This is the combination of 2 winding roads with 48 switchbacks you'll have to ride when you go to and from Lake Chuzenji, Senjogahara, Nantai Mountain, Lake Yu and Yumoto Onsen. Each turn is named after one of the 48 letters in the ancient Japanese syllabary.
There is one road with one way traffic going up and one road with one way traffic going down.
By bus, it takes 1 hour 15 minutes to normally get from JR Nikko Station to Yumoto Onsen. Irohazaka is where the traffic jams start. Irohazaka is normally 20 minutes of the total, but since you have to make so many turns speeding up and slowing down will climbing or descending 400 meters.
Also, there are 2 major highway exits between JR Nikko station and Irohazaka that fill the roads quickly on weekends, holidays and during the autumn leaves season.