I have been at a lot of races, many/most of which I have been involved in some way either officiating or counting.  As the fields get larger counting becomes more and more difficult.  One of the hardest parts of counting cars is the actually see the car approaching and identify it.  Cars that blur together or into the track are the cars likely to be miscounted.  The hardest colors to see on the track are black cars and silver cars.  The easiest colors are bright (red, green, yellow, pink, etc). 

It is something to think about as you are building.

Aaron