What happens if your kids decide to start walking around to places? To school, with friends, or to do stuff? In that time you've signaled to the market that you want larger vehicles on the road, which makes it progressively less safe for your children to walk anywhere while slightly more safe in your vehicle.

John Neighbor follows the market and has bought an even larger car. Now you are in the position of needing to buy a larger car as well in order to maintain your relative safety standards.