Every 15 minutes is a sweet spot as it gives people good connections with the national rail network from their local station...........A fifteen minute service interval means you should be able to make a reasonable connection, be it to local or more distant destinations.
Obviously a higher frequency would be even better..!
I'd regard every 20 minutes as a "turn up and go" rail service frequency but perhaps I'm in a minority there. Don't get me wrong here, I'm even more in favour of 4tph but if operationally, 3 tph can be achieved significantly earlier then I'd say such a line had achieved "metro service frequency".
A 20 minute frequency leads to timetable complications where it has to share a line / resource with a half hourly service. Not saying "good" or "bad" - just saying. Personally, I would love even an hourly service each way at my local station - but whilst we are part of the
WECA» commuter area, I can't really say "IN Bristol" which this thread is about.