There was no reason for the two women to stop the car to kill the walkers. The road was not blocked and there was only one walker when they decided to kill the walker by hand. If they weren't already stopped they would have just kept driving. Likewise, they could have proceeded slowly without damaging their vehicle. At this point in the apocalypse, the living should be risk averse and only be killing walkers when necessary since every kill, no matter how easy, involves danger and increases your risk of being bitten.