There are several busses from Phnom Penh to Sihanoukville. KHR 10'000 one way or KHR 18'000 return. The trip takes about 4 hours with a short stop in the middle. They show an Asian movie and play loud music, so it might be a good idea to bring along ear plugs. A 500 ml bottle of water is included.
Most roads are in a very bad condition, though upgrading is under way.
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