Does your transit agency have a recent history of operating speed that looks like this?
This is the “boiling frog” problem of bus operating speed. In a dense and growing city, it’s not unusual to see speeds falling by about 1% a year, as in this data for Portland’s busiest lines.
If you’re going to analyze how service levels relate to ridership, you have to think about speed. Speed is not just a disadvantage for the customer; slower speeds are also a service cut.
You, the customer, want to go a distance, but the transit agency will pay for your service by the hour. So the quantity of service you experience will be governed by how easily hours turn into distance — in other words, by speed.
So if a transit agency budget grows by 1% a year but speed is falling 1% a year, the customer should expect slower speed and no other growth in service.
When measuring the service quantity that affects ridership, then, look at service miles or km, not service hours.