Would like to visit Luang Prabang, Laos by driving from Chiang Mai to Chiang Rai, crossing to Laos then driving down to LP. Is this permitted by Laos law and if so, how about the roads?

In short YES. DO NOT listen to those that say otherwise. Enjoy and have a safe trip. http://www.thaivisa.com/forum/topic/654447-closest-border-crossing-to-drive-to-laos/

Yes; drive up to Chaing Khong and cross there over the new bridge.

drive to Luang Namtha 4-5 hours, good road, than on the LP ( bad road about 80 kms)

return to Thailand via Vang Vieng and Vientiane ok road

u need the car passport ( purple book)

make sure car is in ur name

u have the cars blue book with you.

Temp export docs done thai side

car is stamped out of Thailand an into Laos on the cars passport and vs when u return

buy insurance when u enter,

drive safe