Service interval
What a service interval is
A service interval is the gap in time before the next bus on the same route comes round to the same stop. It is expressed in phrases such as "roughly every 10 minutes through the middle of the day", and it is the basic frame on which a service schedule is built. Intervals are tightened and extra buses put on through the morning and evening commuting peaks, then widened during the daytime and at night, so even on a single route the interval shifts with the hour.
From the rider's side, the service interval is the plainest yardstick there is for how usable a route is. On a route with short intervals you can walk to the stop without looking up the timetable and the next bus will be along after a short wait. On a route with long intervals, by contrast, missing one bus costs you a long wait, so the smart way to use it is to check the times before you leave home.
Intervals can be thrown off
Because route buses run on ordinary roads, traffic signals, congestion and the accumulated time taken up by boarding and alighting can all stop them from holding the planned interval. When the bus in front falls behind, the widened gap behind it fills with waiting passengers, boarding takes longer still, and the delay swells: one link pulling the next. Where the chain ends up is bus bunching, with the following bus catching the one ahead. Our article on why buses run late sets out the whole mechanism behind delays.
What this means in practice is that "it is a route with a 10-minute interval, so a bus is bound to come if I wait 10 minutes" does not always hold. Acting on the assumption that there will be some wobble is the knack of getting along with buses.
Use it alongside approach information
Knowing the service interval pays off most when you pair it with approach information. Once you can see where the buses are running right now, you can read situations such as "the intervals have been thrown off and the next one is a while away" or "there is another bus just behind this one". It also opens the choice of letting a crowded leading bus go and taking a seat on the lightly loaded one right behind it.
The interval written into a timetable is only a planned figure. Take your bearings from the plan, then confirm how things actually stand with approach information. Once that two-stage habit is second nature, the time you spend waiting around at the stop for nothing drops sharply.