Years ago I worked at Denver Center. We began using a metering program that assigned crossing times at fixes about 150 miles from the airport. We worked a "4 post", so the computer would look at traffic over Arizona, Utah, Nebraska and Kansas and set up the arrival order. As a result, the aircraft arriving from, say, the southeast might be "following" an aircraft from the northwest.
See how it works?
Frequently pilots, when slowed to meet a metering time, would ask, "Where's my traffic?" Try to explain to a guy in western Nebraska that his traffic is just east of Salt Lake City, Utah.
The better informed the pilot is, the more cooperative he'll be - usually - but some situations are too complicated to explain on frequency.
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