A surface road is a regular road whose surface is at the same level (elevation) that the roads it crosses and thus is not avoiding direct crossroads by using bridges, viaducts and possibly tunnels like fast traffic roads do.
I would then suggest réseau secondaire, route secondaire or réseau traditionnel as opposite to autoroute / voie rapide (freeway/expressway/motorway).
A very common name for these surface roads is also petites routes:
"On prend les petites routes pour découvrir les choses. On vient d'aller à l'office du tourisme pour regarder ce qu'il y a dans le secteur" explique-t-il. "En passant par les petites routes, on peut voir les paysages !" complète Stéphanie. "On va aller à l'île Bouchard avec la petite route pour bien découvrir la région" poursuit-elle.
France-Bleu, 2020
Surface road (petite route):
Image par LeFrolin de Pixabay
Non surface roads:
Photo by Kelly Lacy from Pexels
Photo by clemthi2 from FreeImages
Reference: What does surface road mean? EnglishForums.com
Note that translating "surface road" by route plane doesn't make sense. On the opposite, a non surface road is more likely to stay plane when for using bridges and flyovers while a surface road will go down and up following the slopes of the land.
Here is an obvious example showing a surface street or road is not necessarily flat:
Aerial view of Mulholland Drive, Los Angeles, California. United States:
Highsmith, Carol M, photographer. California, Los Angeles, 2012. Photograph.
And a document characterizing Mulholland drive as a surface street:
Environmental Impact Analysis
[...]
An extensive surface street network, including Mulholland Drive and Topanga Canyon Boulevard, provides access to and from the project site and surrounding uses.
Source: saveoaksavanna