A scheme for the transliteration of Thai by means of Roman characters was devised by the Royal Siam Society and approved by the Ministry of public Instruction in 1939. In its spelling of the resort, I believe the Department of Highways is in accordance with the Society's transliteration scheme.
The society's transliteration scheme is, however, not so practical. For example, one of the characters of Thai alphabet (the one associated with a plate) to be represented by a letter from the Czech alphabet. This is handy if you are translating Thai in to Czech and use a word processor. But mostly you will be translating in to English.
Another problem is that all p-let-ters of The Thai alphabet must be represented by "ph". Most westerners associate this combination with the sound of 'f' (like in photo). So when seeing the road signs, they may think the beach resort is called fattaya.
The society's transliteration scheme does not provide for rendering the tones of Thai syllables.
It is interesting in this connection to note that Vietnamese is a tonal language too, and that the Vietnamese in their colonial past have adopted a kind of Romanised script that indicates