  We are conducting a redshift survey of 177 flat-spectrum radio sources in 3
samples covering the 5GHz flux ranges 50-100, 100-200 and 200-250 mJy. So far,
we have measured 124 redshifts with completenesses of 80%, 68% and 58% for the
bright, intermediate, and faint flux ranges. Using the newly determined
redshift distribution we can derive cosmological limits from the statistics of
the 6 gravitational lenses in the JVAS sample of 2500 flat-spectrum radio
sources brighter than 200 mJy at 5GHz. For flat cosmological models with a
cosmological constant, the limit using only radio data is Omega_0 > 0.27 at
2-sigma (0.47 < Omega_0 < 1.38 at 1-sigma). The limits are statistically
consistent with those for lensed quasars, and the combined radio + optical
sample requires Omega_0 > 0.38 at 2-sigma (0.64 < Omega_0 < 1.66 at 1-sigma)
for our most conservative redshift completeness model and assuming that there
are no quasar lenses produced by spiral galaxies. Our best fit model improves
by approximately 1-sigma if extinction in the early-type galaxies makes the
lensed quasars fainter by Delta m = 0.58 +- 0.45 mag, but we still find a limit
of Omega_0 > 0.26 at 2-sigma in flat cosmologies. The increasing fraction of
radio galaxies as compared to quasars at fainter radio fluxes (rising from ~10%
at 1 Jy to ~50% at 0.1 Jy) explains why lensed optical emission is common for
radio lenses and partly explains the red color of radio-selected lenses.
