3-wheelers will keep you dry and out of the wind (big advantages over a scooter). They also look cool, would be fun to drive, and can be fuel efficient, making them a good alternative to a big car for short commutes or hops to the grocery store, library, local brewpub/coffee shop.
You could have a bigger car, or maybe rent or share a bigger car, for longer trips with the family and friends.
I'm thinking something like what a meter maid drives, or the guys who pick up the trash (vehicles like a mini-pickup), with a nicely shaped light fiberglass or composite material body.
The challenge would be to design one that's relatively safe, comfortable, reliable, costs less than half what you'd expect to pay for the cheapest small car (a Chevy Aveo is $10,000), and gets significantly better gas mileage than a fuel sipper like a Toyoto Corolla that gets 30 mpg around town and 40 mpg on the highway.
Small and light doesn't necessarily correlate to good gas mileage.
People don't necessarily buy a motorcycle for fuel economy, but just as an example a 130 hp, 1200 cc, BMW K1200GT motorcycle is estimated at 32 mpg and a scooter like the 360 pound Honda Big Ruckus with a 250 cc engine only gets 58 mpg, plus the Big Ruckus MSRP is $5500.
3-wheelers.com is a web page with over 1000 pages of information on 3-wheelers. It's created by a gentleman from the U.K. by the name of Elvis Payne. It's got some funny pictures and a lot of historical information.
Bajaj 3-Wheelers has street legal vehicles for the U.S. market. Think how fun it would be to take your friends to lunch in this. These rigs aren't cheap at around $6500 a pop. I couldn't find the estimated miles per gallon, on the tech spec page, and I'm kind of guessing the reason is they aren't that impressive.
The California Commuter got 157 mpg on a trip from L.A. to San Francisco in 1980 and could cruise at 55 mph.
We've come a long way since 1980 with advances in composite materials and gas/electric hybrid technology. It will be great to see what smart entrepeneurial people come up with in the way of personal commuter vehicles in today's world.
A basic set of design criteria might be -
Fuel Efficient - at least 100 mpg in town
"Safe" - visible to other cars, good handling qualities, good brakes.
Low Price - less than $5000
Seats 2 (optionally could be a 1 seater...since the vast majority of cars are only carrying one person today)
Reliable - expected life of at least 5 years before major repair/replacement of parts.
Carrying capacity of 400 pounds
Top speed 45 mph and able to maintain 35 mph on hills fully loaded.
Ergonomically designed - easy to get in and out of and comfortable to ride in
DC power jacks and space for a nice portable sound system and a laptop or tablet computer for navigation/mapping/meet-ups etc.
Good coffee cup holders - holders deep and wide enough to hold a cup.
Various compartments/cubby holes for easy storage/retrival of papers, keys, change, makeup, sunglasses etc.
Fun! - easy to customize with lots of colors and various body shapes you could make or buy as after-market add-ons (rocket, fish, clown car, covered wagon, locomotive, tractor, horse, etc.., option to purchase the car without a body, or a blank-canvas body, for those who wanted to make, or paint, their own.
I'd go for designing a new around-town vehicle since a highway vehicle brings in design constaints that might cause you to end up back at a Corolla (not a bad car..but boring...it's already been done).
