Khao Yai was the first national park in Thailand, set up in 1962, and today it forms the heart of a vast forest complex, Dong Phayayen-Khao Yai, that UNESCO listed in 2005. It stretches across mountain ridges, evergreen forest, grassland and the sources of several rivers, and it protects some of the richest wildlife in mainland Southeast Asia: wild elephants, gibbons whose calls ring across the valleys at dawn, hornbills with their huge casques, macaques, sambar deer and, in the thick of the forest, a few tigers and bears that almost nobody sees.
The park road, Route 2090, climbs from the plateau town of Pak Chong, crosses the high ground and descends the other side toward Nakhon Nayok, a ride of about forty kilometres with a continuous series of bends, hairpins and gradients through tall forest. The tarmac is good, but the surface can be wet, and the real issues are wildlife and visitors. Elephants wander across the road, particularly at dusk, and when they do you must stop at a distance, switch off, and wait, because a wild elephant can easily be dangerous. Monkeys will steal from a parked bike. Speed limits are low and enforced, and there is an entrance fee for the park, with a fee for vehicles.
The higher parts of the park stand at roughly a thousand metres, much cooler than the plains, with mist in the mornings and a surprising chill after rain. Two waterfalls are the best known, Haew Suwat, which featured in a film, and Haew Narok, which drops in three stages. Both are reached on short walks. In the dry season, from November to February, the air is clear and the nights are cool, while from May to October the rain is heavy and leeches appear on trails.
Nakhon Nayok, the nearest big town, has fuel, restaurants and shops, and Pak Chong on the other side has a larger choice, along with the roadside restaurants known for steaks and the vineyards of the area.
Early morning is the best time to ride and to watch wildlife, with mist on the grass and gibbons calling. Bring a rain jacket, insect repellent and a lot of patience for animals on the road.