Reduce what has to be displayed in the first instance. This can be achieved with careful map design. Plan routes that don't involve dense areas with many adjacent streets. Avoiding situations where many lines converge on the same place will help with this.
Remove anything that does not have a good chance of being seen from the driver's cab
Reduce level of detail on side streets/those not on the bus route.
Choose objects and splines carefully so that too many textures have to be loaded
Use smaller texture file sizes
Use fewer number of ground textures
Avoid curvy splines - consider chopping up splines (will increase tile load time), or convert to objects and reduce polys in blender (more effort)
Carefully plan AI buses so that not too many of them turn up in the same place at the same time