The trails listed at the North American Auto Trails site are organized into two groups based how many states and provinces a trail enters. Trails found in one or two states are listed under Single- and Two-State Trails, and those existing in more than two states can be found in the list of National Trails. I decided to divide the trails into two groups in order to keep the lists of trails from getting too long. Over 450 trails are catalogued in this site, and I felt that one list was far too long.
I realize that this method of organization tends to obscure the relative importance of some trails, since many trails found only in one or two states were far more important than some of the so-called national trails. However, beyond a few well-known routes (for example, the Lincoln Highway, the National Old Trails Route, and the Dixie Highway), little is known of the importance and use of most trails. I did not want to guess which ones were more important than others, so I decided on a more geographical distinction.