Any person/cargo goods has to reach its intended destination withing 20 minutes. Otherwise ti just won’t start going there.
In practice, if you have an oil well sending oil to the refinery, the time it takes to get there has to be under 20 minutes, including waiting times in the station , loading/unloading times and transit times. If it takes more tean 20 minutes, there will be no demand for that good and the line won’t start producing.