  We present a comprehensive study of (quasi-)simultaneous radio:X-ray
observations of stellar black hole binaries during the spectrally hard X-ray
state, finding evidence for a strong correlation between these two bands over
more than three orders of magnitude in X-ray luminosity. The correlation
extends from the quiescent regime up to close to the soft state transition,
where radio emission starts to decline, sometimes below detectable levels,
probably corresponding to the physical disappearance of the jet. The X-ray
transient V 404 Cygni is found to display the same functional relationship
already reported for GX 339-4 between radio and X-ray flux, namely S_radio
\propto S_X **(+0.7). In fact the data for all low/hard state black holes is
consistent with a universal relation between the radio and X-ray luminosity of
the form L_radio \propto L_X **(+0.7), implying that - as L_X decreases - the
jet becomes more radiatively efficient with respect to the X-rays. Under the
hypothesis of common physics driving the disc-jet coupling in different
sources, the observed spread to the best-fit relation can be interpreted in
terms of a distribution in Doppler factors. Monte Carlo simulations show that,
assuming little or no X-ray beaming, the measured scatter in radio power is
consistent with Lorentz factors < 2 for the outflows in the low/hard state,
significantly less relativistic than the jets associated with X-ray transients.
When combined radio and X-ray beaming is considered, the range of possible jet
bulk velocities significantly broadens, allowing highly relativistic outflows,
but implying therefore severe X-ray selection effects.
