Do electric vehicles (EVs) get rear-ended more often than conventional cars do?
They certainly should.
That’s my conclusion after trying out the “one-pedal driving” mode on my new (used) 2020 Chevy Bolt. And working through the logical consequences of it.
The practical bottom line of this post is that you should think twice before you tailgate an EV in traffic. Because the chances are good that they can stop a whole lot faster than you can. And may give you less warning when they do.
Not convinced? Keep reading the parts in red, below.
Words do not do it justice: An accurate description of one-pedal driving mode.
Source: Yeah, I know it’s a front-wheel-drive car. The Gencraft AI doesn’t, though. Almost all pictures here are courtesy of Gencraft.
Here’s your typical bland one-sentence description of one-pedal driving mode: “With one-pedal driving, the car has enhanced regenerative braking, and will begin to slow as soon as you ease up on the gas (accelerator).”
Before I bought a Bolt, my reaction to that was, big deal. Almost all modern cars do that, to a degree. Anything with an automatic transmission slows when you take your foot off the gas. All hybrids use regenerative braking, that is, they slow down by generating and storing electricity, reserving the friction brakes (pads pressing on rotors) as a last resort.
Some EVs can now do it more? Whoop-te-doo.
Now that I own a Bolt, I know that description is missing a key word: Abruptly. Or, rapidly. Or, with great force. Take your pick.
Taking your foot off the gas in “one-pedal” mode is nothing like taking your foot off the gas in a normal or hybrid car. You don’t coast, at all. You stop, pronto. Not quite a wheels-locked panic stop. But far faster than I normally stop, and far faster than anyone would reasonably expect me to stop in traffic. In the Bolt, in one-pedal model, take your foot off the accelerator and you pull a few tenths of a G worth of deceleration. Enough to pull you forward in your seat. Enough that there’s no way I would engage that mode in snowy or icy roads. Enough that I’d think hard about it before I turned one-pedal driving on in a driving rain.
Enough, already. You get the point. Here’s a more accurate description of one-pedal driving mode:
The act of lifting up on the accelerator, in one-pedal driving mode, is equivalent to pushing the brake pedal. Hard. Your (lack of) accelerator pedal is your brake pedal. It’s not 100% as much force as you can get, if you actually do mash down the brakes. But it’s an appreciable fraction of it.
You may again think, so what? So you can, in effect, actuate the brakes, without hitting the brake pedal. What’s the big deal?
Keep reading.
Brake lights? We don’t need no stinkin’ brake lights.
But wait, it gets better.
Prior to mid-2023, some EVs would do that — stop fairly abruptly, in one-pedal mode — without turning on the brake lights. And no, I’m not kidding about that. (Reference).
The worst of those were fixed via software update, so now, all EVs on U.S. roads will now show brake lights, at some point, during some level of deceleration, in one-pedal driving mode.
As an afterthought. Does that make you feel better about it?
But even now, an EV manufacturer’s decision on when, exactly, to show brake lights, during rapid braking in one-pedal driving mode, is entirely voluntary, and entirely up to the manufacturer, here in the U.S.A. And for all of them, those lights turn on after the car has started slowing down.
Oddly enough, if you see this brought up on-line, you’ll see nothing but apologists for it. Ah, cars have always had ways of slowing down without showing brake lights. Let off the gas, in an automatic-transmission car. Downshift in a manual. Or, if you’re a jerk, hit the parking brake to stop, to fake out the folks behind you.
But those events were either mild in nature (automatic transmission), or rare and mild (nobody in the U.S. drives a manual these days, and nobody in the last 50 years has been dumb enough to wear out their clutch rather than brake pads by routinely slowing the car by downshifting). Or required outright malice, like using a hand brake to stop.
Now, by contrast, you’re putting out a whole fleet of cars, for Joe and Jane Driver, all of which are designed to be driven without touching the brakes. Designed to allow for substantial rates of deceleration without using the actual brake pedal. And for which the decision about whether, or when, to turn on the brake lights at some point during that one-pedal deceleration, is an option for the manufacturer to decide.
Let me offer a clear contrast to what you are used to, in a traditional gas car. There, the brake lights are designed to light the instant you rest your foot on the brake pedal. Brake lights are actuated by a switch that typically sits directly above the metal bar holding the brake pedal. That switch has a fine adjustment on it. You literally fine-tune-it so that the tiniest movement of the brake pedal closes the switch. Even the lightest possible braking pressure will turn on your brake lights. Properly adjusted, you literally turn on the brake lights before the brake pads make contact with the rotors.
So we now have a mixed fleet of cars on the road. For 99% of them, the brake lights illuminate as soon as the driver puts on the brakes. For the remaining 1%, the lights may come on at some point, after the driver has “put on the brakes”, assuming the rate of deceleration exceeds the manufacturer-specified threshold.
Yeah, what could possibly go wrong with that?
Braking distance versus stopping distance.
Definitions: Both terms apply to panic stops. Braking distance is how far your car travels, from the moment that you’ve firmly stomped on the brakes, until you reach a complete stop. Stopping distance, by contrast, is that, plus the distance you travel during your “reaction time”, that is, the time it takes to say “oh shit”, move your foot off the gas, and hit the brakes.
Honking the horn is optional, but highly recommended here in Northern Virginia.
Now for just a bit of math.
1: It takes about three-quarters of a second to lift your foot off the gas, and put it on the brake, in a panic stop. That’s in addition to the initial reaction time — the time it takes you to realize you need to stop quickly. (Estimates vary, that’s my reading of the literature on the subject.)
2: At 30 miles per hour, in that amount of time, a car moves about two car lengths. (Calculated as (30 MPH *5280 FT/MI *(0.75/(60*60) HOURS) = ) 33 feet.
3: EVs in one-pedal driving mode can initiate an abrupt stop without moving their foot to the brake pedal.
My takeaway from all that is that EVs in one-pedal driving mode should be able to panic-stop somewhere around a couple of car lengths shorter than traditional cars. That’s not due to better brakes, or better drivers. That occurs because they begin to brake rapidly before they even move their foot to the brake pedal.
Yeah, what could possibly go wrong with that?
Summary
Shorter stopping distance is just dandy if you’re driving an EV in one-pedal mode. But maybe isn’t such a plus for the person in a standard vehicle, tailgating an EV in one-pedal mode.
If you are in traffic, behind an EV in one-pedal mode, and the EV in front of you makes a panic stop, you need to be aware that, compared to a conventional car or hybrid:
1: That EV is inherently capable of stopping faster.
2: That EV will give you less time to realize it is stopping.
And nothing about that car will give you the faintest hint that those two factors are in play.
You’ve been warned.
Background: Regenerative braking the Toyota way, or why Bolt one-pedal mode does me more harm than good.
We changed the brake pads on my wife’s 2005 Toyota Prius sometime around 140,000 miles. Up to that point, the brakes hadn’t needed any attention.
The crazy thing is, that’s not even brag-worthy. Going 100K miles between brake jobs is normal for any car with regenerative braking.
The Prius has regenerative braking. To the greatest extent possible, the car slows down by turning itself into an electric generator. It converts the forward momentum of the car to electricity, which then charges the traction battery. Cars with regenerative braking routinely go 100,000 miles between brake jobs. So says the U.S. DOE.
No material efficiency gains — for me.
The reason for the low brake wear in a Prius is that almost all the braking energy is done electrically. In an ideal gentle stop, the friction brakes only kick in below about 5 MPH. (If your rotors have surface rust, and your windows are open, you can hear that happen until you knock the rust off the rotors.)
In an idealized stop from 30 MPH to zero, you can easily calculate the fraction of braking “power dissipation” accounted for by electrical generation versus friction brakes. Kinetic energy goes as the square of the speed, so, in a hypothetical gentle stop from 30 MPH to 0 MPH, where the friction brakes only handle the part below 5 MPH, the fraction of braking energy is:
Friction fraction of braking energy = 5-squared/30-squared = 25/900 = ~3%
Electrical Fraction of braking energy = 1 – friction fraction = 97%.
In other words, with a reasonably gentle stop, in typical suburban traffic, regenerative braking (Toyota-style) converts about 97% of the car’s forward momentum to electricity. You don’t get to keep all of that, because there are losses in the electric motor/generators, the wires and charging electronics, and in charging the battery. Maybe you keep 80% of that, or so.
One rationale offered for EV one-pedal driving is that it improves efficiency by recapturing more of the potentially available energy from braking the car. That’s because you can literally bring the car to a full stop, and so, in theory, capture 100% of the car’s forward momentum and convert it to electricity. Of which, again, you might be able to keep and use maybe 80%, after all the relevant losses are factored in.
And that’s the main reason that Bolt one-pedal driving does more-or-less nothing for my driving efficiency. Because, despite what you may read, the Bolt’s regenerative braking does more-or-less the exact same thing as the Prius, during moderate stops. In normal (not one-pedal) driving, when I take my foot off the gas, the car begins to recapture energy through regeneration. And when I push gently on the brake, it begins to capture even more energy through regeneration. Just like a Prius. (All you have to do is look at the dashboard, as you brake, to see that this is true.) And in a normal, gentle stop, with rusty rotors, you can hear the Bolt friction brakes engage at about the same speed as the Prius — about 5 MPH.
I guess if you drive like a bat out of hell, regenerative braking can improve your efficiency somewhat. Plausibly, those who routinely make quick stops can benefit from converting more of the stop to electricity, before the friction brakes kick in.
But my driving habits were formed during the Arab Oil Embargoes/energy crises of the 1970s. And I’m cheap, to boot. So I try to avoid rapid stops.
My gut reaction, from reading about this, is that the real fan-boys for one-pedal driving are, in fact, those who want to drive like a bat out of hell. They like it for the “sporty” feel, and how it lets them zip around all that much faster. Which, to me, makes the whole “efficiency” argument kind of silly. If you drive that way, clearly efficiency isn’t your goal. You’d get more miles per KWH by not trying to drive the Bolt like a sports car.
So, from my perspective, as far as efficiency goes, one-pedal driving provides a marginal improvement in efficiency, for those with habitually inefficient driving styles. Turning that around, if you’re a laid-back driver by nature, you ain’t going to get much additional efficiency out of one-pedal driving, beyond what you get from regenerative braking in “normal” driving mode.
Extras for experts, 1: There is one weird final twist on this, in that, in a hybrid, regenerative braking doesn’t much matter. It might typically add just 2% to the vehicle’s overall efficiency. That’s from a combination of factors. First, even with the efficient Atkinson-cycle engine of a Prius, you start off by wasting 60% of the energy in the gasoline. Second, with relatively small electric motor/generators, and most importantly a relatively small battery, the amount of regenerative braking force — the amount of current you can safely generate and squeeze into the battery, without damaging anything — is highly limited. So for the U.S. EPA drive cycle, with its extended periods of fast stop-and-go driving, you tend to show only a modest amount of energy recapture, as a fraction of the total energy used by the vehicle.
In an EV, by contrast, regenerative braking is a much higher contributor to overall vehicle efficiency, as the Federal government measures it. First, unlike a hybrid, all the inefficiency in converting fossil fuels to electricity is “off the books”, so to speak. That occurs at your local utility, not in your car. The calculation of overall car efficiency starts with charging it, so as a whole the vehicle appears to have vastly less total wasted energy, than a hybrid does. Second, with large motors and much larger battery, you can safely put more current into the battery. Thus, in a hard stop, an EV can likely capture more of the energy than an hybrid can, prior to applying the friction brakes.
Old dog, new trick — look ma, no brakes!
The first thing about Toyota-style regenerative braking is that it’s absolutely seamless. In the best case, you wouldn’t even guess that the car had this feature. Only if you listen very closely, and brake very slowly, can you discern the point at which the friction brakes are engaged.
The second thing about Toyota-style regenerative braking is that hybrids with regenerative braking behave exactly the same as any non-hybrid car with automatic transmission. Take your foot off the gas, and the car begins to slow just a little bit, just like any other automatic-transmission car (then) on the road. The harder you push on the brake pedal, the more braking force you get.
Regeneration in the Bolt, by contrast, feels nothing like a normal car in this regard. It is far more aggressive, even in normal (not one-pedal) mode. Take your foot off the gas in a Bolt, and you slow much faster than you would in a standard car with automatic transmission, or in a typical hybrid. I have already had to break myself of the long-learned habit of lifting my foot from the gas when I see a red light ahead. On the roads around here, If I were to do that in a Bolt, I’d come to a dead stop long before I make it to the light.
But I can live with that. I lift my foot, eyeball the dash, and look for the something close to zero KW going into or out the battery. It’s hardly a life-changing difference in driving technique. Not after I had to re-learn driving for the Prius Prime, and its preference for constant-power (instead of constant-force) acceleration (Post #1618: There ain’t no disputin’ Sir Isaac Newton).
But switching to one-pedal driving has one potentially life-changing difference: You may lose the instinct to put your foot on the brake. If you never need to panic stop, you can literally drive the car in one-pedal mode and never touch the brake. (Some one-pedal fans brag about doing exactly that.)
So do I, as a 65-year-old guy, now want to train myself to drive in one-pedal mode? This, when the approach to driving is so different from our other car (a Prius). And this, where driving in this new style means basically to ignore the brake pedal.
Short answer, no. Sooner or later, in NoVA traffic, I’m going to have to do a panic stop. And when that happens, that panic stop happens on instinct. It took me close to 20 years to get used to ABS, and to lose the instinct to release the brakes in response to a skid, and just keep my foot mashed to the floor. I really, really don’t want to lose the instinct that tells me to hit the brakes in an emergency-stop situation.
So, it’s not that I couldn’t learn this new trick. It’s that I probably shouldn’t. Not with driving two different cars. And not with my recent entry into geezerhood. Better to leave sleeping dogs lie.
The Prius Gene
This is a true story. We bought our first Prius in 2005. The same week that we bought ours, hundreds of miles away, with no communication between us, one of my brothers also bought his first Prius.
We’re now a two-Prius family. I think my brother and his wife have been a three-Prius family, with one going off to Prius heaven as a result of a freak highway accident.
My brother says the exact same thing about his Prius, as I say about ours: It pushes all my buttons, in just the right way. From the super-smooth acceleration with no gear shifts, to the dashboard feedback on mileage, pretty much everything about the car says “relax, chill, enjoy the drive”.
Maybe we both like that because that’s pretty much the way my dad drove. Maybe we inherited the genes that give us that bent. In any case, it seems to run in the family.
It takes some work to drive a Bolt as if you were puttering along in a Prius. But for whatever reason, by golly, that’s how I choose to drive it.
So, no one-pedal mode for me. It’s insufficiently Prius-like.
