The car ahead is approaching a check point (a power pole, road sign, etc.).
Begin counting as the rear of the car ahead passes the check point.
If it takes less than three seconds to reach the check point, you’re too close.
Three seconds (one-thousand-one, one-thousand-two, one-thousand-three) is correct for ideal conditions. Allow more space in poor conditions. Keep well behind the vehicle ahead even when you are stopped. This will make it easier to get out of the way if someone bears down on you from behind.