Thanks for the suggestion Peder. That's something we can look into. I've added it to our Feature Requests list (number 108) where people can vote for it.
There are lots of different surface types on OSM, so they would need to be grouped. What groups would you be interested in? Just "hard surfaces" and "soft surfaces" or other ones?
Would it be possible to toggle an extra map layer showing road surface in a color code, e.g. tarmac in red and gravel in blue?
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