For exploring the more rural, less-trafficked corners of Thailand – Ayuthaya Historical Park, Pai, Sukhothai Historical Park – bicycles are a great way to get around. They can usually be hired from guesthouses for as little as 50B per day, though they aren’t always high quality.
Elsewhere, lack of infrastructure and dangerous roads mean that cycling isn't generally recommended as a means of transport for the casual tourist. Exceptions are the guided bicycle tours of Bangkok and some other large cities that stick to rural routes.
Yet despite the risks, bicycle touring is an increasingly popular way to see Thailand, and most roads are sealed and have roomy shoulders. A good resource for cycling in the country is Bicycle Thailand (www.bicyclethailand.com).