Since 2013 foreigners are allowed to cross the Thai-Myanmar boarder at Myawaddy.Traffic between Myawaddy and Hpa An goes alternatively (one day each) west (Myawaddy to Hpa An) and east (Hpa An to Myawaddy) as the road is too narrow to allow both direction driving. Try to obtain information on the driving directions beforehand and plan your border crossing accordingly.
The border closes at 18:00 Myanmar time and the immigration procedure is quite simple. Coming from Myanmar foreigners don't have to join the queues but can go right into the office while locals are queuing outside, and entering Thailand you can go straight to window number 4. Pick-ups and taxis are easily available on both sides.
The Myawaddy - Mae Sot border crossing is a moderately popular destination for tourists and expats in Thailand who want to make a "visa run" (ie, get a new Thai entry permit stamp by making a short visit to another country).
The time in Myawaddy is 30 minutes earlier than in Mae Sot, and the traffic (mostly) drives on the right which makes an interesting scene on the bridge where cars from both ends have to switch over to the other side.