What...?

I would guess traffic volumn from all roads leading into the intersection.

Books made into movies, of course, are all "Major Motion Pictures".

I don't think there's a strict definition. "The next major intersection" could vary, as it is a comparison to other nearby intersections. In a small town, the "major intersection" could be the one with a stop light; in a city, it could be the one with six lanes of traffic rather than four.

Los Angeles used to - and may still for all I know - have intersections graded from A to F. I used to live near an F, and let me tell you, it was trouble to cross the intersection by foot, by bicycle, or by automobile.

Their transportation website probably has a category description. Alas, I don't remember the name of the transportation department. Maybe DOT.

One definition of an intersection. A noun, the act of intersecting a place of intersecting; specif., the point or line where two lines or surfaces meet or cross the place where two or more roads meet or, esp., cross Math. the set containing all the points common to two or more given sets

A definition of major: An adjective meaning. greater in size, extent, or importance: the major part of the town.

For example where I-70 ends at the I-5 is a major intersection in California.

A major intersection definitely does not have to include any highways.
Usually there is a traffic light. The main feature of a major intersection is that it includes main or major roads relative to the other roads in the area. Often one or both of these roads goes through several towns, depending on the type of area involved.