I'll agree with you there, the more national parks the better, within reason of course. It also depends on the purpose of the trip. I plan most of my trips to take in as much scenery as possible, but there are times when I just need to get from A to B as quick as I can, which is where the highways come in. I still wouldn't go through Khorat though. Normally 304 is one of my favourite roads in this area, but as you say, the roadworks have turned it into a muddy drag strip for impatient 4, 6, 10 and 18 wheeled idiots. I'll revise my interesting route to cut across to Pak Chong from 21, on 2243 or 2247. From there you can cross Khao Yai on 2090 / 3077 (same road) all the way to Prachin Buri. Then 319, 331 and 3240 to Pattaya.