Generally, aircraft want to fly on the most direct route between their points of departure and their destination except when severe weather phenomena are encountered.
However, because of conflicting requirements from many different users, it is usually not possible to establish the most direct route and therefore it is necessary to find a compromise between the different requirements and the offers.
As a result, air traffic control services are required to manage effectively the different requirements with the aim of satisfying the requirement in the best possible way.

When ATS routes are established, a protected airspace along each ATS route and a safe spacing between ATS routes shall be provided. The width of ATS route may be 5, 10, 15 or 20 NM or others depending on the available navigation aids or the applied air navigation systems. A reduction of route spacing may be applied, taking into consideration procedural or radar services and associated controller experience and workload.

Number of ATS Routes should be kept to a minimum but should be in line with the traffic requirements and in respect of ATM capacity and direct routing.

Although it is accepted that a large number of ATS routes can improve route capacity and can reduce sector capacity, it is also recognized that a large number of crossing points, especially in congested areas, need to apply more accurate separation and increase the controller workload.

Planners should optimize capacity by introducing new routes with as few crossing points as possible and these crossing points should be well clear.

When warranted by density, complexity or nature of the traffic, special routes should be established for use by low-level traffic, including helicopters. When determining the lateral spacing between such routes, the available navigational aids should be taken into account and the available navigation equipment carried on board helicopters.

Pre-defined routes of flight constitute one of the major elements used today to channel controlled air traffic mainly for the en-route using a network of ATS routes, but when necessary, in particular at the busier aerodromes, standard departures (SID) and standard arrivals (STAR) may also be established in accordance with provided guidelines.