Most people seem to come here on guided bus tours. For independent travellers, blue songthaews run through the Golden Triangle from Mae Sai 45 min, 40 baht to Chiang Saen 15 min, 20 baht and back again every 20 minutes or so until around 15:00. If you miss the last songthaew, you will be at the mercy of the tuk-tuk and motorcycle taxi drivers that will charge around 70 baht for the 10km trip from the Golden Triangle to Chiang Saen.
