Both of them lasted nigh on 8 hrs, and both were hell.
The first was largely because of the driver rather than the road. The driver, who shall forthwith be known as the Idiot, had no business being in possession of a license and absolutely no business whatsoever being "responsible" for tourists.
The Idiot was a brake-pumping, tail-gating, steering-wheel-jerking, law-breaking, cell-phone-blabbing menace. I kid you not, he spent 70% of an 8-hr drive on his phone. Even when we pulled up at the place where we were supposed to eat lunch, the Idiot, with the phone stuck to his ear, got out of the driver's seat, walked around the van, slammed open the passenger door, and walked off still blathering on his phone without telling us what the heck we were doing, how long we had, etc. (We all figured it was a lunch stop, but the building looked more like a hotel plonked down in the middle of nowhere.)
Six hours later, at our hotel, repeat scenario. No one knew where we were, if we were all supposed to get out, etc. etc., but the Idiot couldn't be bothered to respond. It was basically, "Get your bags and get lost; I need to get back to Chiang Mai."
The Luang Prabang-Phonsavan driver was a little more serious--I mean by that that he didn't talk on his phone. He did however seem to take particular delight in passing cars in blind curves on a very steep, windy road. He also had the brake-pumping, steering-wheel-jerking problem.
Not a good time to be carsick, let me tell you... Poor Julien. I was not a fun traveling companion.
He had pity on me, and instead of taking an 11hr bus from Phonsavan to Vientiane, we flew. And tomorrow, instead of an overnight bus to Hanoi, back to the airport we go.
Amen for technology. And um, sorry for the planet. :(