That's ridiculous. There's nothing "unrealistic" about 4.5 seconds of stopping distance - how could there be, are you thinking that the highways physically wouldn't fit all the cars spaced at that length?

It's unrealistic if you expect the existing infrastructure to meet both that requirement, the capacity requirements, and the time requirements of people travelling. In the real-world, there are tradeoffs. People who don't understand that tend to have superficial understandings of the problem because their mental models show little nuance.