I believe that even in the city it simply maps a path as suitable for cycling and that's the limit of the optimization. It then becomes a "shortest/fastest trip" calculation. They don't mark roads and paths for specific types of cycling and you're not asked to provide such info. So whether it's asphalt, cobblestone, or a dirt road even in the city it will make no difference as long as it's suitable for some bikes. Found out the hard way a couple of times...
At first glance your site shows the local "cycle highways" which is really cool. However I accidentally clicked on the map twice while scrolling around, and then couldn't figure out how to clear the route so I had to reload the page.
On closer inspection I noticed a button called "close route" which seemed to do what I wanted, but I'm not sure if it has other side effects
I do agree with 'overlordalex' on the 'Close Route' button. I couldn't find it at first and 'close' seems like a weird verb to use for 'clearing' the current route. Unless I am misunderstanding what the button is supposed to do. I would personally have a 'Clear' button somewhere close to the 'Get route' buttons (but then again I am no UI/UX expert :)).
Yep. It's clear that Google Maps optimises for city riding and that's cool, but it does fall down badly on longer tours. To a large degree this is inevitable - only OSM actually has the level of surface quality information required for this sort of planning.
With my site, cycle.travel, I've taken the opposite tack: generating quiet, safe routes for leisure and touring rides, while still being as fast to generate routes as Google Maps. Sure, I want it to be usable in cities but it's not the main focus.
People have used it successfully to plan month-long tours across Europe and the US. One of my favourite bits of feedback was https://www.cyclechat.net/threads/swindon-to-orkney-a-wet-we... , where someone just punched in a start and end point at opposite ends of the UK, and rode the route it suggested without any tweaking.
https://cycle.travel/map if you want to play - always happy to hear suggestions/feedback (and thanks to Jake for including it in the post!). Currently Europe/North America/Australia/NZ only.