commuter belt
/kəˈmjuː.tə ˌbelt/noun
the area around a big city where many people who work in the city live
пригородная зона, «спальные» пригороды
Это скорее устойчивое сочетание, чем идиома.
“House prices in London’s commuter belt keep rising.”
“They moved to a village in the commuter belt.”
“Trains from the commuter belt are packed every morning.”