Inter-car distance is always a safety factor but can immensely reduce lane throughput. There is a compromise. I remember a few decades ago, in California at least, they suggested the drivers keep a one-car length distance from the leading car for every 10m/h speed. Funny that I had to convert it in km/h.
Inter car distance is highly irrelevant as the overall speed will quickly drop due to excessive braking due to tailgating. Maximum throughout is around 60kph. You keep 2s distance if you want to be safe. So at highway speeds that is around 50 to 70 meters
I am sure this rule of thumb was suggested after thoughtful weighing of traffic accident risk and jams in Cali's 8-lane highways. Everything is a compromise, often versus life.
