In the past I favored to drive from Korat (Nakhon Ratchasima) down south via 304 (through Khao Yai) on the way to Pattaya.

I was warned by a mate that there is (already) major construction work going on and lot of congestion to expect.

I should better take the motorway route.

Does anyone have recent experiences going this route?

Is there construction going on to expand the two lane sections?

Just drove there on Thursday. Just a bit congested over the mountain itself. Takes about 15 minutes extra to get over, but rest of the road is pure silk. Great asphalt all the way!

That does not sound bad.

I really don't like the highway/tollway route (#2, #1, #9, #7).

When I had a Buriram GF I drove up every month: 331 - 304 - 24. I used to know where all the police ambushes were. What I hated most about it were the traffic lights. I counted: there were 18 sets (at that time.) On average one would think the major road would show green more often than not. Some journeys I got a red as many as a dozen times whilst the police had rigged the lights to enable a captive audience for the vendors. I've waited 2 or 3 minutes to allow one pick-up and a food vendor to cross. Anything up to a 100 vehicles wait, burning fuel or buying flowers.

It got so bad I changed my GF.