Traffic on the Kancamagus Highway varies significantly depending on times of the year.
For the Fall Foliage season there are a lot of road-trippers visiting the Kancamagus, so the traffic tends to peak along with the foliage. Columbus Day weekend is normally the heaviest weekend for traffic on the Kanc, but the weekends prior are a slow build up to this big weekend. Also the weekend or two after Columbus Day weekend are also fairly heavy for traffic, but not as bad as Columbus Day weekend. During the fall foliage season it is not uncommon to see stop and go traffic on the Kancamagus.
During other times of the year there can be quite a lot of people visiting the Kancamagus but there is not usually traffic backups.
In the Spring and Winter there is not much traffic on the Kancamagus at all. You might see a few hardy people hiking, snowshoeing, cross country skiing, or ice climbing but those are usually just a few cars in parking lots here and there.
Holiday weekends in the Summer also brings a lot of campers to the Kancamagus. Visitors normally arrive on Friday and some leave on Sundays. With the campgrounds full that can mean a lot of the attractions are busy as well. But get there early, or around dinner time and the campers are normally back at their campsites.
Best time to avoid traffic on the Kancamagus is when its raining, cold, or in the winter or spring.