I agree and disagree.

I understand the points made about ratios and likelihood of getting on a certain service rather than another, but I think there's also another side to it. I speak from my own experience, and from things I have witnessed in the past.

Chester Road in Sunderland is another example. Up until the cutting-off point (at the Royal Hospital), there are 17 bus services - demonstrated by the timetable shown here. Admittedly, some of those are peak time services only, but a great deal aren't. From experience, I can't say that passengers alighting at the bus stop given on that timetable show a great deal about customer loyalty to a service, its succession, or anything like that. With my example, I can't make a correlation between frequency - varying from every 10 minutes to every 30 minutes - and succession rate.

Further up the road, there are 9 bus services - again shown in the timetable here. The same thing applies.

I understand your point that, if a journey is unplanned/unscheduled, a customer is most likely to get the next bus that turns up that goes to the place they want to go. The exception? If the infrequent bus due after it manages to get to the other place first. Your example of Newcastle Road is a prime example of what I'm talking about - see here for timetable, you can see journey times. The 35 takes 17 minutes to get to Park Lane from Newcastle Road. The 9, E2 and X3 take between 10 and 12 minutes, but in my experience, can get there even sooner than that. If I miss the infrequent 9 service and a 35 is due in 5 minutes or so, with an X3 due 10 minutes after that, chances are, I'll just wait for the X3. From knowledge of the service, I know it can often get to Fulwell a few minutes early, and can get to Park Lane sooner than it is scheduled to arrive. I'm saving all of a minute. People may not want to do that because they know the 35 has arrived whereas there is always the chance the X3 will be late, and as such, the 35 will arrive before the X3, but I've always done it that way.

If planning my journey, I'll always choose the fastest option available and plan my journey around that - providing that the fastest option can still get me there before the slower option including waiting time at a given place! The 21/X21 is a prime example of that to Chester-le-Street or Durham.