Two days ago, I described in this article a frightening incident in my Tesla Model S. Approaching a downhill intersection, I inadvertently stepped on both the brake and accelerator pedals simultaneously. The car barely slowed, and it appeared I was headed for a collision with another car.
Fortunately, I realized my error. A second and more accurate stab at the brakes stopped the car, just in time.
I wrote that the close pedal spacing of the Model S had contributed to my mistake--a phenomenon known in the industrial world as “design-induced operator error.” (The fact that I was wearing heavy winter boots also played a role, along with the car's reduced regenerative braking in cold weather.)
I suggested that Tesla could mitigate such double-pedal driver errors by automatically cutting off power to the wheels when both pedals were pressed. But subsequent reader comments claimed that, in fact, the Model S already does this.
So I took my car out again, and with my right foot holding the speed at a steady 40 mph on a gentle uphill, I put my left foot on the brake. Sure enough, the power immediately cut off--and the car slowed normally.
I was puzzled, to say the least. What had happened back at that intersection?
It turns out I didn't test enough variables. Alerted by an astute comment on the Tesla owners' forum, I tried the same test again--but reversed the order in which I applied my feet to the pedals.
While coasting down a steep section of road, this time I kept my left foot gently on the brake, then pressed the accelerator pedal. The power immediately surged from zero to about 50 kilowatts, and the car strained forward against the brake. In other words, I had precisely replicated the conditions of my near accident.
[EDITOR'S NOTE, Mar 26: What follows is the original article, with a new section added, called "Halfway solution," that reflects the author's additional testing and information.
Some readers may ask what Tesla Motors itself has to say about the matter. We don't know. Regrettably, Tesla stopped responding to queries from Green Car Reports some months ago. We have roughly a dozen outstanding questions in to the company, but its Communications staff no longer returns our e-mails or phone calls.]
Like most accidents, it happened fast, with no warning.
I'd just bought a gallon jug of cider at a local apple farm, and was driving my Tesla Model S down the farm's exit driveway, which exits into a 40-mph two-lane suburban road.
The narrow driveway, surrounded by woods, is steeply downhill, and I had to stop at the bottom where it met the road.
Only mild braking
A few yards short of the T intersection, going perhaps 15 or 20 mph, I stepped on the brake pedal.
To my surprise and consternation, nothing much happened. The car kept going. It was hardly slowing down at all.
I pushed harder on the pedal. Still the car kept going, with only the mildest of braking action.
Something was very wrong. I was about to overshoot the driveway and burst out into the middle of the road.
I sometimes have a dream in which I'm driving a speeding car, trying desperately to slow down. I'm pressing with all my might on the brake pedal, my quadricep quivering with effort, and the car refuses to slow down.
2013 Tesla Model S in Florida, during New York to Florida road trip [photo: David Noland]
This was a mild version of that nightmare.
Out of the corner of my eye, I could see a car--a white Mustang, as I recall--approaching fast from the left. It was clearly on a collision course if I didn't get stopped in time.
I quickly lifted my foot off the brake pedal and slammed it down again.
The brakes grabbed, and the Model S lurched to a halt with its nose a couple of feet out into the road. The Mustang swerved slightly and blurred past my windshield, its driver probably cursing the idiot who nearly pulled out right in front of him, with potentially fatal results.
I sat there trembling. What the hell had just happened?
Design-induced pilot error
As an aviation journalist, I've analyzed and written about hundreds of small-plane crashes. In most of them, there is no single cause, but rather an unlucky chain of events that leads to disaster. In the end, though, the official "probable cause" is usually determined to be pilot error.
In most cases, that official judgment is correct. Like clockwork, inept private pilots continue to run out of fuel, forget to put the wheels down, botch crosswind landings, and put their planes into stalls.
But savvy aviation accident investigators keep an eye out for something called "design-induced" pilot error.
Sure, the pilot may have screwed up. But maybe there was also something about the design of the airplane that contributed to that screw-up.
A plane with poor directional stability is trickier to land in a gusty crosswind. A plane with multiple fuel tanks and fuel gauges makes it harder to keep track of the fuel.
Accident rates triple
Certain types of planes have pilot-error accident rates double or triple that of others--not because they attract incompetent pilots, but because their design makes pilot errors more likely.
And once those inevitable errors occur, certain planes are more forgiving than others. Accidentally stall a Cessna 172, and the plane essentially recovers itself. Stall an AA-1 Yankee, on the other hand, and you must react quickly and correctly to avoid a deadly spin.
It's the same with virtually any other type of human-operated piece of machinery. By their design, certain types of automobiles or chainsaws or anesthesia machines are more likely to be mishandled by their operators than others.
And when those operator errors inevitably occur, the design of the machine can either mitigate the errors.....or not.
2013 Tesla Model S electric sport sedan on delivery day, with owner David Noland
My near-accident is a classic case of an odd chain of circumstances that linked up at just the wrong time. But I also believe it's an example of design-induced "pilot" error.
Looking back, it's pretty clear that I screwed up. But if you ask me, the design of the Model S helped induce my error--and then did precious little to help bail me out after I committed it.
Here's what happened:
In a nutshell, I inadvertently pressed both the brake and accelerator pedals at the same time. Quickly realizing my mistake, I was able to stab only the brake pedal on the second try, and stop the car in the nick of time.
As with most plane crashes, my near-accident was the unlucky convergence of a series of circumstances:
I have big feet, size 13. I was wearing clunky winter boots. The Tesla's brake and "gas" pedals are close together. It was a cold day, so the Tesla's regenerative braking was disabled. The driveway was steeply downhill. There happened to be another car coming at high speed.
It all added up.
Before the near-accident, I was aware of the Tesla double-pedal syndrome. That's what saved me from disaster.
2013 Tesla Model S electric sport sedan [photo by owner David Noland]
On two or three previous occasions during my year of Model S ownership, I'd inadvertently pressed both pedals and gotten the same result--a marked decrease in the rate of slowing. All had occurred when I was braking harder than normal.
No harm had been done in those cases. I eventually figured out what was going on, and had mentally rehearsed what I'd do if it happened again: resist the natural urge to press harder, and instead lift my foot off the brake and quickly replace it in a better position.
(The all-too-human natural instinct to press harder on the pedal when your car fails to brake as expected is the cause of virtually all unintended-acceleration accidents. Drivers later can't believe they were mistakenly pressing on the accelerator rather than the brake.)
Bad pedal design?
In 50 years and perhaps half a million miles of driving dozens of different cars, I had never inadvertently pressed both the brake and accelerator pedal. Not once.
Now, over one year and about 15,000 miles in the Model S, it had happened three or four times.
Brake and accelerator pedals of 2013 Tesla Model S [photo: David Noland]
Driver error? Of course. Design-induced? I think so.
Looking at the brake and accelerator pedals, it seemed to me that they were fairly close together, both laterally and vertically.
Curious, I took out a measuring square and set it on the "gas" pedal. The sides of the two pedals were 2.3 inches apart, and their surfaces at rest were vertically separated by 1.6 inches.
Thus a shoe or boot on the brake pedal overhanging 2.3 inches to the right could touch the "gas" pedal after 1.6 inches of downward brake travel--less, if the shoe happened to be tilted to the right.
Tighter than most
How do these numbers compare to other cars?
Armed with the measuring square, I visited four new car dealers and measured the lateral and vertical distances between brake and gas pedals for 22 different car models, ranging from a Honda Pilot to a Chevrolet Camaro.
The lateral distances between brake and gas pedals ranged from 1.8 inches (BMW 433) to 3.1 inches (Honda CR-V). The average was 2.7 inches.
At 2.3 inches, the Model S fell near the narrow end of the spectrum. Only a pair of BMWs, the 433 and M3 , were narrower.
Clearly, this comparatively narrow gap between pedals makes it easier for an overhanging shoe or boot to touch the accelerator pedal inadvertently as well.
Small vertical gap
But more telling is the vertical distance between the surfaces of the Model S pedals--a scant 1.6 inches.
This turned out to be notably less than any of the 22 other cars I measured.
They ranged from a low of 1.8 inches (the M3 and CR-V) to 2.9 inches (Chrysler Town & Country minivan) The average was 2.2 inches--nearly 40 percent greater than the Model S's miniscule vertical separation.
If we look at the sum of the vertical and lateral distances, the Model S has tighter overall pedal spacing than all of the 22 other cars except the BMW M3, a specialty high-performance model.
Clearly, the Model S pedal spacing is, overall, tighter than the vast majority of other cars on the market.
From that simple fact follows the no-brainer conclusion: the Model S is going to be more susceptible to inadvertent double-pedal incidents than most cars.
Ironically, the Tesla’s strong regenerative braking tends to disguise the car’s susceptibility to double-pedaling.
During the first nine months of my ownership, when the weather was typically warm and the regen rarely limited or disabled, I hardly ever used the brake pedal at all.
2013 Tesla Model S electric sport sedan [photo by owner David Noland]
When I did, it was typically just a light touch as the car came to a final halt. A firm, full application of the brake pedal was something rare indeed.
As a result, I perhaps got into the habit of overhanging my foot on the right side of the pedal. But I got away with it because I virtually never pressed the brake pedal down the required 1.6 inches to contact the “gas” pedal.
But when the temperature dropped these last three months, regen was often disabled, and much deeper brake-pedal travel was required--sometimes more than 1.6 inches.
And of course, winter is also the season I’m far more likely to be wearing big, clunky boots.
Two more links in the chain.....
Tesla engineers apparently understood and anticipated the susceptibility of the Model S to double-pedaling, because they devised a warning for just that situation.
Warning message on dash display of 2013 Tesla Model S: 'Both Pedals Pressed' [photo: David Noland]
When both pedals are pressed simultaneously on the Model S, the car gives a quick three-beep warning and a dashboard notice that reads “Both Pedals Pressed.”
In my case, the triple-beep reinforced my quick recognition of my predicament, and helped trigger the rehearsed foot-off-and-back response that saved me from a serious crash.
(To check out this warning, watch the Drag Times video of its timed acceleration runs of the 60-kWh Model S. The driver, who handily beat Tesla's published 0-to-60-mph time, apparently used the launch technique of holding the brake while flooring the accelerator, thereby triggering the warning.)
But in an emergency, the warning only helps someone like me who's already familiar with the problem and primed to correct it.
A driver unaware of the double-pedal syndrome would have no idea what the beeps meant as he frantically pushed harder and harder on the brake pedal. In a real-life emergency, there would be no time to look down at the instrument panel to read--and react to--the "Both Pedals Pressed" warning.
A better solution
Sure, the Model S helped me out by beeping its warning. But this driver-error mitigation could have been made far better.
Why not simply disable the accelerator pedal, or otherwise cut off power to the wheels, any time the brakes are activated?
The double-pedal sensor is already in place; Tesla would simply need to change the software to cut the power instead of just activating the warning beeps and message.
2013 Tesla Model S electric sport sedan [photo by owner David Noland]
Moreover, a similar system for cutting off the power with brake application already exists in the cruise-control system.
I can think of no circumstances under which a driver would want power going to the wheels while pressing the brake pedal (other than perhaps achieving the lowest possible 0-to-60-mph time).
But note that in the Drag Times tests of the Model S 85 and 85P, the double-pedal launch technique was not used. Both cars still crushed the published times.
If Tesla feels inclined to curry favor with the drag-racing crowd, it could make the power cutoff an on-off option, as it has with idle creep and regenerative braking.
Ironically, the Model S is already programmed to cut off power to the wheels in case of an inadvertent double-pedal situation--but it only does so part of the time. And unfortunately, it's the wrong part.
If you're driving along at 40 mph and happen to step on the brake pedal with your left foot--a highly unlikely situation--the power will indeed cut off.
But if the brake pedal is pressed first, followed by the gas pedal--the usual double-pedal scenario, and the one that happened to me before the near-accident--power to the wheels is not cut off.
The electric motor, which produces peak torque at low rpm, thus strains against the brakes, which greatly increases the stopping distance.
The software logic of the Tesla system seems to be "whatever pedal is pressed last gets the priority."
The human logic? I haven't a clue.
In my opinion, the tight placement of the Model S brake and accelerator pedals--particularly their short 1.6-inch vertical separation--is a potential safety hazard.
I was lucky. Another driver less familiar with the car's double-pedal syndrome may not be so fortunate.
Eventually, somebody's going to get hurt.
The solution is a quick and simple software update: cut the power to the wheels whenever the brakes are applied.