OK I'm back and we got around on buses, boats, motos and tuktuks instead of renting a car. I wish I had rented a motorcycle but that wasn't on the cards.
As far as traffic goes, I found it to be fairly mellow in and between towns, even in Phnom Penh. Nothing aggressive or scary. The system is different to most places though, if you're on a road the traffic entering it from either side seems to have the right of way and won't stop or sometimes even look. But that's OK because everybody goes slow enough to just let the traffic flow through itself. If you're a pedestrian (or loose livestock) you just do the same.
I didn't see any police activity.
Thanks, everyone, for your thoughts!