Automatic Braking on Technology (BRAKE!)

That's really odd. The "BRAKE" sign, when operating correctly, will come on for at least a short time BEFORE it slams the brakes on.

But it does prime the brakes for a MASSIVE stop if necessary and when you touch the brake pedal it will act instantly and savagely.

But "at least 100 yards" - I would hazard that it hadn't even seen it at that point. It must have been something else which put the wind up it.

Maybe you need it checked. Are all your sensors clean?

 

When the car "sees" something ahead with the radar, it would display it on the screen in the dash. Do you recall if that came up on the screen before the BRAKE message?

I have never had my Prius slam on the brakes on the highway at highway speeds regardless of the amount of traffic or whether they were moving it in or out of my lane.

Something is definitely funky with the radar.

 

I have had the "BRAKE message" twice on the HIGHWAY though. One where a motorbike crossed across in front of me much too close through the gap which didn't exist between me and the car in front (on RADAR CRUISE at the time). The other just recently, when a UBER didn't see me and came into the "non-gap" in front of me.

In both cases, I touched the brakes as the message came on, and they were savage.

 

Does this model use optical detection, or just radar?

** A simple radar system can measure speeds only receding or approaching, it cannot measure cross speeds. It would need either an optical camera system, or a more advanced rotating/sweeping scanning radar to measure cross speeds.

** Even if it does know the cross speed, for safety reasons it cannot assume that an object will continue moving out of the way, because occasionally (if very infrequently) the object will stall or halt to avoid hitting a newly found obstacle in its own path. The system must allow for this possibility, and begin stopping in sufficient time to avoid a collision.

Too many drivers assume the path will clear and thus don't begin slowing down, instead cutting it very very close. This is an unsafe driving practice, and sometimes doesn't work out as expected.

If you are not slowing in time to avoid other emergency action, possibly leading to a quick and possibly ill-considered lane change (far too often misjudged), then you are exacerbating the hazard.

I would recommend a defensive driving refresher class for the pilot.

Collision avoidance systems will advance in future revisions with better hardware (and software too), but that is not what you have in this car.

 

But - would either of these happened at "at least 100 yards" as happened to the OP?

 

Very many people's judgements of distance on the road, as expressed in common measurement units, are notoriously inaccurate.

If it was close enough that his considered emergency action was already to steer around because of lack of stopping distance, than it was obviously too close.

Remember also that from an overall community health perspective, a square-on rear-ender is less lethal than a T-bone. And as more and more vehicles deploy effective collision avoidance systems, the frequency of sudden rear-enders will drop proportionately.

 

Fourth-generation Prius cars built in 2018 have the Toyota Safety Sense-P system, which uses a monocular camera and millimeter wave radar; the latter has a phased array antenna for electronic scanning. As a paper about the system (Ono, R., Ike, W., and Fukaya, Y., “Pre-Collision System for Toyota Safety Sense,” SAE Technical Paper 2016-01-1458) explains, the scanning (± 25° in the horizontal) helps to detect crossing pedestrians.

The paper also describes some of the system’s countermeasures against rear-end collisions, a hazard of which its designers were well aware. For example, TSS-P applies “a slight deceleration of about 0.7 m/s² before” pre-collision braking, lighting the stop lamps to warn other drivers. Indeed, Toyota did tests in a driving simulator, showing how this reduces the braking time of a following vehicle; the paper includes a plot of the observed relationship. The system also does feedback control of the deceleration, so the braking force doesn’t have to be much greater, or the stopping distance much shorter, than actually required to avoid a collision with the detected target.

 

Optical too from a camera above the rear view mirror.

 

At a highway speed of 70mph it would take only 3 seconds to travel 100yds.
Assuming you're not taking any evasive action like easing off the gas, braking or steering away from the object, I don't think it's unreasonable for the car to think you're going to hit something that may have been within its detection area for 3 or 4 seconds as it crossed it.
If the car crossing in front had stalled, then your thinking distance and braking distance would probably be longer than 3 seconds by the time you realised that it had stalled.

 

I had this happen once on the highway when my car detected a tire that was on the shoulder and slightly protruding into my lane. If this happens and it’s a false detection, or something you can control and don’t need TSS-P to manage, you can very lightly tap the brakes and that should acknowledge the signal without causing you to come to a flying halt. Steer slightly away from whatever it’s detecting. Also, you can adjust the sensitivity of TSS-P if it’s flagging too often. As for sensors being dirty, that’s conceivable. Usually dirty sensors will flag an alert and let you know that TSS-P is disabled until you clean the sensors.

 

Come to think of it, the only time I have it happen is if I'm going into a curved section of roadway and there are bicyclists in that curve. The car will start to brake then, BUT not display the BRAKE message.

 

I don't have rear traffic sensors. It still sometimes looks like Safety Sense on the MID shows when cars are approaching from behind. The one false positive I noticed - which I'm not clear I fully understand what the computers were detecting - was at 40 mph on 4 lane road approaching a curve of perhaps 20 degrees.
No traffic in front or behind going in my direction. 3 or 4 cars approaching going in the opposite direction and the MID displayed the Car in Front graphic. The graphic I see when in dynamic cruise in the prime is approaching a car in front of me. I looked around to try to see what the sensors were picking up and the only thing that made any sense to me anyways was the cars approaching from in front, but going in the opposite direction.

I usually now hold the cruise button in until I get regular cruise, since I don't want to have to play around with changing safety sense setting and possibly forgetting which setting I have it set to. Even though it only takes a couple seconds to change the setting using the steering wheel button.

 

I've only used "regular cruise" once - turned it off after 2 minutes, went back to RADAR Cruise which I use all the time. It works well. Only time it doesn't is in very heavy rain, where it's blind to cars.

 

I don't mind using dynamic cruise, I've used it a lot since getting the car. I really like the braking feature when holding the stick down for about 5 seconds the cruise will regenerative brake fairly stiffly down to around 32 mph from whatever speed the car is set at. I just prefer the regular cruise for how and when I use it.

 

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That's odd - it's not on mine. I have to do a long press on the button at the end of the stalk to bring it in - then next time it's back to DRCC again.

 

You are correct and I was mistaken. It has been a while since I referred to the manual for the car.

 

Anyone ever use that DRCC stick brake? stiff regenerative braking like a tesla with regen set on Plaid..

 

I had this happen to me once on a secondary road and once backing up on my 2020 LE ,prius. In both cases, the brake warning sign came on the screen several seconds before the brakes activated, and I was not paying attention while talking to my wife. There might have been an accident w/o this wonderful safety feature.