Stopping Distance: Is the 3-Second Rule Wrong?

You know how long it takes your car to go from 0 to 60. But do you know how long it takes to go from 60 to 0?

Most automakers tout their cars’ ability to go from 0 to 60 in X seconds flat. The Bugatti Veyron, for example, can hit 60 mph in 2.4 seconds, making it one of the fastest cars in the world. While all that torque, muscle, and power may be impressive to some, at Esurance we’re more concerned with a car’s ability to stop on a dime.

After all, when it comes to avoiding a car accident and staying safe on the road, it’s the stopping power that can make a difference between a near miss and an oh-no!

So what does stopping power depend on? And how long does it take to go from 60 to 0?

Stopping distance: how long it takes to go from 60 to 0

For the average car with reasonably good tires, here’s how it breaks down:

1922.6-Stopping-Distance

In normal driving conditions, it takes roughly 4.6 seconds — also the time needed to read or send a text — to stop safely. And that means if you’re texting while you drive (not good!), you could very easily collide with the car in front of you.

Add rain or snow to the mix and you need even more cushion because wet surfaces reduce friction, making it hard for your tires to grip and slow down.

The physics behind stopping distance

I’m not a physicist. But I happen to know someone with a master’s degree in mechanical engineering (who, coincidentally, now calculates risk as a homeowners actuary at Esurance).

According to Joe F., coming to a complete stop from 60 mph requires turning kinetic energy (or the energy of motion) into heat through friction. For a car, this involves brakes transmitting stopping force to your tires, which then grip to the road to slow you down until you come to dead halt. So, whether you drive a Bugatti or a Buick, the physics of deceleration is the same.

But coming to a full stop doesn’t just depend on the laws of physics. It also depends on your car’s brakes and tires. And, of course, the condition of the road and your ability to react play a role as well.

The 3-second rule: Is it unsafe?

Given that it takes more than 4 seconds to stop safely at highway speeds, does following the 3-second rule actually put you at risk? It would seem so.

To be safe, AAA recommends that you give yourself a cushion of 4 or more seconds when driving at freeway speeds. Of course, when the roads are slick with rain or slushy with snow, give yourself a cushion of 6 or more seconds.

Related link

Following too closely: a common reason to get pulled over

6 Responses to “Stopping Distance: Is the 3-Second Rule Wrong?”

  1. Mel
    November 11, 2013 #

    I think this is a great simplification of the problem. If you are travelling at 45 mph, that is 66 feet / second. Substituting the value of deceleration into the equation of distance then if you stop in 4.6 seconds you will travel in those 4.6 seconds 152 feet. The reason that it is no 304 feet is because as you decelerate the distance travelled per second decreases by 50% as an average. You wont travel very far in the last second because you are almost at a stand still. If your starting speed is 66 feet / second and the final speed is 0 feet per second the average is 33 feet /second and that times 4.6 seconds is 152 feet. Also the .33 seconds recognition time X 66 feet / second = ~28 feet, the .66 seconds reaction time x 66 feet / second is ~43.5 feet. So assuming that your car can actually stop in 4.6 seconds the total distance traveled is 223.5 feet.

    This is still rather simplified however more accurate that the blog post

  2. Mel
    November 11, 2013 #

    By the way the 3 second rule is definitely wrong. At 80 miles per hour or 118 feet per second you should have at least 6 car lengths between you and the car ahead, assuming that you both slow down together. A brick wall does not take any time to slow down so it is your car that is going to need more time to stop. e.g. a freeway overcrossing pillar. I would say the rule should be the 6 second rule.

  3. Seer
    November 11, 2013 #

    To be honest, if you start braking when to see the brake lights of the car traveling ahead of you flashing you're already late. In other words you're not a very good driver. A good driver is watching what's happening three to five cars or even further ahead. The same applies to what's happening behind you. Do this and you're actually braking EARLIER than the car ahead of you and you don't have to brake hard giving the car driving behind you much more time to react.

  4. charrua
    November 12, 2013 #

    Stop adding seconds… there are no walls appearing instantly in the highway. The car ahead of will need also 4.3 secs since the time the braking light goes on until reaching a full rest (assuming it has the same breaking capability). In theory, 1 second difference should be enough… but you can't blink. More important than the distance it will take to full stop is how far ahead you are seen. That said: Don't drive close to things that block your view ahead (trucks, SUV's a car full of balloons inside). If that's the case you have to leave more than 6 seconds (may take you time to realize that the vehicle ahead of you is doing a full stop and by then it may be too late). Even better, move to the next lane. Also, you may be going downhill and it will take longer to stop. Best way to avoid crashing on a highway: drive 1-2 mph slower than the cars around you… you will reach your destination practically at the same time, and will have a ride with a safe, nice, unobstructed view of the traffic ahead of you.

    • smart idea
      April 3, 2014 #

      Thank you,
      Stop adding seconds – I drove today at 65mph and gave myself a 2 second cushion, which is a great time react – I drove 65mph with the 3 second rule, I had to ask why am I so Far behind this car – I drove 65mph with a 4 second gap WOW did I look like an idiot on the road I could have time to stop and turn around if anything happened in front of me. On a clear day that is.

      My question is why don't we have Stripes across the road as for a Green strip for the speed limits at 3 second intervals – a Yellow strip for half the speed limit at 3 second intervals – and Red strips for stop and go Rush Hour — SO SO SIMPLE no guess work or counting SO SIMPLE

  5. Korben Dallas
    October 6, 2014 #

    The typical stopping time for a car moving at 65 mph on dry pavement is 5-6 seconds (that includes recognition and reaction times). So from that point of view the 3-second rule does not provide you with a reliable safety cushion in a sense that 3 seconds is not enough to avoid collision with a stationary object that suddenly appears in front of you without any warning.

    However, it is important to understand that the 3-second rule was not designed for such situations. It was designed specifically for the "car in traffic" situations. It implies and riles upon the fact that the obstacle in front of you (another car) cannot stop instantly. It implies that the car in front of you will also decelerate to a complete stop gradually, for 5-6 seconds. And during that period of time the car in front of you will signal its deceleration with bright red brake lights. If you react to these lights and also begin decelerating, you will be able to stop safely without crashing into the car in front of you. This is what is guaranteed by the 3-second rule. And the 3-second rule is perfectly enough and perfectly safe in that context.

    Moreover, the 3-second rule works well even in slippery conditions. Yes, your braking time will be greater in slippery conditions, but so is the braking time of the car in front of you. Your braking time (and distance) will be greater, but the car in front will also leave a greater amount of space for you to do your braking. (Yet, I'd still recommend leaving yourself a bigger safety cushion in adverse weather).

    But in situations when a completely stationary object unexpectedly appears on the road in front of you the 3-second rule will not save you. If, for example, a concrete block suddenly falls onto the road at 3-seconds distance ahead, you WILL crash into that concrete block. It is not possible to avoid that collision with only 3-second safety cushion, since, as I said above, the braking time is 5-6 seconds at least.

    In other words, 3-seconds is a perfectly safe rule in a sense that it is as safe as it can possibly get. It is safe within the bounds of what you can do. Everything else is beyond your power. You cannot predict a meteorite striking the highway in front of you. You cannot predict a truck dropping in front of you from a highway overpass above. You cannot predict a semi moving in opposite direction bursting through the center divider and jackknifing in front of you. Anything like that, when it happens, that cannot possibly be avoided.

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