The stringent separation requirements are because of the long braking distances of trains, cars have much shorter braking distances - however even those are routinely violated by drivers. In Germany, the rule of thumb is "half your speed", i.e. 25 m for 50 km/h, 50 m for 100 km/h etc.

The US used to use a rule of thumb like that, but counting seconds is a lot easier than estimating distances.