A compact bike is a portable bike, but the best of both worlds would be a bike that folds. If you're looking for a bike that folds you should know that there are a wide variety of them out there, each with their own unique attributes. Some bikes that fold are made to fold in such a way that they are ideal for storage and travel. Other bikes that fold are made so compact that they can be taken absolutely anywhere. If you own a bike that folds you no longer have any excuses not to ride a bike, as they have no limitations. "I can't take a bike to work." Sure you can, as some models of bicycles that fold get as compact as a brief case. Surely you can take a brief case to work? "I'm too old, my knees are bad." All bicycles that fold come in a variety of speeds, making them easy to pedal. "I'm too heavy, one of those bikes won't hold me." Unless you're an extreme case like John "Earthquake" Tenta of WWF fame, these bikes will support as much weight as a standard bike. "I'm too stupid to learn new things, let alone a million steps to fold a bike. I had Transformers as a kid, those things were impossible." Even the most mechanically uninclined person can fold one of these bikes. Generally, bikes that fold will fold down in anywhere from one to four quick, intuitive steps. If you can tie your shoes you can fold a bike, and often just as fast.
An example of a bike that folds in such a way that it is compact enough to be taken virtually anywhere would be the Brompton.
Some bikes that fold in a range of compactness between the Brompton and the Reach would include the line of Birdy bikes and the line of Dahon bikes. Bikes that fold in this range are suitable for storage and travel purposes and can be taken into most places. You'll find yourself using that feature quite often.
Bikes that fold are perfect for all areas. A bicycle that folds works great in congested city areas in New York and California where space is an issue. Bicycles that fold also come in handy in areas where space is not an issue. Residents of Florida or Texas might have plenty of garage space, but what about in the office? Where a regular bike is off limits, bicycles that fold show just how useful they can be. With a bicycle that folds you are never limited by your bike. In fact, the opposite is true: a bicycle that folds opens up a world of opportunity to you.