This is some really nice-looking and smooth-scrolling engine so far... but there are two things:

If you right click with the mouse under the button corner the game crashes (or exits?) when the car reaches this point outside the map & screen.

If you select several car at once and then direct them somewhere, they move all in one line this looks kind of funny.
(but i now that this is not an gfx-engine problem...i think you just didn''t implement it further)

really good work

It''s coming really nicely
The only problems I saw so far is the non-"depth"-perfect object selection and the simple AI (I don''t really see it as a problem since you''re at early stages of development).

With depth-perfect object selection I mean, I moved two trucks to a tile and they got one half over the other, when I try to select the visible half of a the truck, both get selected.
And the AI, well, I moved all the trucks I found (like 12 or more) to a single place and then, after selecting them all, moved far away, I could see how they made a nice spider and everyone was following the same path. But this is not really important at the moment

You''re doing a great job!