We present results from a parsec-scale jet kinematics study of 409 bright radio-loud AGNs based on 15 GHz VLBA data obtained between 1994 August 31 and 2016 December 26 as part of the 2cm VLBA survey and MOJAVE programs .
We tracked 1744 individual bright features in 382 jets over at least five epochs .
A majority ( 59 % ) of the best-sampled jet features showed evidence of accelerated motion at the > 3 \sigma level .
Although most features within a jet typically have speeds within \sim 40 \% of a characteristic median value , we identified 55 features in 42 jets that had unusually slow pattern speeds , nearly all of which lie within 4 pc ( 100 pc de-projected ) of the core feature .
Our results combined with other speeds from the literature indicate a strong correlation between apparent jet speed and synchrotron peak frequency , with the highest jet speeds being found only in low-peaked AGNs .
Using Monte Carlo simulations , we find best fit parent population parameters for a complete sample of 174 quasars above 1.5 Jy at 15 GHz .
Acceptable fits are found with a jet population that has a simple unbeamed power law luminosity function incorporating pure luminosity evolution , and a power law Lorentz factor distribution ranging from 1.25 to 50 with slope -1.4 \pm 0.2 .
The parent jets of the brightest radio quasars have a space density of 261 \pm 19 Gpc ^ { -3 } and unbeamed 15 GHz luminosities above \sim 10 ^ { 24.5 } W Hz ^ { -1 } , consistent with FR II class radio galaxies .