The world's highest speed limit sign:
The Rocket Sled High Speed Test Track at Holloman Air Force Base, New Mexico, USA.
Tests conducted here have led to the development of pilot ejection seats, parachutes, nuclear missiles, and seat belts. This track was laid in 1949, and is still functioning after upgrades.
Testing the F-22 front section (left) and the N-25 Snark (right):