If There Were a Next Time
The only significant problem we have with the vehicle is that we never really had good control over the speed.   The pulse width modulator gives us reasonably good control over the acceleration (effective voltage on the drive motor), but does not give us good control over the speed.   Given a chance to build the car again, we would have used another photo-sensor to read markings on one of the rear wheels to determine the speed.   We would then have used a feedback control system to keep the car close to a target speed.   That way, our heuristic could really vary the speed of the car rather than just the voltage on the drive motor.