30 seconds footage @25fps will be 750 images needed - @30fps 900 images gives average of 4.5 hours
So you'll need to set the camera to take a photo every 21 seconds or 18 seconds.
You also need to consider how fast the buses are moving.
The bus in a frame has to be identifiably the same bus as the bus in the previous frame, which, if it's had time to go 100 yards round a corner, won't be the case.
If the interval is too long, you'll get the buses vanishing from one location and appearing somewhere else, without any illusion of smooth but fast movement.
I'd suggest that you try out various intervals, taken over periods of half an hour or so, and see what they look like, before committing to the full 4-5 hours