I have no ideahow fast they will go, but it's going to be bracket style. So slower carts will get a head start based on qualifying times. So even the slowest cart could win.
Heads up start would have been a lot easier on wiring. As it is, I have one controller that starts a sequence of 3 yellow lights and the green light (.5 seconds apart). The controller sends a signal to a second controller to start the sequence of lights for the other lane based on the time difference of the carts.