This is a new one we've only started seeing today.
When you visit a new street that's just been unlocked through project completion, the map is not able to construct a path to get you somewhere else.
Usually, you should be able to leave by a signpost and then create a path from there. At the worst, teleport out or, if you're really stuck, ask for dev help in the in-game help channels.
Pathing should resolve itself within an hour, or sooner if we notice it and give the maps a little kick.
We're looking into resolving this asap, promise!