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You often see heavy haulers on interstate highways and the speed at which they’re traveling can be a mystery for other drivers. Some drive the speed limit or more, while others are driving at a more sedate speed. There are actually calculations that are used to determine the speed that a heavy hauler travels.

Safe Speed

Rule of Seconds

Tractor-trailer loads require more time to stop than other vehicles and use the “rule of seconds” in terms of time to govern their speed. If you’re moving at 40 mph, you need to maintain at least 1 second of distance for every 10 ft. of your vehicle length. If you’re driving over 40 mph, you need to add at least 2 seconds and the seconds increase depending upon the length of your vehicle. For instance, a 60 ft. tractor-trailer traveling at over 40 mph should add 7 seconds of distance for every 10 ft. of vehicle length.

Application of the Rule

It can be difficult to judge speeds of other vehicles, particularly on interstate highways. To calculate following distance, watch the vehicle ahead of you when it passes a fixed point such as a specific mile marker, sign, fence, or overpass. Count how many seconds it takes you to reach the same fixed point. Reaching the same point before the applicable number of seconds means you’re following too closely.

Hazardous Conditions

When driving in hazardous conditions such as rain, snow, ice or even high winds, you’ll need to expand the rule of seconds. Depending on road conditions you may have to expand the rule anywhere between 4 and 10 seconds. If you need to come to a full stop in any of those conditions, it will take significantly longer. Your reaction time and braking distance will need to be factored into those situations.

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