  It is widely believed that at high Reynolds number (Re) all turbulent flows
approach a state of "fully developed turbulence" defined by a unique,
Re-independent statistics of the velocity fluctuations. Yet direct measurements
of the velocity fluctuations have failed to yield clear-cut empirical evidence
of the existence of fully developed turbulence. Here we relate the friction
coefficient ($f$) of rough-conduit flows to the statistics of the velocity
fluctuations. In light of experimental measurements of $f$, our results yield
unequivocal evidence of the existence of fully developed turbulence.
