A lot of cars have pressure sensors under the front seats.  So the car knows if anyone is sitting in the passenger seat, and only shows the seat belt warning if there's someone in the seat, but the belt's not done up.

The emergency braking is usally based on a radar in the front of the car.  If it detects something in your path, it may slam on the brakes to at least reduce the speed of the impact.

My newer car occasionally flashes up a big scary warning on the dashboard.  But it hasn't actually applied the brakes yet.

Maybe the car can sense when a seat is occupied (I know some do - and bleep unnecessarily if there some luggage on a seat without the belt fastened), so perhaps what they're trying to say is that if there's a person present in a front seat without a seatbelt, then it won't do the harsh breaking and risk throwing them through the windscreen? (Presumably they're not so worried about untethered rear passengers being flung into the back of the necks of restrained front seat passengers ... which wasn't an uncommon issue in the days of optional seatbelts.)

   - Andy.

From July 7, 2024, the EU made make additional assistance systems mandatory for new registrations of passenger cars (class M1) and light commercial vehicles (class N1).

The new Opel LCV vans will in future be equipped with up to 21 assistance systems, so I guess that will include Citroen and Vauxhall as well as they are basically the same vehicles, and they will all use the same parts and technology.

www.media.stellantis.com/.../new-assistance-systems-mandatory-from-july-opel-models-are-ready

From what I can make out there’s different categories of assistive braking using various inputs (e.g. camera sensor, radar sensor, brake pressure, vehicle speed) to calculate the probability of a frontal collision.

What I cannot understand is why mine apparently only works if there is a front seat passenger wearing a seat belt, the only reason that I can think of is if the car detects someone is sitting in the front passenger seat, but not wearing a seat belt, it will not make the car brake harder. 

But I am reading it as it will only work if there is a front seat passenger, not when I’m in the car alone.

It says:

Depending on the vehicle configuration and the detected objects, there are several operational speed ranges.

On vehicles equipped only with front camera, the active emergency braking operates from 5 km/h to 80 km/h when a vehicle has been detected.

On vehicles equipped with radar sensor and front camera, the active emergency braking operates from 5 km/h to 140 km/h when a vehicle has been detected.

Active emergency braking only works when the seat belts of the front passengers are fastened.

The system includes:

• brake preparation system
• emergency automatic braking
• smart brake assist
• forward collision alert
• front pedestrian protection

But I am reading it as it will only work if there is a front seat passenger, not when I’m in the car alone.

That's the trouble when documentation gets written by wordy people rather than the original engineers, and the wordy people seem to value clarity of expression over clarity of meaning. Then as things are translated, the normal subtleties of normal local usage often get lost. “Active emergency braking only works when the seat belts of the front passengers are fastened.” could be interpreted several ways. Note plural "passengers" - I presume there's no facility for a 3rd person in the front - so they must be including the driver in that number (otherwise it could never exceed 1 and so the singular would have been the natural choice). So they're not referring solely to the LHS passenger, but all the people in the front. So one driver, plus seatbelt, would meet the definition; as would one driver plus one LHS passenger, both with seatbelts. (other interpretations also available)

  - Andy.

 AJJewsbury 

The plural "passengers" could be referring to when it's a van and there is a double passenger seat with two passengers sitting side by side in the front as well as the driver. 

I had a discussion about this a couple of weeks ago with a guy who worked for an insurance company, I said I didn't want to try it out. 

The guy said when he worked for the insurance company he was often invited to test tracks by car manufacturers to try out new car technology and they had an inflatable wall to drive at. Despite knowing he was safe, it took him three attempts to reverse at high speed towards the inflatable wall before he did it without putting his foot on the brake. The system in that test car stopped it, the system in my car might only slow me down so the impact is not so hard hitting something head on.