Keep Your (Braking) Distance: More Than Just Slowing Down


  • Braking Skid

    Braking Skid

    Burning rubber — literally. Brakes are the primary safety feature but are often overlooked. | March 18, 2010

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Although we at Edmunds.com spend a lot of time writing about rpm, torque, 0-to-60-mph acceleration, etc., nothing is more important than your car's ability to stop itself. Knowing something about braking distances (how much ground a vehicle covers before it can fully stop) can make for safer and more enjoyable driving.

Let's start with the basics. A vehicle traveling at 60 mph covers 88 feet per second. But stopping that vehicle takes over 4.5 seconds and covers a distance of 271 feet. Why? Because there's more involved in braking than the actual time your brakes are applied to the wheels (called "effective braking"). In particular, "perception time" and "reaction time" add considerable distance to stopping your car.

Perception time is the three-quarters of a second it takes for you to realize that you need to brake. Reaction time is the three-quarters of a second it takes to move your foot to the brake pedal. When you combine perception and reaction time, a full 132 feet will pass before your car even begins to slow down from 60 mph. So from the time you perceive a braking situation until the time your car comes to a complete stop, a total of 4.6 seconds elapses. During that time your car travels — it bears repeating — a total of more than 270 feet. That's almost the length of a football field. Of course, the faster you go, the more time and distance it takes to stop.

There are other factors as well, such as road conditions. When weather is bad, your braking distance grows exponentially. On wet pavement, total braking time increases from 4.6 seconds to 6.1 seconds, and total braking distance shoots up from 271 feet to 333 feet. And it gets worse. In snowy conditions, even with snow tires, total stopping time jumps to 10.6 seconds and 533 feet. As a basis of comparison, this is roughly the same distance — actually, a little further — as the same vehicle coming to a complete stop from 90 mph on dry pavement, an effective doubling of the braking distance. Let us repeat that: a 100-percent increase.

So what do we do with all these numbers? There's nothing we can do about the weather or about road surfaces, but we can do something about the way we drive. Arming ourselves with knowledge can prevent the loss of property and human life.

First, if you drive a truck or SUV, be especially cognizant of your speed in bad weather. Sitting higher off the road than everyone else only means you'll have a better view of the passing countryside as you slam sideways into a snowbank.

Second, remember this law: That which makes you go won't make you stop. If you drive a four-wheeler, you're not immune to the laws of physics, in fact you're a bit more susceptible (if for no other reason than your overconfidence). Whether you drive an Escort or an Excursion, it doesn't matter. In fact, the heavier weight of a truck or SUV means it will take much longer to come to a stop, given its greater momentum. Repeat: four-wheel drive does not help you stop. We're tired of seeing you folks spun around on the side of the road facing the wrong way. Slow down before you hurt somebody.

Third, remember to keep a "space cushion" around your vehicle at all times — ahead, to the sides and behind your car. This can be difficult to accomplish, especially in heavy traffic where everyone is darting in and out. How close is too close when it comes to following the car ahead of you? There's a handy "3-second rule." When the vehicle ahead of you passes a certain point, such as a sign, count "one-thousand-one, one-thousand-two, one-thousand-three." This takes about 3 seconds. If you pass that certain point before you finish counting, you are following too closely. We suggest a 4-second (or more) cushion in inclement weather.

Fourth, the tires you choose and their condition are another important, yet often overlooked, factor. See our articles, "Tire Safety: Don't Ignore the Rubber on the Road" and "Tires: Traffic Safety Tips" for all the details on tire selection and maintenance.

There are a few other factors that affect braking distances. As stated before, the heavier your vehicle is, the longer it will take to stop. Bear that in mind when you shop for a car or when you load it up. Also, the looser the road surface (gravel, dirt, mud), the harder it is to stop.

Finally, we strongly recommend that buyers choose a car equipped with antilock brakes (ABS), which, with few exceptions, help decrease braking distances on any road surface and in any weather. Whenever a driver slams on the brakes (and it's happened to everyone), the tires have the potential to lock up, sending you skidding. In a skid, tires have little traction, you lose steering control and braking distance is greatly increased. Antilock braking systems are designed to prevent tire lockup by automatically and rapidly "pumping" the brakes, potentially decreasing braking distances in extreme situations.

Of course, in order to get the most out of ABS in emergency braking situations, you have to know how to use it. And really, it couldn't be easier; you just stomp on the pedal. Some drivers are inclined to ease up on the brake pedal when they feel the vibration (and hear the noise) of the ABS doing its work, but it's important to maintain constant, controlled pressure. Aware that people often don't supply enough braking pressure, many manufacturers now supplement their antilock systems with "brake assist," which senses panic braking situations and automatically provides full power braking to shorten the stopping distance.

Many new cars come with antilock brakes as standard equipment, but you must often purchase them as an option on low- to moderately priced cars. And on some models, you may have to step up to a higher trim level to get ABS. Regardless, antilock brakes are a worthwhile feature and we highly recommend that you spend the extra money to get them.

What about disc brakes? Do they make a difference? Today we usually find four-wheel-disc brakes as standard equipment on most midpriced coupes, sedans, wagons and SUVs. Many economy vehicles and pickup trucks, however, continue to utilize a front-disc/rear-drum brake setup, which in most cases provides adequate performance for the general consumer. Nevertheless, vehicles with four-wheel discs usually deliver shorter stopping distances and are less susceptible to fade (loss of braking performance due to heat).

Whether you're reacting to sudden slowdowns on the highway or to a child darting into the street, nothing is more important than safe, well-maintained brakes (and the tires that work with them). Have them inspected according to the maintenance schedule in the owner's manual, and don't wait to have them checked out if you notice a pedal vibration or excessive noise when braking. That squeal you hear is probably telling you something — something that would be cheaper to fix now rather than later.

Additionally, being aware of all the variables — your proximity to other vehicles, weather conditions, road surface — will help you judge proper speed and give you time to react to whatever comes your way.

Comments

  • rojego rojego Posts:

    This is a great article but I disagree with you about Perception Time and Reaction time. I realize that everyone would not be the same time for either. Last Sunday my 77.6 year old wife and I went to see a movie and she was driving West on a 2 lane road about 35 mph, traveling 51' per second, staying about 100' behind the car in front. On the way, a car darted out from a strip shopping center on our left and turned West in front of us. My wife hit the brakes in about 1/10 of a second and avoided a collision. On the way back home driving East at 35 MPH about 1/2 block from the near miss going West, a car darted out of a side street on our right starting about 25' in front of us. My wife again hit the brakes in about 1/10 of a second and missed hitting their car by a matter of inches. According to your Perception Time and Reaction Time, we would have had a collision both times. And on the last one she would not have gotten her foot on the brake pedal in 1 1/2 seconds before a crash. Perception and Reaction can be trained to be as fast as my wife's action that prevented 2 collisions. I taught my wife and my time is slightly faster than her speed and I am 88 years old. An example of super perception and reaction speed, a few years ago I saw an interview on TV of a man who teaches fast draw and shooting to actors for wild west movies. Two round objects about 3" in diameter were dropped about 50' from him. He drew his 6 shooter Revolver and shot both of them while they were in the air and you could only hear one shot sound. The Perception Time, draw pistol, and 2 shots Reaction Time only consumed less than one second. Fast Perception and Fast Reaction should become a requirement for a drivers license. Jeff Goolsby

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