I tried it out this afternoon on a straight dual carriageway that was clear of traffic. 

If you are going fairly fast and start changing lanes without using the turn signal indicators it will turn you back into your lane.

If you are driving slowly and/or using your turn signal indicators it will not do anything. 

It feels a bit like kerbing the wheels, but you can just steer through it.

Yes that is my experience too. It is only active above a certain speed (about 30mph I think) and nudges rather than forces. Of course if you have your hands on the wheel, no problem. If you indicate, it cancels. It is a little strange at first but you soon get used to it. Doesn't always activate, possibly because it can't detect the edges/lanes so appears to be a little cautious.

Other systems are looking ahead with the intention of preventing a crash. One pops up a little symbol that you are too close. Another actually slams the brakes on if an obstacle is detected. The first time this happened was a shock. I knew the car ahead was turning off, I was slowing, my foot was over the brake but the car decided a crash was imminent! Another car close behind me would have had a shock.... It's a bit "trigger happy".

Ha!

I have had a funny with this sort of thing in a hire car - cruise control set to full legal (+) on a dual carriageway ,but the forward radar maintained a safe distance to chap in front. And keeps doing so as the traffic slows in a jam as the exit approaches,  and then slower still as we pull off into town. At about this point the cruise control setting is forgotten about.
After a few mins of following on at 25-30mph the car in front turns off, and the car I'm in takes off like Niki Lauder - presumably trying to re-attain the now highly inappropriate cruise control setting.... A few moments  of ' what the !"£?' and braking before regaining composure.
M.

That was an interesting interaction of systems   I guess that the car had an automatic transmission, otherwise disengaging the clutch to engage a suitable gear for the reduced speed would have turned off the cruise control.

What would the correct algorithm for this situation be? If you had remained on the dual carriageway you would have expected the car to return to the preset speed when any obstruction went away (although maybe not at full throttle). How could the system determine that you had moved to a different situation?

I have had a similar although not so scary situation. I had set the cruise control to 80 kmph when travelling through a reduced speed roadworks area. At the end I accelerated up to the speed limit, 120 kmph and continued on my way. At my exit I steered onto the exit ramp and decelerated. The speed dropped to 80 kmph and stayed there   A quick dab on the brakes turned off the cruise control and all was well.

I wonder what performance level, if any, is specified for these systems? If you are in vehicle with an automatic transmission and the cruise control remains engaged you have to hope that the engine stop button works (but maybe it is disabled when the vehicle is moving?) or that the brakes can beat the engine

Maybe something similar took place here???

https://engx.theiet.org/f/discussions/30096/runaway-electric-car

 I presume the car that reads the speed limit signs is is the solution to that, except that round here it needs to recognise that lamp-posts and no signs mean drop to 30. I can see how the situation arose, and once bitten twice shy - and to be fair to the car it was not a problem again - a large part of the issue was the strangeness of a hire car. (and I normally drive a car with manual gearbox, manual handbrake so I was slightly out of comfort zone already)

Getting out of the situation only takes a second, but it is an un-nerving second.

M.

 I presume the car that reads the speed limit signs is is the solution to that, except that round here it needs to recognise that lamp-posts and no signs mean drop to 30. I can see how the situation arose, and once bitten twice shy - and to be fair to the car it was not a problem again - a large part of the issue was the strangeness of a hire car. (and I normally drive a car with manual gearbox, manual handbrake so I was slightly out of comfort zone already)

Getting out of the situation only takes a second, but it is an un-nerving second.

M.

 mapj1 

Different rules in Wales, street lights without speed limit signs means there's a 20 mph speed limit. 

There should always be a 30 MPH speed limit sign as you enter a 30 MPH zone, streetlights or not.  But you're expected to remember that once you're in that zone.