In "scenarios where vision fails" the car should not be driving. Period. End of story. It doesn't matter how good radar is in fog, because radar alone is not enough.
Too bad conditions can change instantly. You can't stop the car at an alpine tunnel exit just because there's heavy fog on the other side of the mountain.
If the fog is thick enough that you literally can't see the road, you absolutely can and should stop. Most of the time there's still some visibility through fog, and so your speed should be appropriate to the conditions. As the saying goes, "don't drive faster than your headlights."
