Yes, all cars ecxept "special" ones (police, swat, etc.)
I don't know
For example early in the morning when it is foggy, going 20 in a 40 zone is illegal because you can't see 10 feet in front of the car.
Is the car going to take into account atmospheric conditions to limit the speed of the car?
if it doesn't, what good is the system?
I think there are too many problems with the concept to be practical, and many of them have been pointed out already. "Safe speed" is just far too dynamic, and overautomation leads to complacency. (we're starting to see problems with this in the aviation industry)
A simple governor that just caps driving speed at 80 mph or something might work, but I'm not convinced this would appreciably add to highway safety.
One of you will end up here next!
There are exactly zero legitimate reasons for any civilian driver anywhere to go faster than 90. At most, that is how fast their cars should be able to go.
In 2005 Mercedes decided to respond to earning an reputation that their cars were too heavy and too low on power by producing a 1000 horsepower, 1000 ft/pound torque rocketship they also shaved 1000 pounds off via titanium, carbon fiber and aluminum, that met EPA standards, still got 20 mpg and with a quiet exhaust - then downtuned it to 604 horsepower and 738 ft/pounds torque - out of 6 liter motor (385 CID). At the time, it was the most powerful production motor ever made, was very expensive and didn't sell well, for which after making 194 it was discontinued as a money loser. Reviewers loved the car for being eccentric for the radical power in a luxury 2+2, plus also explained the power was beyond what could be used.
As of 2007, every major manufacturer had the ability to get 1000 horsepower and 1000 ft/pounds torque out of 6-liter range motors. But mega torque and horsepower creates mega design and components issues, plus warranty and very challenging safety issues from handling to tires to cornering to brakes. GM could produce 1500 horsepower Co5vettes if they wanted to. Ford could do the same with Cobras and Chyrsler the same with Challengers.
The old 60s to 1971 muscle cars could put out a lot of horsepower, but didn't have overdrive and tended to have "drag packs" (low rear rears) limiting their top speed to around 120. The famous drag-pack 426 Hemi had a redline top speed of 114 mph due to the low rear gearing. Cars of that era didn't come close to having the aerodynamics, brakes and suspension for ubber fast speeds.
Manufacturers are increasingly putting in computer limiters while at the same time upping horsepower and torque potential, so their advertisement tend to be deceptive as are the speedometers. If the computers are "unlocked" they will realize their true horsepower and moreso torque (they mostly limit torque to protect components and to stay within traction control abilities). However, they are making it increasingly difficult to unlock them for actual driving. For example, few now allow you to actually drive the car while the motor is in "dyno mode" - which is maximum power output. 95% of cars are top speed limited - even super cars. However, for super cars that is due to tire safety and aerodynamic stability safety reasons - not power reasons.
Back in the Jimmy Carter era, federal law required speedometers not go past 85 - though the cars could. It has a TERRIBLE effect on car sales and was absurd. However, in that era most cars also were computer limited to about 100 mph. Buyers would be pissed to learn their big motor car couldn't go over 97 mph.
The latest thing of manufacturers who want cars dealer serviced is to have the computers monitoring servicing. While that use to make for warning lights coming on that only the dealer could reset, now it also starts depowering the car and even limiting the transmission. If too much so, it may only allow you reverse and 2nd gear so you can "limp mode" home.
That is annoying but maybe needed at some level. Annoying for sure as you have to pay the dealer whatever they want whether it needs it or not. The Merc we have was starting to depower based upon time passage from the prior servicing - although not driven 100 miles. On the other hand, the non-super exotic super speed cars are increasingly affordable on the used market. A person can now buy a used high mileage 185 mph Mercedes for under $20,000. Even less if the interior bad, lots of miles on it, bad interior and paint etc - but still 185 mph capable. To put it into perspective, at 185 mph a car is traveling the distance of a football field in 1 second. But while the motor can take the car that fast, it can't do it on its tires and WILL wipe out and that 5000 missile would take out everything in it's path.
I think cars should be top speed limited by manufacturers to the maximum speed the car is capable of being safely operated at - meaning limited on speed for the design capabilities. The car in the picture above for its aerodynamics, suspension and brakes is good for maybe 130 mph. Beyond that, it can not be driven safely as it would become aerodynamically unstable (car shapes are basically a lifting-wing), nor are the brakes good for anymore. Product-liability lawsuits now see that standard of speed limited to design abilities happens by the manufacturers.
An example is the Bugatti Veyron. It will go 256 mph. But computer limited to 216. IF you want to go 250, you have to completely stop the car and enter a code. The computers checkout the car. Has it been serviced? How many miles on the tires? Brakes ok? The computer will only allow two 250 mile runs between tire replacement - which costs $70,000 a set. IF the computer approves of the 250 mph run, it drops down the rear wing to eliminate aerodynamic drag and drops the whole car down about 2 inches to stop air piling up under it. Then and only then can you take the Veyron to the maximum speed.
All supercars now have such systems, and the ZR1/ZO6 and other of the new mega-cars have similar, though more passive automatic limiters they don't tell you about. The car potentially may have 600, 700, 800 hp and maybe a potential 800 ft/pounds torque or more, and may have the potential to go 200 mph. That doesn't mean the car's computer is going to allow you to really have those traits in operation. And it is no easy trick to get around those limiters. It is taking 3 companies to get them off my Merc. and it is not cheap. Then again, it is all eccentric as there is absolutely no way to use the must power under 80 or 90 mph.