The Lonely Planet has a road atlas for SE Asia (1:1 000 000).
While it's fine as an overview, it is more of an artist's impression of the road network, and not an entirely true reflection of the actual system.
In use, it is not ideal either. There are no overlap-zones at the edges of the pages, making it frustrating when moving between pages, and, as the place names are in Roman text only, you cannot show the map to a local, pointing to the place name you want in his language.
A poor publication for an organisation like Lonely Planet.
For Thailand, either obtain the locally-available road atlas (shown left), or the superior Michelin sheet map.
Ironically, I have been unable to find the Michelin map on sale in Thailand itself. You'll have to bring it from home.