It is set by .. . The driver???
Similar to how on much of the German Autobahn you can drive as fast as you want, but if you are significantly above 130km/h and an accident happens, the fast party is the one deemed guilty (unless clearly proven otherwise).
I would really like it if my car would warn me that I'm exceeding the speed limit of a stretch of road, and update dynamically. Google displays the speed limit, but doesn't warn me, and is wrong ~40% of the time.