Since their recent realizations, cold reactions have become well known for strong quantum effects that dominate absolute reaction rates, such as resonances. On the other hand, for particularly fast reactions, where the probability for a reaction approaches unity, these effects are expected to be completely suppressed with the reaction rate governed only by the long-range interaction. At this limit universal behavior was predicted by Paul Langevin more than a century ago for ion-atom reactions, and in the 1930’s, Gorin established that the rate any fast reaction involving neutrals will follow E^1/6 collision energy scaling law. Although near unit probability inelastic and reactive collisions have been observed at ultra-cold temperatures, the scaling law has not been confirmed experimentally. We report the direct observation of E^1/6 universal scaling, consistent with the theoretical long-range interaction strength, in strongly anisotropic atom-molecule reactions over more than three orders of magnitude of collision energies. We demonstrate that a the lowest collision energies the classical Langevin theory breaks down by observing transition to a constant reaction rate as predicted by the quantum Wigner threshold laws. Although treating molecules as isotropic collision partners cannot be generally justified, we show that long-range anisotropy is averaged out by molecular rotations in the universal rate limit.