I think i've run in a bug : i pressed enter and the camera stopped following the bus, like i was back on the "scavenge" mode. I pressed the arrow keys and i could move the camera around. (the zombies and the bus were still moving) and when i brought it back to the bus and pressed enter, i controlled it again.
also the level up menu popping while you're driving cuts the "immersion"
The countdown doesn't trigger anything while in scavenge mode i think (i thought i would get found if i didn't go fast at first)
Otherwise, the driving, killing zombies and getting upgrades is fun, and catching the gears needed for upgrades gets tricky when you also need to drive between zombies walking over blood (they blend in well).