First, I am assuming you mean an embedded system like Cars Go Quiet, not a set of headphones optimised for a city bus. The latter would not be on topic here.
There's great information (albeit somewhat technical) contained in "Is active noise cancellation useful in vehicles" which may shed [eventually] the answer you are looking for, but I will try to explain the major caveats in simple terms here:
The problem is maintaining the correct anti phase relationship between what is likely the entire vehicle as a sounding board, and one or more "point source" generators (speakers).
The cancellation effect can even be done with light (lasers) MIT Destructive Interference
HOWEVER, there are some very key criteria. The energy must by culminated, focused, single-sourced, not combined with reflected waves, and ultimately maintain strict phase stability in the desired environment. This is almost never true with real light, and only marginally the case with a carefully benched and controlled split-mirror laser beam. Sound is a lot slower and simpler, and more stable as well. The multi-source and multi-phase issues are still a problem, and exponentiate depending on the volume of the space you want to control noise.
On a set of headphones, this is easily achieved, with only the space in the ear canal that needs to be carefully regulated.
On a car, this can be done to some extent, but will be most effective in the lower "exhaust rumble" frequencies.
On a bus, there is likely too much volume to address the multi-phase and multi-directional sound, as well as expect a single point source to address this volume effectively front-to-back.
The only simple solution for a bus is to wear a set of headphones.