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    Thread: How long does it take to stop?

    1. Member Rukh's Avatar
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      09-03-2010 05:46 PM #1
      There might be some people left in the forum who don't know that I am a driver education teacher. I teach driver education in the classroom, on the street, and I am also a state-certified license and permit tester.

      One of the things we teach in our classroom sessions is stopping distance. The method we've been told to use, and the formulas that permit test applicants are to base their answers on, go something like this:

      1. Reaction Distance (RD) - Speed in mph multiplied by 1.5 gives speed in fps, multiply that by 3/4 second (an average driver's reaction time*), and you get distance traveled before a driver hits the brakes.

      mph x 1.5 = fps
      fps x .75s = RD

      2. Braking Distance (BD) - Braking distance for an average car at 20 mph is 20 feet. As speed increases, braking distance (at maximum braking capability) increases as a function of the square of how much faster than 20 mph our driver is traveling.

      (mph / 20 mph)^2 x 20 feet = BD

      3. Total Stopping Distance (TSD) - Reaction distance, when added to braking distance, tells us how long it will take to stop an average car from the moment a hazard is identified to the time that the vehicle comes to a halt.

      RD + BD = TSD


      So, I've been reading magazines, and trying to verify that these figures are still correct, wondering how old these formulas are, and whether they are still relevant to modern vehicles. Sadly, many publications no longer print braking performance, and instead focus solely on acceleration times. One stalwart that I have noticed is Motor Trend, though they only list 60-0 mph distances. Per our calculations above, the average braking distance for a car from 60 mph should be around 180 feet, but I am seeing that even relatively large vehicles (a Cadillac Escalade, for example) are around 130 feet, and sporty cars like the M3 are closer to 110 or less, while the 997 GT3 comes in at 94 feet.

      The CO state driver handbook claims that it takes ~200 feet for a vehicle to stop from 55 mph, but if we go off of actual performance numbers, it looks like that should actually be closer to 170; and as speed increases the differences would be even more greatly exaggerated from what we have to teach and reality.

      For example, we teach that it takes ~410 feet for an average vehicle traveling 80 mph to stop, but if we use 13 feet as our multiplier for braking distance instead of 20 feet, we come to the conclusion that it'd take about 208 feet to brake, and about 300 feet (298) total to stop - from the moment that danger is first noticed to the moment the car stops.

      (mph / 20 mph)^2 x 13' = BD

      I suppose the question would be: How long does it take an "average" car to stop at 20 mph today, and so on for 40, 60, 80, and possibly even 100 mph? Do the equations need to be adjusted? I have a sinking feeling that the numbers we are teaching are way off, and it's already hard enough to convince my students that it actually takes over a football field and a third to stop from 80 mph, without it being inaccurate.

      Does anybody know of any sites or publications that list braking figures? Have you heard of any new research or alternate formulas to calculate braking distance in modern vehicles?

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      *Apparently, the Colorado State Patrol did a study about four years ago, and it concluded that an average driver's reaction time is closer to 1.5 seconds. This is actually quite amazing to me, and I hope that the state trooper who quoted me the study was incorrect.
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    2. Member Clean1.8T's Avatar
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      09-03-2010 06:14 PM #2
      There is no simple way to accurately determine how long it will take for a car to reach a complete stop. This is simply because too many factors come into play which not only depend on the vehicle but also the conditions it is attempting to brake in and the driver that is braking. These factors constantly change and include the total mass of the vehicle and its contents, the co-efficients of friction on the road and tires, how far the brake pedal is pressed, if the brake pedal pressure is even constant throughout the whole situation, the temperature of the brakes, the slope of the road and even the direction of the wind. Now if you go back to your perception-reaction time you will see that a car travels quite a distance in only .75 seconds, so if your variation in braking time varies from 2 to 5 seconds, you can easily see how you will end up with a large variation in braking distances.

    3. Member adrew's Avatar
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      09-03-2010 06:19 PM #3
      The 0-100-0 metric is a popular method for comparing overall vehicle performance. The car mags do them occasionally, and they always include the distance (and time) required to stop from 100 MPH.

      Example:
      http://www.roadandtrack.com/tests/co...0-0_mph_page_6



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      09-03-2010 06:41 PM #4
      Quote Originally Posted by Rukh View Post
      Does anybody know of any sites or publications that list braking figures?
      Consumer Reports lists 60-0 dry braking and wet braking distances for all of the cars that they test. The magazines are readily available in libraries.

      For example, the various current generation Honda Accords that they tested took 137-140 feet to do 60-0 dry, 148-154 feet to do 60-0 wet. The worst braking that they found recently was in some cars without ABS (e.g. Nissan Versa without optional ABS at 163 feet dry, 187 feet wet) and some 3/4 ton pickups (e.g. Chevrolet Silverado 2500HD at 171 feet dry, 204 feet wet).

      Of course, their tested cars were bought new, and have only some thousands of break-in miles on them. A "typical" car on the road may have worse braking performance, due to poor maintenance (how old is that brake fluid? what's that grinding noise when braking? what's that funny light in the dash that looks like a tire? oh, and why are there bars across the tire tread?).

      Quote Originally Posted by Rukh View Post
      *Apparently, the Colorado State Patrol did a study about four years ago, and it concluded that an average driver's reaction time is closer to 1.5 seconds. This is actually quite amazing to me, and I hope that the state trooper who quoted me the study was incorrect.
      Probably depends on whether "average driver" means the average driver's capability if s/he is paying full attention to driving, or an average driver on the road distracted by cell phone conversations, getting lost, road rage, or impaired due to tiredness or intoxication, etc..

      Also, a driver may see brake lamps light up ahead, but may not realize until a second later that s/he must stop as quickly as s/he can, rather than the more common case of a gradual slowdown. (And even if s/he does stop quickly, s/he may be rear ended and pushed into the vehicle ahead that s/he initially avoided crashing into.)
      Last edited by tjl; 09-03-2010 at 06:49 PM.

    5. Member Some cats and a Rabbit's Avatar
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      09-03-2010 09:21 PM #5
      Car and Driver lists 70 to 0 braking distance in the road tests in the September issue I have laying around my office. They show a 2011 Grand Cherokee stopping in 188 ft, an E550 Cabriolet in 162 ft, an RR Ghost in 164 ft as examples. I think the biggest variable in real world braking is reaction time and how hard the driver hits the brakes. The idea behind brake assist in cars like my Rabbit is that if I hit the brake pedal quickly the electronics sense that I want a panic stop and pile on full force braking even before I apply full pressure to the pedal. Testing by the various automotive companies has shown that this system reduces braking distance in panic stops.

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      09-03-2010 10:06 PM #6
      Quote Originally Posted by Some cats and a Rabbit View Post
      a 2011 Grand Cherokee stopping in 188 ft, an E550 Cabriolet in 162 ft, an RR Ghost in 164 ft as examples. I think the biggest variable in real world braking is reaction time and how hard the driver hits the brakes.
      True, but one would assume that reaction times for a specific situation and driver would be very close. So, consequently, the Jeep GC is going to plow harder into an object than the E-Class. This might mean the difference between a 45mph collision and a 40mph collision, but every bit helps -- massively.

    7. Geriatric Member BRealistic's Avatar
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      09-03-2010 10:55 PM #7
      Quote Originally Posted by Rukh View Post
      There might be some people left in the forum who don't know that I am a driver education teacher. I teach driver education in the classroom, on the street, and I am also a state-certified license and permit tester.

      One of the things we teach in our classroom sessions is stopping distance. The method we've been told to use, and the formulas that permit test applicants are to base their answers on, go something like this:
      .
      I wonder what data traffic engineers use to calculate times for yellow lights?
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