Brake light on black car

The minimum total following distance under ideal conditions should never be less than six seconds.

This article is from the Winter 2016 issue of The Quill. To view the full issue, visit The Quill archive.

Properly maintaining speed and following distance are crucial to safely operating a commercial motor vehicle, but even the most experienced drivers commonly follow too closely or do not properly control their speed. These behaviors could lead to a serious collision and should be avoided.

All drivers should always obey posted speed limits. This is especially true for drivers of commercial vehicles due to the increased stopping distance required by the vehicles’ larger size and weight. Many different factors must be considered to determine a safe speed. Posted speed limits only apply when conditions are favorable. Visibility, road conditions, traffic, work zones and the vehicle’s length, condition and weight all factor into determining a safe speed.

Some states have also raised speed limits to 75 mph or higher. These speeds are excessive for a tractor-trailer and most truck tires are not rated for sustained speeds above 75 mph. Ultimately, a safe speed is one that is below the posted limit, allows the driver to easily control and stop the vehicle under current conditions, and does not exceed the limits of the equipment. 

Stopping distance is one of the most important factors in determining a safe speed and following distance. It is calculated by adding the distances traveled while perceiving a hazard, applying the brakes and braking. For example, an average, alert driver under ideal conditions can perceive a hazard in 1.75 seconds and apply the brakes in 0.75 to 1 second. At 55 mph, the vehicle would travel 142 feet while the driver perceives the hazard and an additional 61 feet while the driver moves to apply the brakes. Once the brakes are applied, it will take about 216 feet to stop the vehicle if the brakes are in good condition and the vehicle is traveling on dry pavement. The total stopping distance at 55 mph, under ideal conditions, adds up to a minimum of 419 feet. This distance can be drastically increased by conditions that affect the performance of the driver and/or the vehicle, such as distractions, adverse weather or road conditions, worn brakes or tires and other factors.

Under good conditions, the time it takes for an alert driver to perceive a hazard and apply the brakes will be a constant. As such, speed has a large impact on stopping distance. Increased speed not only increases the distance traveled before the brakes are applied, but also increases the amount of energy required to stop the vehicle. A vehicle traveling at double the speed will require four times the braking distance, because it has four times the energy. The same vehicle traveling at three times the speed will require nine times the braking distance.

Vehicle weight is another important consideration for stopping distance. A heavy vehicle requires more energy to stop; however, a properly loaded vehicle can stop faster than an empty vehicle due to increased traction. 

Speed should always be reduced and following distance increased whenever the stopping distance of the vehicle is increased. Drivers should plan ahead for conditions that may require increased stopping distances, including wet, icy or snow-covered roads and areas that have a higher potential for road hazards such as work zones and mountain roads. When faced with poor visibility at night or during adverse weather, drivers should reduce their speed to a point that will allow them to make a safe stop in the distance that they are able to see ahead. Drivers should also use caution and slow down when approaching highway or railroad crossings, especially while following school buses, hazmat trucks or other vehicles that are required to stop at rail crossings.

When faced with poor visibility at night or during adverse weather, drivers should reduce their speed to a point that will allow them to make a safe stop in the distance that they are able to see ahead.

Maintaining a proper following distance is just as important as properly controlling speed and will ensure the driver has enough time to slow down or stop the vehicle as necessary. To maintain a safe following distance, drivers need a way to easily measure this distance. The best way to do this is by using time. Drivers should look for an easily visible, stationary object that is in front of the vehicle they are following. Reflective road signs and lit overpasses are good objects to use for this procedure because they are easily seen during the day and at night. Once the vehicle the driver is following passes the selected object, the driver should begin counting in “Mississippi seconds” until his or her vehicle passes the same object. This amount of time is the driver’s current following distance. The driver should then calculate the proper following distance by adding the following: one second for every 10 feet of vehicle length (rounding up to the next 10 feet) and one additional second each for traveling over 40 mph, driving at night, poor visibility and poor road conditions. The minimum total following distance under ideal conditions should never be less than six seconds.

For example, if a driver is operating a tractor-trailer that is 64 feet long and is traveling at 55 mph at night, the driver should allow seven seconds of following distance for the length of the vehicle, one additional second for speed over 40 mph and one more second for traveling at night. If the driver’s current following distance is less than the calculated following distance, the driver should adjust his or her following distance accordingly.

The longer following distances required for tractor-trailers at highway speeds often provide enough distance for other vehicles to move between tractor-trailers and the vehicles in front of them. Drivers should slow down to constantly maintain their following distance when this occurs. They should be mindful to turn off cruise control if they find themselves following other vehicles too closely. It should be noted that drivers with five years of safe driving experience are more likely to be involved in collisions than less experienced drivers due to gradually reducing their following distance over time.

Training your drivers to properly maintain speed and create safe following distances, along with reducing speed and increasing following distance when in doubt, will go a long way toward preventing collisions and will lessen the severity of collisions that do occur.

  • Categorized in:
  • Sharing the Road
  • Driving Techniques
  • Transportation Safety