I've got a compact folding bike that I take with me on public transportation if I have to or anywhere really since it folds into a compact carry on. I'm always in shape and would never want to go back to driving a car again unless I'm forced to. Great fitness, great spirit, very civilized, extremely frugal, the fastest and most pragmatic way of transport in the city. There's a social bonus as well: almost every day I have a chat with a stranger (yes, chicks too) who get impressed by the idea. Highly recommended.
That's what I use. It does not fold and is slower than most normal bicycles. But very light weight and it's mechanics are easy to understand and repair even for me.
What kind of speed can you attain on one of those? I have one with a 26" tire (mountain-bike size), because I've pretty tall, but I still haven't quite got the hang of riding it.
By the way: My recumbent bike has even smaller tires (20") but it is way faster. The size of the tires is not the limiting factor in unicycle speed --- it's the fact that you have to keep balance and input power using the same system.
Right, which is why my speed tends to very briefly approach terminal velocity.
I have a fixed-gear mountain bike with 700c x 42mm ("really big") tires, and that doesn't really affect speed much either. The wheels very clearly have a sort of flywheel effect, though: once I'm moving, they hold a lot of momentum. (It's excellent for commuting in snow, BTW.)
I chose the very basic I could find: a single speed Dahon Boardwalk. It has a steel frame so it's not that light, but I'm OK with the weight. The bike is simple so there's not much to break and for a year and a half of (ab)use it has been solid. Where I live it's mostly flat, so it may not be the best choice for hilly SF for example, although I hear fixies are popular there. Bike forums are best to discuss these matters.
Fixies are terrible for hilly areas, but they're trendy as hell ATM, so that probably explains it.
Still, they're nice: there's really not much to break (or spend time maintaining). They are also incredibly easy to keep stable on ice and snow, provided you have reasonable tires. If the wheels start to lose traction, the pedals lurch, and you reflexively stabilize them.
