This is a good question. As an accident attorney, I can say this is an important topic as tailgating and other aggressive behavior accounts for at least half of the car accidents on the road. Additionally, with the increase in driver distractions, all drivers must be prepared to stop suddenly and safely, because the person you are following may not be paying attention to the road; and that requires following at a safe distance.
There are many factors that go into deciding how much space to leave between you and the car you are following: speed, reaction time of the driver, weight of the vehicle, and road conditions. For more information, please see, Four Factors to Keep in Mind When Keeping Your Distance.
In general, the faster you are traveling, the heavier the vehicle, the slower your reaction time, and the wetter the road - the more distance you need to leave between you and the car in front, as these factors all increase the space required to come to a stop.
The old adage used to be - one car length for every 10 miles per hour of speed. Today, with the variety of cars on the road, there is a new rule of thumb, the 2 second rule. This is how it works:
On a dry road, there should be a two second delay between you and the car in front of you. You can do this by finding a fixed object and count how long it takes you to pass it after the car in front of your does, i.e. "one-one-thousand", "two-one-thousand". It is recommended that 2 seconds go by before you pass the object, and some experts recommend 3 seconds to play it safe.
On a wet road, there should be a four second delay between you and the car in front of you.
On icy roads, there should be a ten second delay between you and the car in front of you.
Following at a safe distance and paying attention to the road, just may save your life! A news story that brings home this message: A Tragic Reminder To: Keep Your Distance, Pay Attention, and Buckle Up!