Neat idea and solving the 'puzzles' is kind of fun, but there are number of obvious issues.
One very noticeable one is the lack of background music, feedback noises (like passed an intersection with no further instruction -- it's sort of noticeable with the step marker...), or and sound effect for car crashes.
Another is that you have to zoom in a fair bit to see what direction the car is going (need to see yellow and red dots). It would be simple to add an arrow to the car art so we can see which way that car is going.
Also, the little dashed squares (destinations) sometimes confuse me as to the car number when I am glancing back and forth because they are sometimes right around the car and very close to the little black squares. White and green dashes/text are a little hard to see as well since they blend in with the road edges/markings or the grass respectively (insufficient contrast).
P.S. There's also some UI weirdness regarding tabs and backspaces and if you hit escape you can't click on the buttons. The UI pulldown for copy, paste, select, etc is more a PITA than it's worth. Regular keyboard shortcuts work fine. You can't click play while the stupid pulldown is visible and closing it is non-intuitive. Sometimes when you press backspace to delete a step it instead pulls the selection back to the car number every time which makes it hard to delete multiple steps.
Using W,A,S,D or UP,LEFT,RIGHT,DOWN with some simply square images w/arrows to show in the box would be much more intuitive and less irritating to type out. Even S(traight),L(eft),R(right) would be preferable.
P.P.S. It would be nice if it kept your solutions while you go on to further steps in case you want to go back and refine them. Tangential to that some scoring would be nice, perhaps for number steps AND timing.