Dear Car Talk column about sensors on a car grill

Re: Your very recent article regarding the sensor mounted on the car grill reminded me to send this message.
I am an AARP Driver Safety (CT)Instructor/Carfit Event Coordinator and other AARP related programs. Question came up in a class.
Imagined situation: Driving South on Route 91, driver is maintaining the accepted 3 seconds behind the car in front. Cruise control, Lane departure and Forward Collision systems are engaged. Another car enters Rt 91 using a merge lane from right and merges into your lane between you and the car you were following….Distances between cars are now temporarily changed and closer. Suddenly the car in front dramatically attempts to stop forward motion. Forward Collision system brakes your car. Car following you at the usual “tail gating” distance of 10 feet, slams into your rear in part because the rear braking light isn’t activated by the auto braking system…or is it? ( I can’t observe the tail light operation while driving my own 2016 Subaru Outback.) There is a presumption the light does go on but I couldn’t reliably speak to the challenge. (I checked with the UCONN professor in charge of autonomous car development and he made the same presumption but also wasn’t absolutely sure.)

On this site many people here will not know which article you are referring to . This is probably rude but as an instructor should you not contact someone to have the answer to the question about the brake lights. According to Mr. Google yes the brake lights will activate when the collision avoidance system applies the brakes.

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Well, the two of you could hsve answered the question by going out to your car and having one person drive at something big enough to activate the brakes and the other person observing.

I believe this is the column in question.

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I don’t know what’s the point… that presumption is there whether or not a car has a FCW system or not. Once the brakes are applied (manually by foot or automatically by FCW) the brake lights are assumed to be on.

Any time the brakes get applied, the brake lights are supposed to come on. It’s the law. Here’s a sample of the relevant law from my state:

Stoplights. (a) Any vehicle may be equipped and when required under this chapter, shall be equipped with at least two stop lamps on the rear which shall emit a red or yellow light and which shall be actuated upon application of the service (foot) brake and which may, but need not be, incorporated with the tail lamps and which shall be plainly visible and understandable from a distance of 100 feet to the rear during normal sunlight and at night. (emphasis added)

Any time the service brakes are activated, the brake lights have to illuminate, even if they’re activated automatically.

As for the scenario you propose, yes it’s entirely possible, but isn’t really any different from if you didn’t have the system, and stopped for the guy stopping in front of you.

De-jure, you are right, the guy on the back is guilty if he hits your car stopping for the car in front of you.

De-facto, it is a little bit more complicated.

I have Honda with that auto-applied brakes, and the system works flawlessly in my assessment, but it is still a dumb system.
At least once, a guy in front of me slammed on brakes momentarily and then let go.
The auto-pilot went into the full “brake hard” mode, where I was actually about to correct the speed in more gentle way, but I was overridden.
To compare to my expected correction, it made it more painful to the guy in rear when it was warranted, I’ve seen it in his deep/big/round eyes :slight_smile:

On the contrary, when I see that cars in 1-2-3 cars in front of me start braking, I “blink” my lights by light tap on brakes so the person in the back is prepared I might need to use brakes more energetically soon - that saved me from getting rear-ended few times.

So, while de-jure the auto-pilot is right, de-facto (given driver did not sleep over the developing situation) it may save your front and still give you an increased chances of getting rear-ended.

I disagree that the system is “dumb.” It’s a system of last resort meant to compensate for a dumb driver who is too busy doing other things to notice that cars in front of him are stopping.

I don’t have a lot of experience with the Honda-specific system. I’ve driven my mom’s RDX a handful of times, though, and it’s never activated for me even when the guy in front briefly taps the brakes. This would suggest that if it’s activating for you in that scenario, it’s because you’re following too close in the first place.

I did note that the TLX I drove a number of years ago when I had the timing belt replaced on my TL did tend to flash the big yellow “BRAKE” warning more frequently than I felt it should, but it never actually braked because I was keeping a proper following distance.

I have more experience with the system in my Lexus which also has never activated in panic-brake mode, but the radar cruise control does use the brakes when someone in front slows down. It does a good job of matching their rate of deceleration, with some cushion of lighter braking at first since it also by default keeps a very good following distance. So, no hard braking unless they’re braking hard (and honestly, if that’s the case I’ve usually already overridden the cruise because I’ve been paying attention and know we’re slowing down before the guy in front of me does).

In other words, in my experience with both systems, if the last-resort emergency brake system is activating absent an actual near-collision it’s generally because the cruise control isn’t being used (or isn’t in radar-adaptive mode) and the driver is following too close.

When you consider the liability for vehicle manufacturers that would result from a lack of brake lights when these AEB systems are activated, do you REALLY think that the auto makers would not have ensured that the brake lights are activated when the AEB system is activated?

I do agree on this.

In my example - yes, I followed too close (it was on intersection with a yellow light), it was raining (which probably contributed) and while I do know that “BRAKE” warning light shown by this system, I do not see it often, so I’m not a habitual tailgater.

I still think it applied too much brake force, but it was a borderline with me having too much speed difference for the system to have time to react in that close quarters. The guy in the rear was indeed surprised from my over-reaction.