Effective? In a 5 minute burst just before 10.00 am this morning, 1 car, 20 motorcycles, 1 cyclo and 2 bicycles drove up street 63 in the wrong direction. In considering these figures one must note that there are regularly traffic police standing on the crossroad with Sihanouk Boulevard (there's a big tree giving ample shade), so there is some caution with drivers to disregard this sign. At the same time there were roughly 25 cars and 175 motorcycles using street 63 in the correct direction, which also makes it physically difficult to drive in the wrong direction.
Conclusion, this traffic sign is regarded as a indication, not as a directive. Law enforcement?