Clearance delivery is the position that issues route clearances to aircraft, typically before they commence taxiing. These contain details of the route that the aircraft is expected to fly after departure. Clearance delivery or, at busy airports, the traffic management coordinator (TMC) will, if necessary, coordinate with the en route center and national command center or flow control to obtain releases for aircraft. Often, however, such releases are given automatically or are controlled by local agreements allowing "free-flow" departures. When weather or extremely high demand for a certain airport or airspace becomes a factor, there may be ground "stops" (or "slot delays") or re-routes may be necessary to ensure the system does not get overloaded. The primary responsibility of clearance delivery is to ensure that the aircraft have the proper route and slot time. This information is also coordinated with the en route center and ground control in order to ensure that the aircraft reaches the runway in time to meet the slot time provided by the command center. At some airports, clearance delivery also plans aircraft push-backs and engine starts, in which case it is known as the ground movement planner (GMP): this position is particularly important at heavily congested airports to prevent taxiway and apron gridlock.