the car and you're driving on flat, clear roads, you still have to buy a car big enough for four people, a load of groceries, your luggage, the one time you need to go pick up a really big box at Home Depot, the time you're climbing a hill in the winter, maybe even the time you need to haul a load of gravel or hook up a tow chain.
What's worse, though, is this. Suppose General Motors called up and gave you, as part of some research study, an experimental one-person car, just big enough to hold you and the stuff you need to bring to work every day or a couple of bags of groceries. This car is as safe as any other small car (in reality safety is an issue here but let's ignore it for now), and gets 75 miles to the gallon. Would you use it every day on the way to work? Maybe. My guess is only about half of people would. The other half would be afraid they might need to make an unplanned stop, or give someone a ride, or something, so they wouldn't use it most of the time in service of being prepared for the unforeseen.
And unfortunately I can't entirely find fault with that thinking. I myself routinely carry around a bunch of things I probably won't need 99% of the time so I'll have it that one time I need it. The unlikely happens all the time, because something unlikely to happen today is sure to happen eventually, and because there are so many unlikely things that could happen that it's likely one of them will.
So while it might make sense for an individual to have a pickup truck that gets 10mpg even though most of the time he's got nothing he needs in the back and no one in the passenger seat, when you view this from the point of view of the entire society deciding how to use its resources, we're wasting billions of dollars and countless pounds of atmospheric carbon to move around hunks of unused metal millions of miles a day for no reason. I don't think I've ever heard of any inventor working on a revolutionary new approach to transportation who's even really tried to address this dichotomy.