Or not.

Actually this is a good example of why engineers shouldn't do policy. It would be a perfect solution if cars were black boxes managed by microprocessors, but keeping a fixed minimum distance by hand would make driving incredibly tiring and stressful, and would probably lead to more accidents.

You could also be fined if the driver in front does an emergency stop, which doesn't seem like a good thing.

And it would have to be context dependent - otherwise you'd be keeping the spaces in traffic light queues as you would on the open road.

And wouldn't the optimal distance vary by road type?

Etc.

Fail.