  An analysis of ~19500 narrow (<200 km/s) CIV 1548.2,1550.8 absorbers in
~34000 Sloan Digital Sky Survey quasar spectra is presented. The statistics of
the number of absorbers as a function of outflow-velocity shows that in
approximately two-thirds of outflows, with multiple CIV absorbers present,
absorbers are line-locked at the 500 km/s velocity separation of the CIV
absorber doublet; appearing as 'triplets' in the quasar spectra. Line-locking
is an observational signature of radiative line driving in outflowing material,
where the successive shielding of 'clouds' of material in the outflow locks the
clouds together in outflow velocity. Line-locked absorbers are seen in both
broad absorption line quasars (BALs) and non-BAL quasars with comparable
frequencies and with velocities out to at least 20000 km/s. There are no
detectable differences in the absorber properties and the dust content of
single CIV doublets and line-locked CIV doublets. The gas associated with both
single and line-locked CIV absorption systems includes material with a wide
range of ionization potential (14-138 eV). Both single and line-locked CIV
absorber systems show strong systematic trends in their ionization as a
function of outflow velocity, with ionization decreasing rapidly with
increasing outflow velocity. Initial simulations, employing CLOUDY, demonstrate
that a rich spectrum of line-locked signals at various velocities may be
expected due to significant opacities from resonance lines of Li-, He- and
H-like ions of O, C and N, along with contributions from HeII and HI resonance
lines. The simulations confirm that line driving can be the dominant
acceleration mechanism for clouds with N(HI) ~ 10^19 cm^-2.
