What is the commonly used rule of thumb for following distance?

• Keep one car length back (about 20 feet) for each 10 miles per hour of speed.
  Explanation:

  "Although no rule for following distance is perfect, the most commonly used guideline is to maintain a distance of one car length (approximately 20 feet) for every 10 miles per hour of speed."
• Keep two car lengths back (about 40 feet) for each 10 miles per hour of speed.
• Keep three car lengths back (about 60 feet) for each 10 miles per hour of speed.
• Keep four car lengths back (about 80 feet) for each 10 miles per hour of speed.