Brake lights w/ adapative cruise control?

Yes they do.

 

Quote from the Gen 4 Owners Manual:

When a vehicle is detected running ahead of you, the system automatically
decelerates your vehicle. When a greater reduction in vehicle speed is
necessary, the system applies the brakes (the stop lights will come on at
this time).

This suggests that the brake lights would only come on if braking action were needed to decelerate the car to match the speed of the car ahead of it. Perhaps this is sufficient to warn cars following yours to slow down, but I would prefer that the lights come on with any deceleration.

Your profile shows your car is a Gen 3. I'm not certain the brake lights act in the same manner with your model.

 

I had a KENWORTH much too close behind me one night on the Motorway, and could see my brake-lights clearly reflected in the chrome Bumper/Grille/Roo-Bar. In normal speeding up/slowing with RADAR Cruise, there were no Brake Lights - but if the car in front slowed/braked to the extent that their brake lights came on, my brake lights would too.

 

Ugh. Drivers tailgate me all the time when I use DRCC, even at the closest setting. Just shows that computers are safer drivers sometimes!

 

If so then to be consistent it should also work that way when not using DRCC, since drivers behind you won't know or care whether the deceleration is caused by you or by DRCC.

Actually I have sometimes thought that to be consistent with other cars the brake lights on a hybrid or BEV should come on any time regen is taking place, since regen is the equivalent of lightly pressing the brake pedal on a regular car.

OTOH I sometimes think the brake lights on regular cars come on too often, since the brake light is not based on pressure but simply by touching the brake pedal. For that reason I always shut off cruise control with the switch instead of the brake pedal otherwise the driver behind me thinks I'm braking which could startle them and cause them to hit their brakes.

Ideally the brake lights on regular cars should come on only during moderate to heavy braking but not during very light braking, which I would define light braking as similar to regen on a hybrid. Meanwhile BEV's and hybrids with paddles, anything beyond first level regen should trigger the brake lights.

However I don't see changes happening anytime soon to how regular cars activate their brake lights, so in the meantime I think the goal for BEV's and hybrids should be to mimic as much as possible the brake lights on regular cars in terms of what level of deceleration will cause the lights to come on.

 

I recall an earlier discussion on this forum where a member produced data to substantiate a claim that a greater volume of cars could travel through a highway in a given time period if drivers were motivated to increase their following distance to the point where traffic always continued to flow smoothly and never reached a stop and go scenario. "Granny driving" that Prius drivers are known for actually allows for greater throughput of cars on a highway. For this reason, I don't see a problem with brake lights coming on more frequently if the effect is to slow down following cars, so long as it does not cause them to actually come to a stop.

 

To be fair, when you take your foot off the accelerator on a traditional ICE-only car, engine braking gently slows the car. This is simulated, and closely matched, in a Prius; taking your foot completely off the accelerator results in mild regen, and decelerates the car at a similar amount to a traditional car in the same situation. In both cases, and when the DRCC is being used, such mild deceleration doesn't illuminated the brake lights.

 

I guess it depends on the car, when I first started driving a Prius I noticed that I kept coming up short at stop lights and I had to adjust for the extra deceleration caused by regen. And I got the feeling that cars behind me didn't realize that I was slowing down so much because my brake lights weren't on. Maybe on the newer models they have lightened up a bit on regen during coasting. If so that's good because regeneration is not fully efficient. Sure it's better than friction braking, but even better is no braking. Ideally you would take your foot off the accelerator long before reaching the intersection and coast with no braking of any type (engine, regen or friction) except a little at the very end. But I think by law cars are not allowed to operate in "neutral" so they have to have some braking, but again the less the better. Ironically so many people rank the efficiency of a hybrid by how much regen occurs during coasting, you hear this in reviews and comments, people will laud a hybrid or BEV that has strong regen during coasting, but they complain if the regen during coasting is weak because they think of it as a lost opportunity to reclaim energy. I think this comes from the common misconception that people think the brake pedal only operates the friction brakes and that regen only occurs during coasting.

But yes if a Prius is decelerating at the same rate as a regular car then the brake lights should not come on. I just don't know if any of this is quantified anywhere.

 

From what I understand, this is absolutely true. I often hear people referring to the deceleration on one or more of the previous generations as very "grabby". I've only experienced the Gen4 and it's certainly comparable to all of my previous car's engine breaking in 4th or 5th gear. It's not much at all really.

However, it's not all good. On previous generations there was an easy spot to find on the accelerator where you were just gliding, with no ICE, regen, or EV. A "stealth glide" if you will. I've managed to achieve this on the Gen4, but only for a few seconds at a time. Usually 1 or 2. It's no longer a static accelerator position, but the ECU also takes into account other factors and hence the EV will kick in or the ICE.

 

the real question is, do the brake lights come on during strong regenerative braking - but no brake pedal.

.

 

No - that will never happen. All cars slow down when you cut the throttle (unless downhill gradient), and their brake lights show ONLY when they over-ride the natural deceleration by applying the BRAKE pedal. ONLY then does the brake light shine.

Think about it - hundreds of cars doing 110km/hr on the motorway, with some gentle up/down gradients (NOBODY with RADAR Cruise). Cars will naturally speed up slightly or slow down - PRIUS with Cruise Control (or DRCC) gains speed slightly as you crest a hill and slightly drops at the bottom of a dip. And then adjusts by increasing or reducing speed. You want EVERY brake light to shine when a car reduces speed? The driver behind will react slightly more when they see the brake light, but 3 cars back, when they see the brake lights, they HIT the brake pedal - and 200 metres back - traffic will be stopped.

No, the way they do it is fine.

 

Yes, my short drive of a previous model, it was more grabby - but the one I drove didn't have DRCC.

If you want to "GLIDE" - it's easier if you're in ECO, in PWR the accelerator is too "jumpy", at least for my foot. I agree, I've not managed it for long periods, but then maybe I've not persevered enough.

 

YES. [EDIT - on a Gen 4 with DRCC - I notice you've got a Gen 2, so don't know about them.]

 

No - not on gen ii

 

Others cars also slow down lightly or moderately from engine compression after downshifting, which does not activate the brake lights.

 

Yes, for sure. My first Diesel, a 2 litre, 6sp Manual FORD FOCUS had great engine braking, and driving vigorously on a tight mountain road you just drove mainly on the throttle in an appropriate gear - great car, pity they stopped importing them here.

 

It's an interesting point about downshifting in a manual, but that's generally used for maintaining speed on a downhill, or adjusting to a slower speed like on a turn. Although I suppose some people routinely downshift as a way to slow down, but the downside of using it that way is that someone behind you may not realize that you are slowing since your brake lights won't come on.

However if you were refering to automatics, yes I know that automatics slow down more when in gear than when in neutral. My point was that if regen on a BEV or hybrid exceeds normal deceleration in a non-electric car, the brake lights should come on. If they are the same they shouldn't. Apparently regen on the current gen Prius is very similar to the deceleration of a non-electric car. But that's not true for all hybrids and BEV's.

 

Do remember that automatics can also be downshifted in the very same way as manuals. Some may have only one or two choices, but many allow all lower gear choices.

And some drivers of automatics actually use those choices.

 

I haven't had a conventional Auto for 13 yrs, but I well remember my first, a VOLVO 164. And - I would use the shift regularly, particularly on the mountain ranges near to home. My wife commented "why buy an automatic and still choose to shift gears"? I don't think she read the owner's manual either.

With my PRIUS, I still often go to pull back a gear as I approach a corner.