"Highway" is a generic term that was in use long before the car was invented.
highway
hī′wā″
noun
- A main public road, especially one connecting towns and cities.
- A public road or passage; a way open to all passengers, by either land or water.
- In law, any road or way, whether for foot-passengers, beasts of burden, or vehicles, or all, over which all persons, as members of the public, have a right to pass.
The American Heritage® Dictionary of the English Language, 5th Edition.