> Also, these cars know the speed limits for the road

Does it always get this correct, or does it sometimes read a 30mph sign on a side road and then slow the car on the motorway down to that speed?

Different manufacturers probably use different systems but no. BMW attempts to read the speed limit signs using the frontal camera with a mix of some sort of stored info - it knows that the speed limit is about to change (Mobileye?), but it is very often that it won't catch a sign in the bend or when the weather is bad. Also, it does not recognize time restricted speed limits, for example 30kph from 7:00 to 17:00 Monday to Friday so it would keep driving 30kph outside of those hours while 50kph is allowed. In some places in Germany, it does not recognize the city limits and carries on showing 70kph for a kilometer longer than it should.

I'm not sure how the cars know the speed limit. Maybe someone else knows? My guess is combo of GPS/camera to position correctly on road and the lookup of known speed limit data. Perhaps it reads signs though?

My car shows the speed limit of roads it knows. It uses GPS and stored limits. It also doesn't know the limits of non-major roads and doesn't attempt to show a limit then. My car is a 2013, and I've not paid the $$ to update the maps in that time (seriously, they want $200-$400 to update the maps).

Since I bought my car, Illinois (where I live) has raised the maximum limit on interstates by 10 MPH. My car doesn't know about it. If my car limited me to what it thought the limit was, I'd probably be driving 20 MPH slower than prevailing traffic, a decidedly unsafe situation.

The rental car I am using now certainly a) reads road signs for speed limit information, b) is definitely fooled by signs on off ramps etc.

It’s hard to imagine how speed limit systems would work without some sort of vision capabilities — a database of speed limits would never be up to date with roadworks and so on.

Nobody should be using autopilot driving through roadworks anyways.

Going slower than traffic is actually unsafe and increases the chances of collisions with other drivers.

Going slower than traffic happens all the time. Over the road trucks often have speed governors set to 60-70 mph for example.

Almost nobody drives at or below the speed limit. It’s dangerous to do so in many places.

That’s a feature accommodating realities of driving on public roads, not a bug.

If you drive on a 60mph speed limit highway, no one is driving 60mph, everyone is going around 70mph. If you decide to use autopilot and it limits you to 60mph, you singlehandedly start disrupting the flow of traffic (that goes 70mph) and end up becoming an increased danger to yourself and others.

Not even mentioning cases when the speed limits change overnight or the map data is outdated or if a physical sign is unreadable.

Over the road trucks often have speed governors, some companies limit their trucks to 60 mph because it saves a lot of fuel and leads to a much (50%) lower risk of collisions.

Apples to oranges. Stopping distance of a 16-wheeler is magnitudes larger than that of a typical sedan, so in their case it makes sense.

For specific numbers (after subtracting reaction distance being the same for both):

55mph: car 165ft, 16-wheeler 225ft. 65mph: car 245ft, 16-wheeler 454ft.

As you can see, the gap between a car's stopping distance and a 16-wheeler's stopping distance increases with speed increasing, and non-linearly at that. Not even mentioning the destructive potential of a car vs. a 16-wheeler.

I would agree with your point if majority of the roads were occupied by 16-wheelers, but it isn't the case (at least in the metro area that I commute to work in).

Source for numbers used: https://trucksmart.udot.utah.gov/motorist-home/stopping-dist...

Note: I agree that it would be safer if everyone drove the exact speed limit, as opposed to everyone going 10mph above the speed limit. However, in a situation where everyone is driving 10mph above the speed limit, you are creating a more dangerous situation by driving 10mph slower instead of driving 10mph above like everyone else.