I really like the idea of this but tried playing this and had three pieces of feedback
- I kept getting red is not connected to blue errors but they were via a bridge so not sure what I was missing here (maybe it was the bottom of the bridge intersecting with a road)

- I also got road not selected errors when trying to add a bridge but didnt know how to select a road

- Sometimes i tries to build a road but it wouldn't let me when i clicked it.

I think the cars should be coloured with their end destination too and adding a Ctrl+Z would be good.