cycle.travel will always choose a paved traffic-free path in preference to the equivalent length of paved road, but if it’s much further, it’s more of a judgement call. There isn’t any option to change that I’m afraid (for the usual reason that more options mean renting more servers and renting more servers is expensive!).

I’d like to offer more alternatives but it’s complex algorithmically – here’s a discussion related to implementing it in OSRM, the routing engine which cycle.travel is based on.

Could you link to the section of road that isn’t being shown in the summary?

Thanks very much for your answers. It's great that traffic-free is preferred! The reason I find it useful to see several traffic-free routes (even if a bit longer) is because I will sometimes change routes depending on what bike/scooter I use, weather conditions and road work that may be ongoing. But yeah I understand the constraints, and it's already doing a great job I think.

I'm not sure how to link to a specific section but here's a route example where most of it is unaccounted for:

cycle.travel/map?from=Niort&to=Coulon&fromLL=46.3241132,-0.4649403&toLL=46.3221277,-0.5861466

By the way, could you tell me what the "Routes" mode does? I couldn't figure out what this is about exactly.

The “routes” mode is new and follows signposted routes only – anything that’s marked with dots on the map (= cycle networks in OSM).