If you reach a fixed object ahead before finishing counting to four, it indicates:

• Your following distance is safe, and you can maintain your current speed
• You might not have enough space to stop safely if the vehicle ahead brakes suddenly
• You need to slow down to create a safe following distance
• Both B & C
  Explanation:

  "While driving, pick any fixed object such as a stop sign, light pole, bridge, parked vehicle, etc., in your path of travel and begin counting seconds, ‘one-thousand and one, two-thousand and two, three-thousand and three, four-thousand and four.’ If your vehicle reaches the fixed object you picked before you finish your count, you will not have enough distance to bring your vehicle to a complete stop, thus avoiding the potential hazard."