Thailand has a good network of well maintained roads and highways between all the provincial capitals and major towns and cities in between.
Most roads and highways are in good condition and have two or three lanes on each side, including a majority of the north-south route (between Chiang Mai, Bangkok, and the southern beaches). Road signage follows international convention and is posted in both Thai and English, though some signs are only in Thai (like 'Stop' and 'Give Way'). Buy a decent road map before you set off, though it’s well to remember that Thai words aren't always Romanized consistently (e.g. Petburi Road on the map and Phetchaburi Road respectively spelled on a street sign and a map are one and the same).
Driving at night is not a great idea as many of the truck and bus drivers are overworked and forced to drive throughout the night - and many resort to using amphetamines (yah-bah in Thai) to keep themselves awake. Even sober, they tend to have little respect for cars. Consequently, you should be aware that the bigger vehicle has right of way on Thai roads; be prepared to get out of the way quickly if there is a large truck behind you.
Passing on blind corners is not uncommon in Thailand, nor is it unheard of for cars or motorbikes to drive against traffic on the opposite shoulder or down the wrong side of the road into oncoming traffic. Defensive driving is a must in Thailand (except for in Bangkok where it wont get you out of the driveway).
Bangkok has a poor reputation for traffic conditions and this is justified except on the expressways. The city is very overcrowded with traffic and this makes driving difficult for drivers unfamiliar with the city itself or city driving in general. Whilst Thai drivers are exceedingly courteous and considerate to other road users (horns are used very rarely), traffic directions often change during the day to help with rush-hour traffic flow. These changes are seldom posted on signs, and none of these are in English, hence unfamiliar drivers can end up a considerable distance from their desired location due to enforced traffic flows.
That said, an increasingly extensive network of expressways within Bangkok now makes entry and exit of the city a simple task. Accordingly, renting a car from the airport in order to drive into the countryside, either north or south, is not a difficult task and driving on the more major routes outside Bangkok and around resort areas is both a safe and an enjoyable experience. A self-drive holiday offers the opportunity to experience wonderful Thai culture and the opportunity for travelers to stop as long as they like at the many points of interest in the Kingdom.