brent Most municipal bus routes are changed at most twice a year. This leads to a sizable lag between demand changes and route changes. Switching between single and double is still the easiest thing for them to tune between those route changes. These changes might happen on a week to week basis, but not every day.
But it's also worth noting that every bus has to travel back and forth between the central depot at some point during the day. If all routes travel to and from the same depot on every repetition of the run, it's likely that at some point in that run you will see a bus that is mostly empty. This is a simple byproduct of people switching buses at the depot, not yet being picked up at the start of a run, or having all been dropped off at the end of the run. If there are routes that do not return to the depot until after several repetitions, you might also see a few stragglers taking the bus at a select time of the day back to the depot. You should also consider the fact that any bus in the fleet may be retasked to every route. So you might be seeing a bus that just finished a peak-hours run going back to the depot to switch off to a different route. You should also consider that something like a 4AM run might be made available for people who work night shifts or to serve as a "drunk bus" after last call. I would also say that 7AM Sunday isn't off-hours. People will be commuting to church or even to work as usual. You may also see that bus starting its run with the anticipation of having to pick up many more people later on in its scheduled repetitions. Observations like that are anecdotal at best and rarely reflect the efficiency of the system as a whole.
As for the conundrum, it's nothing really that complicated. Some people may very well opt to use public transit to reduce their environmental footprint, to reduce wear on their vehicle, or because they know that they should not be behind the wheel of a vehicle (e.g. out to a bar, anesthesia, etc.). Now if a majority of people would choose to start using public transit or car-pooling to cut down on their environmental impact, this would of course lessen the impact of their own vehicles, but it wouldn't eliminate it.
