Pick a car model (preferably the one you drive, but can also be any car of your dreams), and find its stopping distance at highway speeds (you can usually find this type of data online). Assuming that the car in front of you suddenly does a hard brake. For simplicity, assume that its braking performance is about the same as yours. Then also assume a reasonable amount of reaction time on your part (the time delay between seeing the brake lights lit up and applying your own brake). In order for you not to run into the car your are following, what's the closet distance you need to keep between the two cars