I have tried the route suggested by Farmers earlier on this thread:
Chachoengsao - Phanomsarakam - 304 Pakthongchai - 24 Nangrong - 218 Burirum - 219 Mahasarakam - Karasin -except for the last part: 227 Phangkhon - 22 Udon Thani.
This is a better way to exit Bangkok than the highway 32 with its confusing lane system.
Here just ride to the end of Ramkhamheng (way past Red Baron)until you hit a T junction. There turn right. Anyway the direction to Chachoengsao is sign posted. Easy. Then, it 's mostly a 2X2 lane road until Buriram. Some part are being renovated and you get 1x1 lane.
For the anecdote, I got stopped by Highway police for speeding between Chachoengsao and Phanom Sarakham... I am positive: I was not riding over 100 km/h at the time and they had no speed camera... Anyway I was not going to stand for Human Rights and Constitutional Law by the side of a Thai road, so I let a red banknote go and save me time.
Scenic? Quite nice when going through Khao Yai National Park. Otherwise the road is mostly straight. Don't think about getting any angle.No fun riding, really.
Bkk-Buriram: about 400km.
Good room in Buriram. Riding out of the center towards the airport you 'll find Thepnakorn Hotel (66-44 613 400-2). 560 Bahts... However try the outer row of the bungalow type rooms. The inner row(closer to the main building) is right against a very noisy machinery that starts very early in the morning. Not a nice wake up eventhough you plan an early departure!
Then Buriram-Udon Thani: 380 km.
219 to Satuk, then 219 to Borabu, Mahasarakam and Karasin. Then 227 to Sam Chai and Wang Sam Mo. Then 2023 to Kumphawapi. From there it's only about 40 km on the main highway 2 before you reach Udon Thani.
Overall, from Bangkok the trip is more time consuming (One day and half) with less heavy traffic to manage. However the local type of trafic (farming vehicles, local pick-up, dogs, etc) you come accross requires all your attention as well.