How to Calculate Proper Following Distance Part 1

Calculate Proper Following Distance

Recognize the Hazards

A key factor in rear-end crashes is improper following distance. In order to determine how much following distance is required to bring a tractor-trailer to a complete stop, drivers first need to know how much stopping distance their vehicle requires. A tractor-trailer driving 65 mph travels at a rate of approximately 100 feet per second. A fully loaded tractor-trailer requires a minimum of 665 feet to reach a complete stop. The table below demonstrates how stopping distance is calculated.

Calculate Stopping Distance

  Seconds   Rate   TOTAL FTS
Perception Time 1.5 x 100 = 150
Reaction Time 1.0 x 100 = 100
Brake Lag 0.75 x 100 = 75
Braking Distance 3.4 x 100 = 340

Perception Time

This is the amount of time it takes a driver to perceive the need to stop, such as noticing the brake lights on the vehicle ahead. It takes a fully alert driver’s brain approximately 1.5 seconds to perceive the need to stop. At 65 mph, a driver will travel 150 feet before the brain perceives the need to react.

Reaction Time

This refers to how long it takes a driver to react and move their foot on the brake pedal. This takes approximately one second to perform. In this time, the vehicle travels an additional 100 feet per second.

Brake Lag

This is the time it takes for the air brake system to function. This is generally three quarters of a second. The truck will travel an additional 75 feet per second.

Braking Distance

This refers to how long it takes the truck to actually stop once the brakes are applied. Assuming the brakes are functioning properly and the tires have adequate tread and traction, a fully-loaded tractor-trailer could take an additional 340 feet to stop.

Total Stopping Distance

Add together the perception time, reaction time, brake lag, and braking distance to calculate the total stopping distance required by a tractor-trailer traveling at 65 mph. The sum is 665 feet.

The information in this article is provided as a courtesy of Great West Casualty Company and is part of the Value-Driven® Company program. Value-Driven Company was created to help educate and inform insureds so they can make better decisions, build a culture that values safety, and manage risk more effectively. To see what additional resources Great West Casualty Company can provide for its insureds, please click below to find an agent.

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