Major highways are congested, even in rural areas. On Friday evenings, weekends and holidays in particular, but not only then.
Petchakasem highway as far south as Bang Saphan at least and pretty much the whole way to the Malaysian border is always full of traffic.
The Friendship Highway (starting as Phahonyothin as far as Saraburi) between Bangkok and Nakorn Ratchasima is very congested, much of the time. You can experience traffic jams in Klang Dong, which is a "small, rural town" at 9 or 10pm any day of the week. Many Major thoroughfares in greater Bangkok are relatively quiet by then, but traffic is often backed up on the uphill/downhill stretches of Friendship Highway.