Suggestion: offer filtering of what historic route types and speed is used to inform "popular" - allowing users to select "popular with cycling" or "popular with running" only, and ideally "minimum / maximum speed of XX kmph for the path"
This would reduce many issues with suggested routes directing users down roads that are actually inappropriate, and where known / statistically identifiable journey types have significantly different routing requirements (e.g. cycling rides will typically be longer, higher average speeds etc.)
Problem: Within Build a Route - the "follow most popular" pathing does not have different data for Running / Walking vs Cycling. Most Popular is often not the most appropriate route.
Plotting a Road Cycling route will direct a cyclist down a very popular path, popular because it is used by runners/walkers - i.e. inappropriate for cycling, creating unreliable route suggestions at best, or routes with more cycling/pedestrian conflicts, or illegal (pedestrian only) or even dangerous route suggestions (high-speed road).
Popular but with much slower average user speeds might also not meet the road-cyclist's aims vs a nearby alternative - where better fine-grain suggestions would help immensely with routing.