No where near the fastest, but it's the fastest that I know I have a picture of (keep in mind it's only km/hr). This was in a Citreon C4 (small diesel engine). This was my parents first trip to Europe and it was pretty fun driving a car that fast with my parents in it
I have designed and studied a bunch of flashing speed limit signs and they do in fact work to slow (some) people down. They are mostly used in school zones or other areas where slower speeds are crucial. And you are right, cops seldom hang out near them, they are not intended to catch people speeding or assist a cop in handing out tickets.