Item between 'minor' and 'major'

Intermediate would work, as it means that it comes between two extremes.

Medium, mid, middle, and median could also work, and are pleasing because they begin with an M so fit with major and minor.

To me, arterial roads means roads within a city that are quite long and used for travel between suburbs or areas some distance apart. I find it hard to imagine why you could need an extra classification of those.
This may be wrong for your purposes, but perhaps your first classification could be between longer 'arterial roads', which pass by or through a number of suburbs or areas, and 'local or suburban streets' which are much shorter. You might then classify both into major or minor depending on the amount of traffic, etc.
Just a thought. I hope it helps.

How about 'median' for your in-between layers?

https://www.google.com.sg/search?q=definition+median&ie=UTF-8&oe=UTF-8&hl=en-sg&client=safari

Or you could use 'interstitial' which means 'occupying the space between'.

https://www.google.com.sg/search?q=interstitial&ie=UTF-8&oe=UTF-8&hl=en-sg&client=safari

How about "Moderate"? Its context is broad yet implicates neither extreme, though could be interpreted as an indication toward the upper-end of the scale between Minor and Major.

Personally, I would vote "Intermediate" as it suggests that multiple instances of the term could apply, e.g. a minor layer, a major layer and many intermediate layers.