A heads up: Brake lights when coasting

A heads up: Brake lights when coasting

I was on the freeway today and a motorcycle was tailgating me. I let off the accelerator while glancing in the rear-view, and this triggered my brake lights, so the guy thought I was messing with him, and passed while making a gesture. Now, I don't know why he didn't just lane split to begin with, it's legal here, but it serves as a caution: the Tesla lights it's brake lights every time you release the accelerator pedal and the car begins to regen/decelerate, and some could misinterpret this as an intentional braking action.

jat | August 29, 2013

If the car is slowing down, why wouldn't you want the brake lights on? Yes, in a manual you could use engine braking to slow down without alerting the tailgater and have him run into you, but it seems like you would prefer to avoid that.

riceuguy | August 29, 2013

And it's not every tine you let off the accelerator; it's when the accelerometer determines that you're slowing down as much as if you were actually braking.

jat | August 29, 2013

In my car, it is exactly at -30kW regen.

chrisdl | August 29, 2013

Nevertheless, for new owners it is important information. You need to adjust your driving style to single pedal driving. If you want to coast, don't take your foot completely of the accelerator but keep it pressed to adjust your speed. (It's not really "coasting", I know.)

And, obviously, you did read the manual, right? ;-)

"NOTE: If regenerative braking is aggressively
slowing Model S, such as on a steep descent,
brake lights turn on to alert other road users
that you are slowing down."

I admit that this text is not very clear (why do they mention the steep descent?), but it is a direct copy paste from the manual.

mrspaghetti | August 29, 2013

I actually don't see the issue, other than that you were tailgated by an a-hole on a motorcycle. And that's his problem.

J.T. | August 29, 2013

@mrspaghettii I agree. Some people seem to find it uncomfortable when their intentions are misunderstood . . . even when they are misunderstood by a jerk.

DFibRL8R | August 29, 2013

I believe in some areas "break checking" or tapping your brakes in response to someone tailgating you could get you fined for aggressive driving (not to mention cause an escalated response/road rage situation).

dtesla | August 29, 2013

On the 17" console, running SW version 4.5, press the Tesla "T" at the top to bring up the release notes screen. The car on this screen is animated and you can see when the break lights are on. My MS turns the break lights on at 15 KWh regen.

Theresa | August 29, 2013

@DFibRL8R, Really? I have never heard this but it would not surprise me to find it is true.

DFibRL8R | August 29, 2013

I have read accounts of drivers who brake check getting cited or found at fault in accidents. That said, I would think there's a difference between slamming on the brakes and the gentle deceleration when you let of the accelerator in the model S. I suppose a tailgater could over-react and run off the road but the bigger issue is inciting road rage.

DallasTXModelS | August 29, 2013

I would hope the person tailgating would be pulled over for not following at a safe distance before ssomeone sowing down. If you let off your accelerator enogh for regenerative braking to begin the brake light should come on longer than just appearing to be a brake check.

J.T. | August 29, 2013

I believe in some areas "break checking" or tapping your brakes in response to someone tailgating you could get you fined for aggressive driving

How is this enforced? Is there a statey that can see that the car in back of you is tailgating and doing nothing about it? How can the statey see that you're brake tapping if he's not behind you? Is he the one tailgating? If not, can he see through the car that's tailgating?

DickB | August 29, 2013

There is no brake light when using the LOW setting in regen. The MS will act just like an ICE vehicle. The STANDARD setting in regen you will have brake lights once you feel your body going forward. The kW reading on regen does not really matter. It all depends on how much you let your foot off the pedal.

Press the battery icon on the top left will give you a large picture of you car, then you will see the brake lights on or off.

moorelin | August 29, 2013

This could have gotten me in trouble this morning.

Driving in to Texas Medical Center through a residential neighborhood well known for aggressive policing (South Side Place for Houstonians), I was first at a red light. I took off pretty fast, but not all the way down, and I got to only a few miles over the 35 MPH limit. I didn't see where he came from, but a couple of blocks later a police car came up very fast (well over the limit) and tailgated me aggressively. No lights, but he was definitely within 5 feet at times. I obviously maintained a safe and steady pace, and he left after a few blocks (ridiculously small jurisdiction for this micro town). Clearly harassment. Very glad a misinterpreted regen brake light didn't make it worse.

shop | August 29, 2013

Guys, there is no issue or difference here - the brake lights come on when the car slows down. Just like any other car.

moorelin | August 29, 2013

@shop - agree 100%. There have been many other threads on this, but it works exactly as it should. And the Tesla icon animation thing can answer any questions.

RedShift | August 29, 2013

I have a slight disagreement on the 'no issue' comment.

On an interstate, ICE cars slow down due to coasting. TMS slows down with brake lights, (with a slight margin, say 15 kw regen) which could be interpreted as brake checking. I have done some investigation about when the lights turn on during what is 'light' slowdown on interstate roads. There does seem to be that thing where you coasted and thought lights weren't on, but thy had come on. May be the software can factor in current speed as well when deciding to tun them on? Increase the margin at anything above 65 and keep it normal below that?

On street roads this is no issue, however.

carlk | August 29, 2013

@Theresa He is right about brake checking. Same as flashing headlight to signal slower moving cars on the fast lane you want to go faster can be looked at as aggressive and provocative behavior and be ticketed in California even though it's a standard practice on European highways. All these are not bad as tailgating of course.

skymaster | August 29, 2013

I would estimate 75% of drivers follow way to close and "tail gate". I do not. I allow plenty of braking distance at any speed...and more on the highway. I also use the "low" regen in the Model S.

The majority of drivers have no concept that they following to close. The things that make you say hmmmmmmm...

skymaster | August 29, 2013

Edit: That they ARE following to close.

DonS | August 29, 2013

Brake lights turn on based on accelerometers, so that is equivalent to regen torque, but not power. Therefore, regen power to activate brake lights depends on speed.

AmpedRealtor | August 29, 2013

In standard regen mode, when you let your foot off the accelerator the car slows much faster than if it were coasting. If you want to be able to coast in Model S without brake lights coming on when you release the accelerator, switch to low regen mode and your "issue" is solved. It will behave just like an ICE vehicle.

However, having brake lights come in standard regen mode is absolutely appropriate due to the heavy slowing exhibited by the car. In Model S, standard regen slows the vehicle at the same rate as if you applied braking force in an ICE vehicle. It is NOT like coasting! When you slow down at such a high rate, then the brake lights must come on because they would in any other car when your forward acceleration slows at such a rapid pace.

NKYTA | August 29, 2013


"On the 17" console, running SW version 4.5, press the Tesla "T" at the top to bring up the release notes screen"

This feature no longer exists in 5.0. I think it is really useful for new owners so I'm sad to see it go.
But once you get used the car, you don't really need it.

PBEndo | August 29, 2013

This isn't "brake checking" i.e. tapping the brake pedal for the sake of triggering the brake lights. The regen is actually slowing down the car. The lights only illuminate in response to the deceleration. This is a much more logical way to apply the brake lights than the way it is implemented in most ICE cars.

AndyZ | August 29, 2013

Well, I disagree on 'non-issue' - here's what happened on 4th of July.

I was driving home after watching fireworks (about 10PM) and suddenly a police car followed me and made me stop. After some chat and checking 'who is who' he said that I was driving 'erratically' as the reason for pulling me over. I didn't get his explanation at the time, but some weeks later I did notice brake lights coming on when I was cruising! I was my first month back in July and I had no idea why police car was following me, but after reading this thread I think some people just don't get the 'new driving style' of an electric car.

I have to be more careful with driving after sunset on my S60 just to not waste time for chats like this !

danej | August 29, 2013

As folks have noted, the system works the way the system should; when you have regen set on the standard level, the auto slows quite a bit, and the lights come on, as they should.

My only point was to give folks a heads up: your tail lights are doing a lot of blinking you may be unaware of, and this may confuse other drivers, the police, etc. Don't be surprised if you get some reaction, along the lines of "why the heck is that guy in front of me braking now??", etc.


larryh | August 29, 2013

I did some testing today and determined the lights come on at a particular accelerator pedal angle and not at a specific regen level.

For example, when going about 15 mph, the lights came on at about 5kW while at 40 mph, about 30kW with Inbetween speeds giving inbetween power. All at seemingly the same pedal angle.

Brian H | August 29, 2013

skymaster | August 29, 2013 new
Edit: That they ARE following to close.

Edit Edit: That they ARE following TOO close.


jbunn | August 29, 2013

I'm not brake checking. It's because I thought a cat was going to run in front of the car, saw a deer in the woods, a piece of debris in the road, or someone in front of me slowing down.

If you rear end me because you are tailgating it's your damn fault, and you're paying for back massages for the next year.

It's the responsibility of the person behind me to keep their distance, and be able to stop in a controlled manner.

AmpedRealtor | August 29, 2013

+100 @ jbunn, couldn't agree more.

The solution to avoiding erratic flashing of brake lights is to not completely let off the pedal in your Model S when you want to slow down. Instead, gently ease off the pedal and control your slowing using just the one pedal. If your body lurches forward, you've let off the pedal too much, so next time ease up less. Once you get the hang of it, your gentler foot motion will lead to more consistent and sensible brake light behavior to those behind you. If you want something close to "coasting", gently ease off on the accelerator until your regen and energy usage are both as close to zero as possible. This will allow for simulated coasting without activating your brake lights.

grega | September 3, 2013

That's an interesting design challenge. I've been wondering about the Tesla's method of regeneration by taking the foot of the accelerator. I assume that the traditional method of using regen for slight braking, while using the brakes + regen for hard braking, hasn't been quite as effective as desired in the EV world since most drivers are heavy on the brake, thus reducing the regeneration.

- and I think perhaps the Tesla design helps break that braking habit.

But when to display the brake light is the challenge. I already drive softly on my brake (normal car though), and notice reactions of people to lack of brake lights. ie: People behind can be annoyed if the car is slowing down towards red lights, and will change lanes and accelerate quickly to the lights to stop first - BUT if I make my brake light show then most will stay behind me, since they perceive me as slowing for the lights rather than them being stuck behind a slow driver (which I'm not).

On the freeway coming out of 65mph to 55mph zones, I allow my car to slow itself down, and I wouldn't want my brake light popping on.

Which is all to say the psychological design of brake lights isn't simple. They send a message to the car behind, including "I am actively slowing down", but also "my foot is on the brake peddle, I might brake quickly" - and neither of those messages is necessarily true on the Tesla, as it adds "the car is slowing more quickly than an ICE car would be expected to slow".

I imagine the designers are already grappling and adjusting when best to show the brake light. Software design could notice when a driver drives on the cusp of having the brake light on and adjust what it shows - though I suspect that driving on that cusp already would mean you're slowing down (and if it isn't that would be an easy design change). And I also have to wonder what the Tesla can do to help the world of peddle-pumping drivers who constantly accelerate then pull their foot off.

Providing a different physical feedback in the brake peddle could be very effective, like very light pressure to keep the car at the point of "slowing a tiny bit" with no brake light, but normal pressure for driving. But they need to have the car effectively brake too when the foot is pulled back, and that would seem to affect that. Another option is to actually have a slightly 'bigger' brake light when the foot is actually on the brake peddle.

jjaeger | September 3, 2013

For me, if the car is slowing down - the brake light should be lit. Other than the car is slowing down, the folks behind can dream up any other rational that they may want to add to this complicated equation.

mrspaghetti | September 3, 2013

My head hurts.

Iowa92x | September 3, 2013

I drive a manual transmission, which slows significantly due to engine braking when I let off the gas. No brake light and no problems getting rear ended.

To those that drive a Tesla and also used to work a stick shift, is the deceleration on regen similar to decel' due to engine braking? If yes, then maybe Tesla doesn't need the light when lifting the throttle. If deceleration on regen is more aggressive than engine braking, the light might be safest.

mdemetri | September 3, 2013


Excellent point.

I would say that the full regen (completely taking your foot off the accelerator) is equivalent to down-shifting in a manual transmission. That is, it is much more than simply letting off the gas in a manual transmission.

IMO, after 6 months of driving, I think Tesla has the regen/brake light issue just about right.

JC1 | September 3, 2013

Anyone know where the accelerometer is located or whether there are multiple accelerometers (i.e. to better detect deceleration on turns such as on a highway cloverleaf)

Brian H | September 4, 2013

Feathering the pedal halfway gives some slowing, not necessarily with brake-lighting. It's G-force controlled. You can learn to do it "by the seat of your pants" if you try. Bring up the car logo and watch the image.

lspitzner | September 4, 2013

If your curious how this works, just try this (worked for me last night). When driving at night, turn your rear camera on. Whenever the break lights kick in from regenerative breaking, you will see the lights turn on in your camera (easy to spot the red). This will give you a good feel for when it does / does not come on during the slow-down process.

PatT | September 4, 2013

TM used this system on the Roadster as well. I discovered it one foggy night almost a year after taking delivery.

AmpedRealtor | September 4, 2013

@ Iowa92x,

Just because engine braking in a stick shift doesn't activate the brake lights doesn't mean that it shouldn't be done. The purpose of brake lights is to warn the person behind you that you are decelerating. That should apply to engine braking as well. The fact that something has never been done before doesn't mean it shouldn't be done today if the technology exists.

I believe the folks behind me should receive a visual indicator of my deceleration. Withholding information on the roadway is never good. Just like in real estate, disclose, disclose, disclose! :)