private void CarExitsCarPark(object sender, EventArgs e)
{
carPark.CarExitedCarPar();
exitSensor.CarLeftSensor();
btnCarExitsCarPark.Visible = false;
btnCarArrivesAtEntrance.Visible = !btnCarEntersCarPark.Visible && !btnDriverPressesForTicket.Visible;
btnCarArrivesAtExit.Visible = !carPark.IsEmpty();
UpdateDisplay();
}