In Vancouver, the cost to operate regular buses, per revenue-hour (time the bus is carrying passengers) is ~$120/hour. The cost of operating shuttle buses is ~$70/hour. [1] This is because smaller buses use less gas, are cheaper to maintain, and have better labour utilization (Time the driver spends on the clock driving passengers, versus time the driver spends on the clock waiting around, switching routes, etc.)

And this is operational costs. Capital costs are another huge factor, that's not accounted for in the operational numbers. It costs a lot more to order an 80-passenger bus (And even more so for a trolley), then it does a 24-passenger one.

Also, this is only anecdotal, from being a rider, but I'm pretty sure that shuttle-buses both perform better in congested environments, and cause less traffic congestion (As they have a much better power-to-weight ratio, better acceleration and breaking, a smaller turning radius...)

[1] https://www.translink.ca/-/media/Documents/about_translink/g...

The capital costs of the bus isn't that much different over the lifespan of a bus. If you need a large bus at all, then it is cheaper to buy one large bus than both the large and small. Small buses cost less mostly because the drivers don't have to be as well trained - I'm not sure I want that difference.

This worked because shuttlebus service was slowly ramped up year over year.

The small buses are solely used for last mile suburban routes, the large buses for anything else.