So is "can you imagine how much infrastructure would cost to ensure everyone drove completely safely"
Like most real-world engineering, there is a cost-benefit balance. Could we design an interstate highway system that allows everyone a 10 car buffer? Sure. Would we like how much it costs, the effects on the environment, etc.? Probably not.
As an aside, your comment seems to go against HN guidelines by taking the least charitable interpretation of the comment.
You might have forgotten the ultimate rule of road safety: Everything is a tradeoff. Safer is only sometimes better. Otherwise the speed limit would be 10mph, because it's quite a safe speed.
