Unfortunately, Online Maps does not have a good way to highlight all the roads, because they are just drawn in the texture.
I think the best way to do this is to make a second map style (using Google Maps or Mapbox), on which roads will have a different color and / or size, and switch it on the fly.
http://infinity-code.com/atlas/online-m … ample.html
But this method is not suitable for all cases.

I'd like to be able to differentiate them from the surrounding map areas in script, and adding a drawn layer for them seems like the best way to go.