Great to see that you've been able to work on this again (I haven't exactly worked on my bigger project till recently either). As for the UI, you should have the X/Diamond go on top of the cars and change color. The X could be red and the Diamond could be green. Maybe do these two in their purest form (As in 255,0,0 for red and 0,255,0 for green) as well, they'll stick out more from the duller tones of the cars.