Bangkok is one of the few part of Thailand where the district boundaries are located within densely populated areas - only that in Bangkok the districts are called Khet and not Amphoe like in the other provinces. The city has erected at the larger streets at the district boundaries, thus these signs are a common sight in Bangkok.
The one I choose for showing here is located directly under Memorial Bridge and marks the boundary between Thonburi and Khlong San, both old and central districts of the former city and province of Thonburi.
These signs are of course only found in the city of Bangkok, however while driving from Surat Thani to Bangkok I had seen several small white signs when entering a new district. But since they were all in the middle of nowhere and we were driving to fast I cannot show any such photo here.