Better to brake fast (late) or slow (early) when approaching a light.

In a Prius it's better to slow down more gradually, especially from higher speeds because the hybrid battery can only absorb energy so fast before the mechanical brakes kick in and start slowing you down by creating waste heat, and the electric motor can only generate it so fast. Some here may be able to shed more light on it than I can, but in my 2008 Prius, I seem to get more actual charge from moderately light braking than just coasting down to a stop because less energy is wasted by mechanical drag and thus gets converted to electricity instead. I don't know where the tradeoff point would be, but it would almost surely be different in a PiP than in my car because of the much larger battery and different battery chemistry.

 

I fully agree with babybird, in fact this braking approach arguably has more effect on your fuel mileage than your acceleration rate does.
As for your traffic light strategy, traffic permitting, you're better off coasting as you approach the decision point then braking or accelerating as appropriate.
From a safety standpoint you're at higher risk blowing through an intersection at full speed a few seconds after the light has turned green anyhow. I saw someone t-bone a motorhome once. The motorhome driver blew through a fresh red when he noticed the yellow light too late. The unfortunate car had it timed perfectly but then couldn't cope with the cross traffic.

 

Drive like you would on icy roads.

 

Was the CarTalk question specific to hybrids, or was it just for traditional cars?

In a hybrid, the maximum battery charge rate would argue for a slow but variable braking.

For non-hybrids, the energy question is mostly a matter of minimizing speed loss. I would argue that the slow early brakers will generally need to scrub less speed. Due to uncertain light timing, the fast late folks will have to come to a full stop, or near stop, more often, wasting more energy.

As for brake wear, I suspect that even for identical energy dissipation, hard fast braking causes faster wear than light slow braking. But I have no proof.

 

Some thoughts:
* The right answer could be found by a competent mathematical analysis. This is not it.
* If you end up using the friction brakes when you didn't need to, you are probably not braking optimally.
* Slowing down sooner, ATBE, reduces the aerodynamic losses.
* Maintaining more distance between you and the car in front, gives you more options.
* Stopping completely is more expensive than other similar sized drops in speed, it should be avoided.
* Brakes do not respond linearly.
* Given perfect knowledge, the optimum would seem to be, brake lightly as soon as possible, and then coast, such that the highest minimum speed was achieved at the transition point where you start accelerating.

 

Brakes? What are these things of which you speak.

 

And then there is the issue of the driver behind you giving you the finger for slowing down "too soon". They somehow consider it a point of honor to get to the next red light first.

 

Yep, had one pass me this morning in just that manner, rushing to be able to stop at the light, which to anyone who regularly travels the route is a notoriously long red for our direction of travel. But he won, and got to idle his big SUV for an extra thirty or so seconds.

 

I thought the idea was to avoid using the brake. If that is indeed the case it also means you should avoid using the acellerator as well. This does not mean you should creep along at 20 mph in 45 mph speed zone. Accelerate up to the desired speed (45 mph) then hit the accelerator pedal as little as necessary to maintain your speed. As I understand it charging the battery via regeneration can only recapture a fraction of the energy used to accelerate the vehicle. If regen fails to recovery 100% of the energy used to accelerate the vehicle then your best option is to coast as much as possible. Of course the first rule is to drive defensively.
I find the the signal light cycle in our area varies between 60 to 130 sec. In several places I can easily time the cycle and make it through the intersection at a reasonable speed and without braking. If it is obvious I will have to stop I try to brake smoothly over as long a distance as possible pissing off the people behind me.

 

slow

 

this is of course true. but, still, some of us, sometimes, still need to brake.
lol.

 

Sometimes there is a fine line between between braking early and timing it right to still be going 10-15 mph when the light turns green...and not rear ending stopped cars...and just coasting or breaking normally. But when it only costs 2-3 cents per mile to drive in EV it isn't worth being unsafe.

Mike

 

I do pretty well, perhaps not as best as possible...

• Light brake to slow the car down to EV mode
• Coast until I think it likely I will require braking
• Light brake (2-3 bars on regen meter.) When I say up to 3 bars I mean that I have just passed 2 bars.

This is a lot easier than what my description suggests. I do not have to think about it much at all.

 

This is a good response. Indeed slowing via brakes takes your energy and spends it heating the brakes. Wearing them out. Think how many mph do you want to put into brakes vs slowing early and keeping more mph.
The question of preserving speed is a balance. There is to early and too late. If you know the timing of the light then you should brake super early and coast just right so you reach the bumper of the car in front of you as they are accelerating. So many variables are involved (some outside your control, ) that it's dangerous and unadvisable to actually reach the bumper. But you can operate with a safe margin. Your remaining choice is whether to let the opportunists take cuts while you're doing the smart thing... and beware slowing someone who is trying to make a turn between you and the light.

 

Similarly, drive like you ride a bicycle. Would you pedal up close to the stop sign/light and then brake hard?

 

Try to drive without using the brakes! I realize this sounds weird, but another way is to pretend you have an egg between your foot and the brake (or drive with bare feet).

As for "Car Talk", I used to listen to that show when it first aired up here in New England back in the 1980s, but like lots of other things, over the years it has become somewhat stale, and has (IMNSHO) degenerated into a veritable chuckle-fest (literally) where any useful content has been diluted to just about 0% and the laughin', chucklin', and cuttin' across each other by the hosts seems to be more important than any content! Thanks gawd my radio has an "OFF" switch!

 

WRT "Car Talk", one of the most important takeaways I got was "shut up and drive", i.e, it's really better not to use one's cell phone in the car, due to the distraction it causes.

I don't use mine for that reason, even hands-free, and I'd like to see others stop using theirs for the same reason.

Posted via the PriusChat mobile app.

 

To give Click & Clack their due, the only useful thing I learned from Car Talk (in over 30 years of listening) was how to set up the mirrors for maximum coverage! It really does work!)

 

I never considered this, but you are definitely right. I time out intersection lights fairly well when approaching, but never considered the downside risks. Great insight

- Lee