Check transit frequency on a route.
Frequency is best expressed as headway — the gap between services — because that is what a passenger experiences. Six an hour is a ten-minute headway, which is the threshold at which people stop consulting timetables and simply turn up; below that, the service starts to dictate their day. For a random arrival the expected wait is half the headway, so a ten-minute headway means a five-minute average wait while a 20-minute headway means ten. That non-linearity is why halving frequency feels much worse than half as good: you lose both the average wait and the freedom to travel without planning.
Transit Frequency
Headway = 60 / services per hour; daily departures = peak hours x peak rate + off-peak hours x off-peak rate; average wait = weighted headway / 2
Every 10 minutes or better. At that headway the expected wait is five minutes, which most passengers will accept without checking a timetable.
Because a passenger arriving at a random moment is equally likely to land anywhere in the gap between services, so the mean wait is the midpoint.