it can be extended arbitrary given the proper dataset. For the current default settings, road class covers everything below 3. This translates to

FREEWAY = 1 # Freeway (Multi-lane, controlled access)

PP_TH = 2 # Primary Provincial/Territorial highway

SP_TH_MA = 3 # Secondary Provincial/territorial highway/ municipal arterial

MC_USP_TH = 4 # Municipal collector/Unpaved secondary provincial/territorial highway

LS_WR = 5 # Local street/ winter road

3 was the sweetspot. The dataset can be explored here in case you want to get an intuition on detail level. https://geodata.bts.gov/datasets/usdot::north-american-roads...