I love this thing!
One quirk i find is that at larger distances (for example 800) i placed a crossing and it appears to lose precision and start z-fighting. The UI for the horn right before that crossing is also jumpy because of the lost precision. Perhaps build the game in a double precision export?
Also to add a new level i have to go to the files and duplicate a level instead of clicking a button. That would be a nice addition :)
Since it's that easy to add new levels, why not make two main tabs: official levels and extra? The official levels are the first three and all the other ones you figure out are high quality, and the extra levels are just for additional content so people can have more to play with.
My custom level was attempting to be a bit longer, ending at 1200 and the point was to see a different behavior of your mechanics: speed. Maybe one of your levels can force the player to go really fast?