Depending on the era you're modeling and the width of roadway, there may be some rail portals that can work. An example is the European ICE portals
Which look just like the expressway ones through the Alps, etc.
I have used a Vollmer single track steam era portal on a 2 lane road mountain tunnel.
Additionally, you could make them either by 3D printer, or use various diameters of pvc white drain pipe and couplings for modern ones.
A square brick portal could even be made of a block of wood or plywood.
Ones that comes to mind are the Fort Pitt and Squirrel HILL tunnels in Pittsburgh. The Liberty tunnel has a square shape with round portals.
The bricks can be painted on or use glue on paper photo sheets, or cast rubber or plastic ones from Chooch enterprises, Busch, Faller, etc. They make different colors and sizes of retaining walls, etc.

Noch are wonderful! You have to cut pvc pipe on a table saw to height in half or by a quarter or a third.

Those Viessmann led ones are cool! About $55 U.S. though. Led strips can be bought very cheaply, as well as resistors.

You can cut it flat, or cut pvc flat also, and the Noch tunnels cut with a knife.