Rear-end crashes are very common, and following too closely is a leading cause. That's why every state's driver manual teaches a following distance rule, and why the permit test asks about it.
How to measure following distance
The method is the same everywhere:
- Watch the vehicle ahead pass a fixed point, like a sign or an overpass.
- Count "one-thousand-one, one-thousand-two..."
- If you reach the same point before you finish counting, you are following too closely.
Related: What to Expect on Your State’s Driver’s Permit Knowledge Test
How many seconds? Check your state's manual
This is where people get caught out. States don't all use the same number. Some manuals teach a two-second rule, others ask for at least three seconds, and some recommend three to four seconds or more. The test will use your state's number, so learn the one in your manual, not the one a friend from another state remembers.
Manuals consistently call for more space in rain, snow or ice, at night, and when following a motorcycle or a large truck. Several manuals say to at least double your following distance on slippery roads.
What makes up stopping distance
Stopping distance is the total distance your car travels from the moment you see a hazard until you stop. It has three parts:
- Perception distance: how far you travel while your brain recognizes the hazard.
- Reaction distance: how far you travel while moving your foot to the brake. Manuals often use about three-quarters of a second each for perception and reaction.
- Braking distance: how far the car travels after the brakes are applied.
Braking distance grows much faster than speed does. Because the energy the brakes must absorb rises with the square of speed, doubling your speed roughly quadruples your braking distance. That's why a few extra miles per hour matter so much.
What else lengthens stopping distance
- Wet, icy or gravel road surfaces
- Worn tires or brakes
- A heavier load or towing
- Fatigue, distraction, alcohol or drugs, all of which slow perception and reaction
Sight distance at night
You should be able to stop within the distance you can see. Some manuals put the reach of normal low-beam headlights at a few hundred feet, which is why they advise slowing down at night and in fog.
On the test
Expect a question on your state's following-distance rule, one on when to increase it, and one on what makes up stopping distance. If a question mentions bad weather, the answer almost always involves more space.