I think "highway" is a very old term dating back to the days of the horse and buggy, and "highwaymen," who robbed travellers. Some states in the US mention "highways" in their traffic laws, but it means different things in different states. For example, in Vermont, it's against the law to back a vehicle into a highway. In California, I was told by a police officer teaching traffic school that there's no such thing as a highway in California law.
In general, I think you'd be safe in calling any non-residential street which is not a dirt road, a highway. It may have only one lane in each direction, or it may have four. In most states, the interstates are called "interstate highways," or simply, "the interstate." In California, they're all called "freeways." All of the freeways are limited access highways. I believe there are a couple of the original freeways in the Los Angeles area which are
not interstates, eg the Pasadena Freeway, which has a primitive form of limited access, but does not meet the specifications of the interstates.
I believe there are a few cases where exceptions are made, allowing interstates to be accessed at traditional stop light or stop sign intersections, but these are in remote, rural areas with very little traffic, and ample warning is given. (There's usually not enough traffic to justify the expense of constructing the necessary ramps, bridges and overpasses.)
- A..