For a bus driver, you could measure productivity by how often the departures are on time, how much fuel is spent for a certain route, etc.

Some of these are out of the driver's hands of course. They can't control how much traffic there is, but someone else in the company can change the routes to avoid traffic, service more people in the same amount of time by optimizing where the stops are based on how many and what demographic lives in the immediate area, etc.

This, in turn, makes the productivity of the bus drivers increase because they now get more work (measured here as passenger-miles per hour) done.