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?
*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.