Downtowns usually represent the largest market for transit service, and often the only market that is large enough to support transit service. However, as cities grow, the amount of travel between non-downtown destinations grows as well.
In cities with grid-based road networks (for example, Phoenix and Houston), this demand is typically met by developing a grid-based transit network and growing it outward with demand. However, in most older urban areas, as well as in many newer ones, the road network is radial and not suitable for a grid-based transit network, or even a partial grid network. In these types of cities, most transit systems provide connections between non-downtown areas by developing a hub-and-spoke-type network in which service to non-downtown locations is provided through outlying transit hubs.