The reality is simple. We live in a world where resources are scarce. Imagine any resource: a short-cut to your work, a city everyone wants to access, an instructor everyone wants to take a course with. The same way we want these resources, others do too. 

These resources, that are basically “service systems,” can be divided into two types: those where the lack of capacity means that some users don’t have access to the service (access to a course, or a flight), and those where the lack of capacity means that the service becomes slower or users experience delays. Roads and transportation systems that are road-dependent (e.g., cars, buses, and taxis) are of the latter type.