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You can hitch just after [https://goo.gl/maps/Kwrmt33Mcm9Nxzhs8 Phrom Bayon Statue and roundabout] on the eastern side of Chroy Changvar Bridge, where National Road 6 starts. You can get here from Monivong Boulevard by bus 1 heading north (1500 riel, cash only, no change given). Make sure to make a sign as first of all you will have a lot of local traffic here, and secondly the roads later split. The NR6 continues northwest towards Siem Reap, but if you want to go to Kratie (Kracheh) and towards Laos you have to take NR7 which starts 70 km north from this spot.

From Monivong Boulevard take the public bus line 1 as far north as it goes which should be somewhere around [https://goo.gl/maps/3oVvGYxYRWLNsrdn9 Prek Pnov Market] and walk further north to find a good spot. There is a Sokimex petrol station with a lot of space for cars to stop. Alternatively take a tuk tuk just north of Chroy Changvar bridge [approx. 11.5872N, 104.91914E], which usually go as far as Udong (about 40km away).