Drivers: Do you know the 'three-second rule'?

You’re driving in traffic when the car in front of you slams on its brakes and … Will you be in a crash?

Keeping a safe following distance can help prevent that. Here’s what you should know.

The ‘three second-plus’ rule

For passenger vehicles, the National Safety Council recommends a following distance of at least three seconds – if you’re driving in ideal conditions. But when weather conditions aren’t ideal and/or other driving hazards (such as heavy traffic) are present, you need to add time.

This is known as the “three second-plus” rule.

Ryan Pietzsch, program technical consultant for driver safety at NSC, gave an example: Let’s say you’re driving in snowy conditions on a busy road during rush hour.

“You would need to add one second for the snow, one second for the road surface condition and one second for the congested traffic,” he said, “for a total of six seconds.”

Something else to consider? How you’re feeling – both physically and mentally.

“If you’re feeling ill, tired or stressed, you would add at least one more second to equal four seconds of total following distance,” Pietzsch said. “If you’re stressed and tired, then add at least two more seconds, one second per condition, for a total of five seconds following distance.”

How to measure following distance

To best gauge your following distance, pick out a fixed landmark such as a road sign or even a crack in the road.

Once the vehicle in front of you passes that landmark, start counting until the front of your vehicle reaches that spot.

Then, you need to count, “One thousand and one, one thousand and two, one thousand and three” and so on. This is important.

“You must say all the words,” Pietzsch said. “You can’t just count, ‘One, two, three … .’ That’s not long enough.”

Stopping distance

“Stopping distance” is made up of three components: perception, reaction and braking distance.

In other words, you have to see the problem, react to it and then apply your brakes.

Depending on speed, a passenger vehicle will travel a certain distance by the time you see a problem, a certain distance while you react, and a certain distance as you slow your vehicle and bring it to a stop.

At 30 mph, that total distance is around 153 feet. At 55 mph, it’s nearly 346 feet. And at 70 mph, it’s approximately 490 feet – or about the length of three Olympic-sized swimming pools.

Large trucks and buses take even longer to stop, so it’s important to give them lots of space and not cut in front of them.

Added benefits

Besides avoiding a costly collision and any potential injuries, a safe following distance can have other advantages.

“The less obvious benefit to choosing a safe following distance is the decrease in stress,” Pietzsch said.

“Following too closely or tailgating is very stressful, not to mention extremely dangerous. Simply choosing a safe following distance greatly improves your chances of arriving at your destination safely, and it reduces some of the stress of driving.”