I don't know why you keep mentioning the overhead signs? The anti-collision radars that are mandated in every new car are mounted on the bonnet level, they won't react to an overhead sign. And yeah, emergency braking in the middle of the highway is not safe at all, but seeing as literally all new cars have to have that system and we're not seeing people randomly braking in the middle of the motorway for no reason I can only guess it's fine. Besides, the system doesn't start braking if you're actively applying throttle, it will start beeping at you but it doesn't start braking until you lift your foot off the gas.