And this is where I take a minor quabble with TC. He thought that GM “over engineered” the volt’s brake light system because it used an accelerometer. But honestly I think it is the easiest way. The only “trick” is to make it stay on once stopped (I’m sure there are conditions when you wouldn’t want them on when stopped though).

In fact it looks like the new standards are going to be defined by the rate if deceleration regardless of the means used to do so. Which means basing the lights on an accelerometer is the most reliable way to meet the spec. It shouldn’t matter how you are slowing down, just that you are.

Why do we need an accelerator when we know how fast the wheels are turning directly? The first derivative of wheel speed is almost always car acceleration. Is this to handle some edge cases like not turning the brake lights on when we're actually sliding on ice with the wheels locked? Seems unnecessary.

Could be as simple as "the controller doesn't have wheel speed on its can bus"

Why is it unnecessary? Isn't the primary goal of brake lights to inform people of deceleration and the secondary goal to indicate when a vehicle is locked stationary?

Showing a speeding vehicle behind you that you've lost control and should slow down is well within the conops of brake lights.

I think there's an agreement on the goal.

I believe the poster is asking why do we need another sensor to achieve that goal? We already have a speedometer based on wheel motion. A trivial computation gives acceleration.

Because tires can block, not all tire turn at the same speed all the time, because redundancy is a good thing... There a lot of reasons. And if the number of sensors in your car bother you, well, the early 80s, with carbs and without ABS, are last model years you can buy.

Many late 80s/very early 90s fuel injection vehicles have shockingly low numbers of electronic sensors also. MAP, O2, TPS, CPS is all you really need. Some of them are even analog.

No need to get snarky :)

Redundancy is a good thing. Do we feel manufacturers are cross-checking accelerometer with the speedometer? If not, there is no redundancy gained.

On the other hand, if your speedometer is faulty, or not working, you'll know pretty fast. If your dedicated accelerometer is faulty, you might have no idea what your brake lights are doing in the back.

Dunno; it just feels "we need to know acceleration, let's add an accelerometer" is a non-imaginative, add-cost, add-complexity idea. Again, if we think ABS and speedometer and the new accelerometer are being cross-checked and sanity-checked, awesome, but I'm just a bit cynical of that, compared to adding the 3rd thing to do the same thing.

> add-complexity idea

You're on a forum where a lot of people think Kubernetes is a good idea.

;-)

No idea what Kubernetes are, and I am serious here, since I am no software dev. On the hardware side of things, there is redundancy and complexity. Those two might look similar on the surface, they are totally different beasts so.

To simplify it as much as possible, Kubernetes is a system that deploys and manages containers. Containers allow multiple programs to run under the same system while being completely isolated.

However, Kubernetes is actually incredibly complex. It's powerful, but holy hell does it add complexity. And yet it's basically a buzzword and people are using it in very basic scenarios that don't call for the complexity that Kubernetes adds.

So that's where my joke comes from. There are people that behave as if adding unnecessary complexity is a feature.

Thank you a lot for the explanation!

An accelerometer will show braking when going at constant speed down a hill. Change in wheel speed is better.

I’d say you are braking somehow if you are going at a constant speed downhill, are you not?

What do you save by not having that sensor? Nothing. What do you lose by not having that sensor? The ability for the system to perform its requirements.

I'm assuming you read the context of loss of traction being a requirement.

Relatedly, having a dedicated accelerometer sensor is a baseline dependency of many of other modern features like cross-comparing compass headings and GPS for map heading information (much less any of the Level 2+ "self-driving" mechanics). Most cars likely want one, anyway, even in base models. It's one of the cheapest sensors in a suite of increasingly standard sensors (in almost any form factor of device, not just cars, but phones/watches/toasters/etc).

> What do you save by not having that sensor?

A $9000 repair bill in 6 years when your mechanic tells you “Sorry your car is immobilized and won’t start, the deceleration brake light sensor went and we need to remove the motor and 3/4 of the wiring harness to replace it”. And for anyone who thinks I’m being sarcastic, try owning a BMW or an Audi and you’ll know it’s the truth.

The problem there is owning an Audi or BMW, not the sensor itself.

A MEMS accelerometer is pennies. If it is critical then add redundancy and don't accept single fault tolerance. If it is optional or the signal can be estimated from other sources with acceptable error then fail gracefully. This is honestly simple stuff. Accepting less from manufacturers is a bad deal.

>>A MEMS accelerometer is pennies.

That's an dishonest argument (perhaps not intentionally). NOTHING is "pennies" to a consumer when it comes to repairing a vehicle. More frequently, you pay $1,500 in labour and parts to replace something that costs "pennies" in some bulk manufacturer wholesale catalogue.

Modern cars are getting more and more awesome, in terms of safety and convenience; but the sticker shock when going to dealership for repair is also becoming bigger and bigger, and it absolutely is intertwined with the significant effort to make 3rd party or even self-repair difficult to impossible.

So again, IFF this is an easily replaceable part that is thought-through and cross-checked intelligently with other existing sensors, brilliant. But can you at any level understand my skepticism that any of these are true? :)

I'm not talking about the price of maintenance but the cost of manufacture. Please give generous interpretations rather than ones that fit an argumentative narrative. Assuming the topic is cost of repair rather than manufacture makes no sense when we're talking about system engineering to accomplish functional requirements.

I have owned a troublesome 2010 BMW for 8 years now. No individual repair has been over $2,700.00. Most are closer to $1,500. I was quoted (by a dealer) $15,000.00 for engine repair once but it turned out to be a spark plug. $9,000.00 sounds like a dealer quote. Find a good independent shop.

> Why do we need an accelerator when we know how fast the wheels are turning directly?

Because the car doesn't precisely know the wheel's radius. The radius depends on the rim, tire, and tire pressure - and the car's operator may have accidentally or deliberately chosen something unexpected for any one of those 3 parameters.

You're not wrong but how much does not precisely knowing the wheel's radius change things?

The operator would also experience an incorrect speedometer. For the purposes of a brake light it'd be off (either lighting up too soon or too late) by some amount but I imagine it'd be "close enough" except for the most pathological cases of tire sizes.

ABS relies on the fact that the computer knows how fast the wheels are turning in relation to each other. Wouldn't seem to be much of a leap to leverage that system to determine if the car is slowing down.

You don't need to know the wheel radius, you just need to the rate of change of the wheels RPM. Then the brake light could be set to some conservative value where regardless of any realistic tire size/inflation scenario the brakes will activate at a reasonable deceleration.

In any case, with standard gas/brake pedals on ICEs, I can barely tap the brakes and have my brake lights turn on without any deceleration on my part, or I can do that and still be pressing the gas and so actually be accelerating with my brake lights on.

If the car doesn't know the size and speed of the tires it also can't accurately display the current speed, which seems like at least as pressing a problem.

Edit: Or is detecting deceleration more sensitive? I guess I don't know how precise it needs to be relative to how precise a speedometer needs to be.

This is not a problem. We know this isn't a problem, because ABS almost always uses wheel speed sensors to do it's thing, and ESC usually taps into the same wheel speed sensors. Hell, those sensors are often used to tell you when your tires are low on pressure, and yet that doesn't prevent any of the other functions from working, or even affect your speedometer enough to care.

Changing wheel size enough to affect readings of speed does not change the broad slope of the derivative of wheel speed.

One more possible reason might be when driving down a hill: When descending a sufficiently steep incline, I may be braking just to maintain my speed! That said, I'm still _feeling_ negative acceleration to maintain a constant speed. (That said, if I'm doing this for more than a handful of seconds, I'll generally downshift, and _that_ doesn't trigger brake lights, despite also causing increased negative acceleration. So maybe regenerative braking is analagous, and fine without brake lights?)

I don’t think it’s such an edge case at all. I imagine it would be disconcerting to the driver behind you if you slammed on the brakes on a wet road, locked the wheels into a skid, and the brake lights went out. (Shouldn’t happen with modern braking systems, but still…)

>The first derivative of wheel speed is *almost* always car acceleration.

There you go, you answered your own question. When it comes to safety features, "almost" always working isn't really good enough.

Yea the Tesla brakes work exactly how you would want. As soon as you start decelerating, even w/ regen, the lights kick in. Not sure how Hyundai messed it up after having an example..

Hyundai/Kia will do the minimum necessary to comply with the local laws.

Interesting, would seem more efficient to comply with as many laws as possible that don't clash and have one configuration, only tweaking for local oddities.

I have a complaint about Tesla on this one. In Europe they ship vastly better tail lights with a different color for turn signals. As I understand, the superior signals are acceptable, though not preferred, by US standards. But Tesla makes a separate, different, worse part to use in the US, rather than giving us the good stuff also and reducing their manufacturing variation.

As an European I always wondered about why in the US they have red turn signals. It seems...odd, so to speak?

Suburban and Americans aren't so good at living with the level of aggression that city driving requires, and so prefer having clearly delineated left turn time to something perceived as fighting in traffic.

It’s cheaper (or was) - early cars had one red light on each side and could blink it for a turn signal, but it was also the brake.

Later a second light was added for the taillight but they did it in the same bulb.

Thanks for the explanation! It still seems odd to me that it hasn't been regulated somehow, having 2 different colors for 2 different actions is safer.

It's a pattern for Hyundai/Kia. The whole thing with "no anti-theft immobilizers" is only where it's not mandated.

Their business is shaving pennies, hence why they didn't install immobilizers in states that didn't require them, in order to save a few bucks per car.

My 2020 Bolt does the right thing also. I only wish the lights would stay on while I'm stopped at a light but that's minor.

Do you not hold the brake when you are stopped? I know electric cars don't have to creep like an automatic ICE, but I drive stick and still hold the brake at a stop because not randomly drifting forward or backward is safer and it also is important in the rare case of getting rear ended

I don't know about GM, but in a Tesla, if you have 1-pedal driving enabled, then it automatically holds the brake. And the brake lights stay on.

> I know electric cars don't have to creep like an automatic ICE

They don't have to, but funny enough, it's an option in a Tesla. My wife uses it simply because it's what she's used to.

The Mustang Mach E is somewhat similar, if I remember correctly.

When slowing down with regenerative breaking it triggers the lights basin on how fast it’s slowing down.

So if it’s just a tiny bit (slowing from 35 to 30 over a few seconds) it won’t kick on, but from 70 to 35 over a few seconds would.

We had manual transmission cars that could slow down from engine braking for decades and that never lit the brake lights. I'm not convinced this is an issue. It is your responsibility to keep a safe following distance from any car in front of you regardless of circumstances. You should be able to safely not rear end the car in front of you even if it immediately hits a brick wall and stops completely, and even if the brake lights are entirely missing.

Yep.

I drive a Jeep Wrangler and between its manual transmission and the fact that it's less aerodynamic than a cow, I can and often do slow from highway speeds to ~25 MPH without touching the brakes. It's never been an issue.

EU regulators were convinced it was an issue. And do many drivers follow you so distantly?

> You should be able to safely not rear end the car in front of you even if it immediately hits a brick wall and stops completely, […]

I don't think that's the usual expectation. Maintaining absolute braking distance is what railways do, whereas cars commonly operate on relative braking distance plus a safety margin accounting for your reaction time.

Over here, both driving schools and other general road safety education talk about keeping 2 seconds following distance (or half your kph-speed in metres, which corresponds to 1.8 seconds following distance), and traffic engineers commonly give the capacity of a single lane of traffic as 1800 vehicles/hour, which again corresponds to 2 seconds distance (this time as measured vehicle front to vehicle front, instead of tail to front of following vehicle).

The BMW i3 is the same way.

This sounds like a recallable defect in the OP’s car’s software.

One drawback of this is the person behind may incorrectly assume you’re brake checking them; i have had a number of tailgating drivers retaliate by accelerating in front of me and slamming on their brakes (which is especially stressful with a kid in the car). Stuff like this makes me deeply regret choosing Tesla.

I would love a control panel option to turn automatic brake lights on/off.

It's really annoying to signal something to others that you don't intend.

I think there are a bunch of things tesla does that prevent you from being a good driver.

some examples:

- you can't flash your headlights (unless you traverse menus on the touchscreen)

- you can't set the wiper interval

- quickly turn on the defrost?

- turn on the hazard lights? (you should google it, it's funny)

newer cars go even further - no stalks, touch controls for turn signals, horn and wipers on the steering wheel

What model and year do you have?

My 6 month old Model Y has a dedicated button for hazard lights on the ceiling.

I can flash lights by pulling the left stalk (I forget if forward or backwards).

Wiper interval I do miss but what I usually do is either hit the wiper button once on the stalk and set via screen or use the voice interface to say “wipers speed 1”.

Can’t you turn defroster on with the voice interface too?

> - you can't flash your headlights

You can flash the high beams which is good enough, IMO.

> - you can't set the wiper interval

You absolutely can. In my Model 3, if I press the button on the left stalk, it makes the wipers wipe once and shows the wiper options on the screen. I can set them to Off, Slow, Medium, Fast, and Auto.

The only exception is when the car is on Autopilot. Then, it's forced to be Auto. The quality of Tesla's auto-wiper software is questionable, though.

> - quickly turn on the defrost?

There's a button for that now.

> - turn on the hazard lights? (you should google it, it's funny)

? It's a button above the mirror?

> ? It's a button above the mirror?

Yep. FWIW, Hazard Lights is the one control that is mandated to be a physical button by US law.

> You can flash the high beams which is good enough, IMO.

flashing high beams at night is a good way to set people off and wind up in a road rage incident. some people interpret that signal very aggressively and will retaliate.

I feel like the psychology behind being a driver is a problem.