I'm hitchhiked from Singapore to Kuala Lumpur a few days ago. I came along a major expressway where I found it very easy to get rides at the rest areas.
Rest areas are places along major long-distance roads where you can stop for a rest, buy petrol/bas, use the toilet, buy food and drinks, or sleep before driving further. Some have only a couple of these features. In some countries there are similar amenities adjacent to toll booths.
I have not been able to find out whether the major long-distance roads in Thailand have rest areas. I've checked Hitchwiki and Wikipedia and Google.
One thing is that actual "expressways" only exist in Greater Bangkok, whereas I'll be travelling in the south from the Malaysian border to Krabi on roads classified as "motorways". Could it be that in Thailand there are petrol stations and restaurants along the motorway without any special rest areas?