To get the Road Warrior achievement in State of Decay 2 do the following:
When you're driving, you can hold X to open your car door and use it as a weapon to slam into zombies. This will only take a few minutes if you have a car with enough gas and durability.
Drive along the roads and open the door for the zombies, like a true gentleman. After 50 acts of chivalry, the achievement will unlock.