We breakdown the safe stopping distance formula and why you’re almost certainly following too closely.
Decelerating your car is about as fundamental as automotive safety gets. It’s a wonder then how nearly all of us aren’t doing it properly despite years of experience. This is because people tend to be poor judges of the basic physics involved in bringing a one-ton (or more) vehicle from speed to a full stop. This, even though that first snowy day of winter or that deer that darts across the road frequently remind us, we’re not leaving enough distance ahead to reliably avoid collisions.
Most of us are taught the rough calculation of one car’s length per ten miles per hour of speed. While this is a decent rule of thumb, it’s really limited to just the mechanical braking under optimal conditions. As we’ll see, there are other factors in play that complicate this basic heuristic.
In fact, your total stopping distance is a combination of three separate numbers: perception distance, reaction distance, and stopping distance. First, your perception distance or the distance traveled between an obstacle appearing in the road and your perception of that obstacle. Next, the reaction distance is the interval between when you’ve identified an obstacle and your ability to react. And third, the stopping distance is how far it takes the vehicle to actually stop once the brake is applied.
The first two are typically estimated at ¾ to 1.1 seconds per ten miles per hour. So, at 50 mph, you’ll have traveled 55 feet for perception distance and another 55 for reaction distance for a total of 110 feet before applying the brake. Add the mechanical braking of 158 feet, and you’ve got 268 feet from an obstacle appearing to coming to a full stop from 50 mph. Recall our car lengths heuristic. The average vehicle on the road today is 14.7 feet long. With a combined distance of 268 feet, that’s just over 18 car lengths for you to recognize, react, and come to a full stop.
It’s also important to remember that stopping distances increases with your speed. Doubling your speed from 20 mph to 40 mph quintuples your stopping distance. Triple your speed, comparing 20 mph to 60 mph, and you’ll need nine times the distance. At 80 mph, you’ll need 16 times the stopping distance you did at 20 mph.
The stopping distance formula breaks down as follows:
Obviously, this stopping distance formula applies in optimal conditions providing for optimal traction. Poor road conditions like dirt, water, snow, and ice all decrease traction and increase stopping distances considerably (double or greater), as we’ve explained in our article on hydroplaning.
Worn tires are another major factor in your stopping distance. We’ve explained here and here, why having the right tires for road conditions makes a big difference in your traction. Having tires mismatched to weather and/or road conditions or having old, worn tires will greatly reduce your traction and hamper your ability to stop. Average all-season tread depth starts at 10-11/32th of an inch. At a speed of 70 mph, your typical sedan with new tires will be able to stop in 195 ft (or 3.7 seconds). Wear those tires down to 4/32ths of an inch and that distance is now 290 feet (or 4.7 seconds). At the minimal 2/32ths (legally bald tires), the distance grows to a full 378 feet or 5.9 seconds to a full stop.
Suffice to say, the vast majority of drivers aren’t leaving enough space between themselves and the vehicles in front of them. And this fact is amplified by your speed. This is true even under the best of conditions. Throw in rain, snow, or ice and you’ve got to further recalibrate your stopping distances. Of all the possible errors in driving, not leaving enough space to safely stop is one of the easiest mistakes to make, but it is also one of the easiest to fix (that and putting down your darn phone to avoid distracted driving).