Miles-in-trail describes the number of miles required between aircraft departing an airport, over a fix, at an altitude, thru a sector, or on a specific route.
MIT is used to apportion traffic into a manageable flow, as well as to provide space for additional traffic (merging or departing) to enter the flow of traffic. Normally MIT is implemented in response to a specific situation.
For example, standard separation between aircraft in the enroute environment is five nautical miles. During a weather event, this separation may increase significantly.
Many delays are directly attributable to MIT. For example, if traffic management has implemented a 30 MIT restriction on aircraft departing BWI/DCA/IAD via J75 that will affect 10 aircraft in a one-hour timeframe, the delays encountered from this restriction would be considerable.
The FAA continues to evaluate MIT and its impact on delays. FAA facilities are expected to provide justification for any requested MIT.
A variation on MIT is minutes-in-trail (MINIT). Minutes-in-trail describe the minutes needed between successive aircraft. It is normally used when aircraft are operating in a non-radar environment or transitioning to/from a non-radar environment. It may also be used if additional spacing is required due to aircraft deviating around weather.
Delays resulting from both MIT and MINIT are normally manifested as departure delays. However, these restrictions can also be put into place after departure, resulting in speed restrictions and possible airborne holding.