Pre Collision System Activated

If there was a slight rise in the road where the plate was, its not unexpected. Some people complain that the PCS doesn't do enough.
Anyhow if you check the manual you'll see that there's a way to turn it off, but there's no such thing as a 100% perfect system.

 

The owners manual has a section about things that might inadvertently set off the PCS. It's a known issue, though there haven't been many reports here of false activations. My personal experience is that the brakes don't come on that hard. Toyota made a deliberate decision not to bring the car to a hard stop, unlike similar systems used by some other car companies. It tightens the belts, primes the brakes, makes noises, and starts applying the brakes if you get close to a perceived obstruction. Personally, I think the safety benefits far outweigh any possible concern about rare false activations. I've seen reports of studies in Europe showing these systems definitely save lives. I also have my own personal experiences that made me a true believer. I won't ever buy another car without a similar system.

See this thread: PCS Saved My Stupid Butt | PriusChat

 

I would try driving over that plate again and see if it repeats.

 

The automatic braking on our 2012 Prius v Five/ATP has not activated inappropriately in the few hundred miles I've driven it over the past two years and my wife has never mentioned it happening but it has happened on the 2014 Sienna with ATP we purchased in April. The Sienna is my daily driver and the Prius v is my wife's.

A couple of months ago, the Sienna braked itself when I was approaching a steel plate on a city street while driving at the 40 mph speed limit and using the radar cruise control. I was on a level portion of the road when it happened and the steel plate was at a point where the street started up-hill. The steel plate was in place for several weeks and I drove over it numerous other times during my daily commute without the problem reoccurring. I have driven over many other steel plates without this problem occurring.

When I told a Toyota dealer service writer about the steel plate incident, he gave me a list from tis.toyota.com describing when this problem can occur. There are 12 situations described on the list including steel plates, narrow metal bridges, road signs and road debris.

I regard this issue as minor, like knowing we have PCS, even if it might help only a little, and I absolutely love radar cruise control.

 

The radar signature of a metal plate would vary with how close to perpendicular the approach was to the span-wise and thickness-wise edge. Approach the plate edge from different aspects/angles and the PCS results would likely differ from "lock" to nothing at all. Would be interesting to know what was the threshold approach angle to trigger PCS.

 

In my experience, auto braking while using the radar cruise is different than pure PCS braking, and seems to have a different threshold for activating. Many of the same type things might cause it, but the radar speed control is far more sensitive than PCS.

 

It is hard to tell where one leaves off and the other begins. If a car cuts in front of me while using radar cruise control, the system seems to "think" for a few seconds before it starts braking enough to increase the following distance to the set interval.

In the case of the steel plate on the roadway ( when I was using the radar cruise control), the brakes were automatically applied with significant force - not "panic stop" force but pretty close.

I didn't mention it but inappropriate and forceful automatic braking in my Sienna has occurred other times that had nothing to do with a steel plate. It those occurrences it happened right after I had slammed on the brakes. Only yesterday on the way to the car wash a driver in front of me slammed on his brakes and came to a complete stop on another 40 mph street while I was using the cruise control. There was no stop light or sign - maybe he was lost. I nailed my brake petal and came to a very abrupt stop and waited until the other driver decided that, yes, maybe he really should turn right. (Duh!) I accelerated back to 40 mph and set the cruise control again. My brakes were automatically and forcefully applied. Since no one was behind me, I set the cruise control over and over to 43 mph (a true 40 mph) to see what would happen. Each time the automatic braking forcefully brought the speed down to around 15 mph where the automatic braking stopped. After I turned PCS off and back on with the switch, the radar cruise control then functioned properly. This scenario is one of the 12 on the list the Toyota dealer gave me. If I am counting correctly there have been five of these "malfunctions" in the four months I have been driving the Sienna.

These "features" of PCS/radar cruise control are not isolated to the Prius and Sienna. Go over to ClubLexus and you will find an increasing number of threads about similar issues now that Toyota is making PCS and radar cruise control available on numerous Lexus models.

 

^^^
I'd expect that the distance to the "obstruction" and the vehicle speed determine the response. The CC reacts to things much further away, where the PCS reacts to things that you're about to crash into. There's no reason why they can't work in concert with each other, but they are separate systems with different parameters for reacting to what's ahead. The PSC screams at you, the CC does not.