The Car and Truck Fleet and Leasing Management Magazine

How Two Extra Seconds Can Reduce Rear-End Collisions

June 2002, by Greg Miller

Review your company's vehicle accident records and chances are you will find rear-end collisions near the top of the list. Not only have your company's drivers had a close unexpected look into the trunks of fellow road warriors, they most likely have been the recipient of such intrusions.

Rear-end collisions are costly. The costs of an average at-fault rear end collision can run thousands of dollars. Rear-end collisions are an epidemic spread by misconceptions and bad habits, which seemingly even the most experienced and skilled drivers cannot escape. Drivers often drive in a false belief that if the car in front suddenly started braking, they would react and end up stopped the same distance apart. The antidote for preventing the spread of this most preventable of accidents is simple: Back off. More than 75 percent of rear-end collisions could have been avoided just by maintaining proper following distances. Spread the word - it takes more to stop a vehicle than just depressing the brake pedal. Drivers must understand the dynamics of bringing their mass of metal and iron to rest.

The Components of Stopping

Let's review the four components of total stopping distance:

1. Human perception time (1/4 to 1/2 second).

Human perception time is how long the driver takes to see the hazard and the brain to realize it is a hazard requiring a brake application.

2. Human reaction time (1/2 to 3/4 second).

Once the brain realizes danger, the human reaction time is how long the body takes to move the foot from accelerator to brake pedal.

3. Vehicle reaction time (1/4 to1/2 second).

There is a slight delay after the brake pedal is depressed before braking actually beings. This is inherent to the mechanical design and physical properties of the brake system and can vary by vehicle type.

4. Vehicle braking time.

Finally, the actual braking or "stopping" force begins. Many factors can affect the vehicle braking time. The condition of the braking system, tires, and suspension, as well as the cargo and/or passenger load can play a key role in braking performance. However, most dramatic is the influence that vehicle speed and road conditions have on the vehicle's braking. Double the speed of a vehicle from 30 to 60 mph and the braking distance quadruples. Wet or icy roads add even more distance requirement, perhaps 3 to 4 times.


Human and Other Factors

The first two components of stopping distance are human factors that can be affected by alertness, medications, alcohol, fatigue, and driver distraction.

Under ideal circumstances the combined human perception and reaction time require about a second. Just one second. At 60 mph this "one second" will mean you travel 88 feet before even pressing the brake pedal.

This is why the tailgating car usually cannot stop in time. When the brake light came on in the front car, its driver had already completed the perception and human reaction periods.

The tailgating driver was at least one second too late in applying the brakes.

Don't be fooled, you cannot drive defensively when you're following too close. As you travel closer to the vehicle in front, you begin to fix your attention on a small window of activity, reacting to the limited information transmitted by the tail lights directly ahead. You no longer have the big picture and your options to react to a situation are limited by the space you have allowed.

Too close for comfort. It seems enough. Many have heard these messages before, yet some will continue as tailgaters even after experiencing the unfortunate consequences. Why? Following distance is a habit. Drivers with a habit of following too closely must work to alter what feels comfortable. Generally, one car length for every 15 mph is considered a safe following distance. Take five of your coworkers outside and ask them to approximate the distance to an object several hundred feet away. You will most likely get widely varying answers. People visualize distance and compare with a known distance to make judgements. Mentally they are visualizing a known size between the object and themselves to approximate the distance. Maybe it is a tape measure outstretched in the mind or maybe the 20-foot boat that one owns laid end to end. The point is, distances can be difficult to approximate, especially when traveling at highway speeds. The varying views from different vehicles can also play a part in how distances are perceived. Following distances in vehicles that sit taller than the surrounding traffic are often misjudged, feeling as though they have greater space in front than what they actually have.


Two Seconds for Safety

The best way to judge following distance is to use the two-second rule. Pick a fixed object ahead and begin counting off seconds as the vehicle in front of you passes it (1,000 and 1, 1,000 and 2, 1,000 and 3, etc.). Stop counting when your vehicle passes the same object and use this simple formula:

  1. Maintain at least a two-second following distance
  2. Add at least one second for highway speeds above 55 mph.
  3. Add at least one second for each adverse condition (night, rain, fog, construction, etc.) and SLOW down. Never overdrive conditions.


Distance Referencing

Another exercise to help drivers improve their sense of safe following distance is what I call distance referencing. Locate a fixed object that all vehicle pass leaving the driver location such as an exit gate, driveway, signpost etc. Create a simple marker and place it 30 feet back from the fixed object and a second 75 feet back.

Attach the markers to cones, walls, fences, or any place that can be clearly seen from eye level, side window as your driver pass by. Produce a communication that asks all to pause briefly as they pass the markers and reference the distance to the fixed point ahead. The "60" marker is the minimum following distance required at 50-60 mph. The "30" is the distance required at speeds 25-30 mph.

After one such exercise, a driver was heard to say "I drive in the real world and there is no way to maintain that kind of distance." The same driver had a rear-end collision 40 days later land was cited for following too closely. Fortunately, most people respond well to guided discovery and the company will likely see a dramatic reduction in the frequency of rear-end accidents after implementing such education.

Recognize that tailgating can sometimes be indicative of broader unsafe driver attributes that require much more remedial action.

Statistically, tailgaters tend to be more aggressive and driver faster. They are more often found in the 4 percent of drivers who have 30 percent of the accidents and many will have at least one point on their driving record. (Let's not forget the uncanny, but unconfirmed correlation between a driver's following distance and I.Q.)

Company policy should view all rear-end collisions as serious and preventable. Drivers who are involved in a rear-end collision should be reviewed carefully, placed on the company's safety watch list, or considered high risk for at least one year after the event.


Double Trouble

It should be no surprise that many rear-end collisions occur just after completing a lane change or when merging into a flow of traffic. Drivers need to pay particular attention to maintaining or restoring their following distance during and after lane changes. During lane change maneuvers more attention is given to the clear space behind and to the side of the vehicle, and prolonged use of the mirrors makes forward focusing more difficult. Space is at a premium and lane changes often result in reducing following distance by 50 percent or more. Despite these odds, a few simple rules can nearly eliminate the risk of an accident during or after lane change.

  1. Signal early, allowing at least six seconds or six flashes of signal time before executing the lane change.
  2. Attempt to enter the new lane with the minimum following distance already established.
  3. Avoid braking into a lane change.
  4. Adjust your speed before you enter the lane. Your speed should be equal to the lane that you are entering at the time you make the change.
  5. Look at least two seconds ahead of the vehicle you are about to trail, note their following distance and the traffic conditions
  6. Recheck your following distance after completing the lane change and adjust as necessary. When practical, use gentle acceleration and deceleration as opposed to brake application.


Greg Miller is director of fleet services for DHL Worldwide Express, San Francisco, CA.


Cover Your Rear

OK, so now you understand the importance of following distance and have adopted new comfort levels to follow those in front. What about the guy that has not yet read this article? You can't see his face, but the grill of his '70-something 4x4 is filling your rearview mirror. The good news is that now armed with your newfound knowledge, you can crusade to prevent the non-preventable. First and foremost, avoid the urge to teach the tailgater a lesson with a quick brake. Depending upon the circumstances, you have several defensive options:

  1. Change lanes
  2. Increase your following distance from the vehicle ahead by two seconds with gentle acceleration and think of the tailgator as a trailer that you may have to slow and stop with your vehicle. This also opens the space ahead for the tailgator to pass. Avoid any acceleration that reduces your following distance.
  3. Turn on your four-way flasher to caution tailgater.
  4. Safely pull off the roadway and let the tailgator pass.


Most company policies will usually charge the driver struck from the rear with a non-preventable accident and require no remedial action or follow up. Best practice, however, is to at least conduct a driver interview and determine if defensive options were available and used.

Though only one element of safe driving, proper following distance should be the cornerstone of your driver education. Driver safety must be everyone's business and accident prevention a priority for a business' success. Accident cost must be viewed as a controllable business expense and proactively managed toward excellence. Collision causes are as identifiable as any other business shortfall and proactive management practices can address accident prevention no differently than profit attainment. A good safety program can do for the fleet operation what a good preventive maintenance program does for the vehicle; stop potential problems before they cause damage and require costly major overhaul.

Expect excellence, a few seconds at a time.


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  1. 1. isaac [ December 03, 2014 @ 09:49PM ]

    I enjoyed your article Greg Miller hopefully i have the chance to meet you in January


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