It all works fine when just having this square, but if I extrude an edge of the road it becomes a mess. Is that adressed in a later lesson? 
Quite possibly, there's an issue that causes this exact problem that i don't notice until the end of the Crosswalks lesson (Chapter 4 Lesson 1) - if that doesn't solve your problem let me know and I can take a look at your file/node tree.
Edit: Just saw your screenshot. Yes, this looks like the exact problem I run into at the end of the Crosswalks lesson so good job, you're already ahead of me!!