Great game. I just got 3 stars on all the levels. I noticed there is a bug though that when you get 3 stars on a level it doesn't always save. I then have to go and redo the level if I want it to register the stars. It does save the correct high score.
I thought a nice idea for the trafic lights would be to have them be red by default. Usually when you place a trafic light you only want it to be on for a few blips so this is just less clicking.
It would also be nice if you can pause or slow down the simulation so you can better understand what is happening.
In the beginning I had some difficulties understanding the details of how the signs worked. Even after a few hours I sometimes get confused why a car can't make it's journey. If you could see the incomplete paths of cars that can't reach their destination when overing over the symbol that could clear things up.
I understand implementing all this stuff takes alot of time so I don't want to put pressure on you.
It's a really fun game and very well made :)