score:4
While not Google maps - this can be achieved on Open Street Maps - eg. https://www.openstreetmap.org/relation/3574625#map=7/34.153/134.934 (It's a long road so this may take some time to load)
You can either do this by directly searching for the name of the road in the search bar, but you'll probably find it easier to right click on a section of the road. Clicking "Query features" and then the name of the road you have just clicked on. And then scroll down to the "Part of" section - clicking any of these options will highlight the whole route.
This does depend on someone having created the relation in Open Street Map, so the more major a road you're looking for, the more likley it is to be present.