Iceman

The difference is:

On the highway the cars that stop, stop in front of you and don't suddenly get out of the way.
Example: While on approach to a traffic light there is a car in front of you that slows down, your car slows down as well. Then, suddenly, the car in front turns right and your car is now speeding up because there is no more car in front (assuming the car in front was the only one that was in front of you). Now, beeing that close to the traffic light (the other car turned right, so you are AT the traffic light), this could be dangerous if you are not prepared for it. Pedestrians might be in danger as well as your car.

To avoid this problem, the car doesn't open hell and speeds up as fast as possible.

And this leads to the problem why you can't really use it in the city, it takes forever to speed up (in the city). On highways it doesn't SEEM to be as bad to you, since the set speed is much higher and a space in between cars is usually MUCH bigger than in the city.

big_ipaq

Since, we're here, Iceman, can you confirm the 80m distance when going at 160km/h at closest setting? Isn't this too much? Almost a footbal field...

Iceman

Legal requirement in Germany is half the distance of the tachs shown speed, that is 80m when going 160 Km/h, correct. The system wouldn't be legal otherwise.
AFAIK they measure it in reaction times... (settings one, two, three, four => x seconds of reaction time, y seconds of reacting, etc...).
So, the minimum is legal requirement and with the longest (four stripes) you have x seconds reaction time more, to avoid the crash. But allways beginning from legal requirement, half the distance of the tachs shown speed.

big_ipaq

I really don't know that well the German laws, but as I know the reaction time should be 1 second. I am also sure the ACC setting, represent time to car in front, not actual distance.

160 km/h means 44 m/s. But it is not that easy...

If the car in front is stopping instantly (crashes into a wall), 80 m is simply not enough. If the car in front, breaks at maximum, one second is teoretically (and maximum legal required in Germany) sufficient to break along with the car in front.

However while speed function is mathematically linear, deceleration (breaking) is not. So we're not talking when ACC is in question about ability to stop, but instead about reaction time, which should be over 1 second. However, in the above case of 160 km/h it is 1,8 seconds.

In 0,8 seconds, there not much to do from 160 km/h.

Worst... This difference is huge when speed is lower...

At 50 km/h this means 13,9 m/s. While you say this would mean a distance kept by ACC of 25 m. Again 1,8 seconds. However, this time, 0,8 seconds could mean the difference between crash and not. 0.8 seconds at 50 km/h is sufficient to decelerate the car while breaking, if the one in front decelerates normally by own means.

In conclusion, I feel that the rule of "half the speedo in meters" is more like a rule so people could easily compute the required distance. But is it in actual low, or is "1 second reaction time" in the low instead?

Is ACC really keeping 80m respectively 50m?!? Or this is just an aproximation and the actual distance is closer to "1 second of reaction time" and not to 1,8 seconds?

Thanks.

abe

I find this interesting. I have a Toyota Sienna with "Laser Cruise Control" which maintains a distance with the car in front, but shuts down below 25 MPH. There are situations when the laser cruise does not function, such as when it's raining (the manual says when the windshield wipers are on) and when you're driving into the sun. So, Toyota gives you the option of defeating it and using NORMAL cruise control.

I'm wondering now if I want this feature on a BMW. Active Cruise Control is not a great feature if you're the fastest driver on the highway. You'll constantly be slamming on the brakes (or at least in a non-gentle fashion) and then double downshift speeding back up when someone moves out of your way. On the Toyota, the system performs badly when the road is hilly, as it can't detect a car that's above or below you. It's also bad when following large trucks as the system can't seem to handle the high bumper on some trailers. Again, this is the Toyota system.

Anyone experienced with the ACC S&G here that can comment on how well it functions?

narvselius

I threw the family into a 750i yesterday, as BMW had some nice cars "On Tour" in my home town. Crammed with goodies the car also had Active Cruise Control. And as you said, the system is far from bullet proof. Just to test it, on a high way exit I took the sharp bend in a "unlogic fashion" so that the car in front of me disappeared for a short while. And, the indicator on the screen in the 750i showed that it had lost contact with the car ahead. And slowly started to accelerate... thanks, but not a good idea in a sharp bend going of the highway. But crusing along the highway the ACC was great and made the trip a very comfortable one.

But I wouldn't use it in city traffic! A car ahead of me before a roundabout turned right, which gave the system a "free shooting range. Suddenly I was charging into the roundabout - only a fanfare like ta-da-da-ta-daaaaam! - was lacking in that scene as the V8 came into live. I can imagine other cars going "Oi, whaddaya think you're doing, you **** , ****, **** BMW-*** driver!"

bardel

The idea of trying to use any form of cruise control in a city with traffic lights / roundabouts seems ridiculous to me? Apart from the scenario's mentioned where the car in front turns off, how do you expect the car to deal with vehicles / objects approaching from the side???

Others mentioned that normal cruise would be better in city traffic when cars pull in front than ACC, but I can't see how, sure normal cruise would mean a) you'd be travelling too close the vehicle in front (especially as you probably have your feet rested on the floor) or b) you're going to plough into the back of the car in front? Maybe the traffic's different outside the uk, but I'd like to understand from a scenario already posted how normal cruise would be better?

I have no experience of the ACC (stop and go) yet, but I've just ordered a 535d with it fitted but in this country I cant see how cruise control is any use without it?

Just a quick question, I presume the normal cruise deactivates the minute you touch the brake? Does the ACC do the same?

Barry

Iceman

Yes.

Touching the brake pedal deactivates the system.
When you push the gas pedal though, the speed remains set and after you take the foot off the gas pedal the car is going back to the set speed.

aybeesea

In sensible countries, vehicles approaching from what would be defined as a "side" have to give way, as you would have to give way if you were entering a main road from a side road.

Of course not all countries have this rule.

For "objects" and unforseen circumstances there's the brake pedal and a resume button.

ABC

Iceman

It's a driver assitance system, ASSISTING the driver, not an autopilot.