If you have more people who want to ride a bus than the bus can carry, then you need another bus and driver. That is a step function that scales with the number

If you have more people who want to ride a bus than the bus can carry, then you need another bus and driver. That is a step function that scales with the number of riders. What you said is only true until the bus reaches capacity, but if people actually use the transit (which is not the case in most of the US, but is the case in the area my numbers are from), then the cost is directly related to the number of riders. Also, most buses don't have 5 minute headways, so if you're going to assume that you may as well assume that every car contains 4 people. Additionally, a 100 person articulated bus coming every 5 minutes is equivalent to 20 single occupancy cars per minute (or 1 car every 3 seconds), which is typical for a busy street. You need really short headways to match a car lane, shorter headways to beat it, and even shorter headways to justify making the lane exclusive to buses.