I meant that a bike can go anywhere that a car can go, and then some. (Here in the U.S., at least, bikes are legal anywhere that cars are, except for most freeways, and even freeways must have a parallel road open to bikes before bikes can be banned from them. Similar remarks apply to walking, which is even more flexible than biking. Possibly other countries have more restrictions on bikes, but there are still plenty of places that bikes can go but cars can't.) So by flexibility, walking > biking > cars > trains > planes, with the last two reversed for long-distance travel in the U.S., and with planes moved up drastically for intercontinental travel and travel to remote regions.

Now, taking a bike on certain roads may not be safe, and it almost always will take more time, but I would count these issues separately from flexibility.