  A comprehensive understanding of the structure of Doppler motions in
transition region including the center-to-limb variation and its relationship
with the magnetic field structure is vital for the understanding of mass and
energy transfer in the solar atmosphere. In this paper, we have performed such
a study in an active region using the Si IV 1394~{\AA} emission line recorded
by the Interface Region Imaging Spectrograph (IRIS) and the line-of-sight
photospheric magnetic field obtained by the Helioseismic and Magnetic Imager
(HMI) on-board the Solar Dynamics Observatory (SDO). The active region has two
opposite polarity strong field regions separated by a weak field corridor,
which widened as the active region evolved. On average the strong field regions
(corridor) show(s) redshifts of 5{--}10 (3{--}9)~km~s$^{-1}$ (depending on the
date of observation). There is, however, a narrow lane in the middle of the
corridor with near-zero Doppler shifts at all disk positions, suggesting that
any flows there are very slow. The Doppler velocity distributions in the
corridor seem to have two components---a low velocity component centered near 0
km/s and a high velocity component centered near 10~km~s$^{-1}$. The high
velocity component is similar to the velocity distributions in the strong field
regions, which have just one component. Both exhibit a small center-to limb
variation and seem to come from the same population of flows. To explain these
results, we suggest that the emission from the lower transition region comes
primarily from warm type II spicules, and we introduce the idea of a
`chromospheric wall'---associated with classical cold spicules---to account for
a diminished center-to-limb variation.
