Sheldon Road at eleven o'clock on a February night is empty, salt-pale, and exactly where the work is. ULTRA UNLIMITED SERVICES, INC. runs the overnight stretch between Northville and Plymouth — plowing lots and drives, cutting a clean way open to every door, spreading salt ahead of the refreeze that always shows up near dawn. Nobody is around to watch a lot get cleared, and that is half the appeal of it. What people see is morning, pavement, and no reason to give the weather a second thought. Northville, Plymouth and Livonia sit along the route.