The Irohazaka climbs from Nikkō, at about six hundred metres, to Lake Chūzenji at roughly twelve hundred and seventy, and each bend is labelled with one of the forty-eight characters of the old Japanese iroha syllable poem, so you can tick them off in order like a very steep alphabet. The road is actually two roads, a one-way uphill route built in 1954 and a one-way downhill route added in 1965, so you meet the bends in one direction only and never face oncoming traffic on them.
The ride is superb and slightly nerve-racking in equal parts. The corners are tight, with a rhythm of braking, leaning and accelerating that never quite lets you settle, and the surface is usually good, though damp, with a layer of fallen leaves. In autumn, the maples and birches turn the whole slope red and gold, and the traffic grows heavy, particularly at weekends in mid to late October, when queues can stretch for hours. Japanese macaques sit on the guardrails and are happy to inspect a parked bike. Winter brings snow and ice, and chains or studded tyres are needed.
At the top, the road delivers you to Chūzenji, a deep lake formed when a lava flow from Mount Nantai blocked the Daiya river. The lake sits at a cold, clear altitude, ringed by wooded hills, and the outflow drops over Kegon Falls, a ninety-seven metre ribbon of white water that is one of Japan's most famous cascades. In the Meiji period, foreign diplomats built summer villas along the shore, and some of the buildings are still preserved.
Fuel is not available on the climb, so fill in Nikkō or Imaichi before you start. The town of Nikkō, below, has plenty of stations and many restaurants, and its yuba, a delicate tofu skin dish, is the local speciality. Allow extra time on autumn weekends and check road conditions in winter, as the pass can close or require chains. Early morning is the best moment for the climb. A cloud-capped Nantai at sunrise is one of the sights of the Japanese mountains.