Hiring a car and exploring Thailand on your own is an outstanding way to see the real Thailand, as hiring a Thai car is a cheap way of seeing rural areas and meeting everyday Thai people. Whether you hire a car to explore around Phuket or to see the countryside around Chiang Mai, renting a car is generally an easy and fairly inexpensive proposition. One way rentals between destinations (e.g. Bangkok-Chiang Mai) are also a possibility, though you should expect to pay a drop-off fee.
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Avis, Hertz and other international car hire agencies are well represented in Thailand, although many rental companies will not rent a Thai car or provide insurance to drivers who do not have an international driving license. While it is technically legal to drive in Thailand with a valid foreign driver’s license, having an international license will make renting and driving a Thai car potentially less problematic.
Furthermore, Thailand has an excellent 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 (from Chiang Mai to Bangkok to the southern beaches). Road signage follows international convention and is in both Thai and English, though some 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 on the street sign are one and the same).