Set aside and refrigerate 1/2 cup whipped topping for garnish. In a large bowl, whisk the root beer, milk and pudding mix for 2 minutes. Fold in half the remaining whipped topping. Spread into graham cracker crust.
Spread remaining whipped topping over pie. Freeze for at least 8 hours or overnight.
Dollop reserved whipped topping over each serving; top with a maraschino cherry if desired.
After reading all your comments we took this recipe back to the Test Kitchen and decided it makes a wonderfully refreshing FROZEN pie. Enjoy!