Arlington County has an extensive network of bicycle infrastructure, including 50 miles of shared-use off-street trails. These multi-use trails are generally 10 feet wide with a solid yellow line striped down the middle to separate users.
Constructive mathematics. Idea. Broadly speaking, constructive mathematics is mathematics done without the principle of excluded middle (or other principles, such as the full axiom of choice, that imply it).
Sometimes it is possible to prove the existence of something mathematical without actually constructing it. Why would you want to do this? Well, it could be that you just cannot think of a constructive proof, or that a constructive proof is very long and tedious.