  Studying Doppler shifts provides deeper insights into the flow of mass and
energy in the solar atmosphere. We perform a comprehensive measurement of
Doppler shifts in the transition region and its center-to-limb variation (CLV)
in the strong field regions ($|\textbf{B}| \geq$ 50 G) of 50 active regions
(ARs), using the \ion{Si}{4} 1394~{\AA} line recorded by the Interface Region
Imaging Spectrometer(IRIS). To locate the ARs and identify strong field
regions, we have used the magnetograms obtained by the Helioseismic and
Magnetic Imager (HMI). We find that in strong field regions, on average, all
the ARs show mean redshifts ranging between 4{--}11~ km/s, which varies with
ARs. These flows show a mild CLV, with sizable magnitudes at the limb and
substantial scatter at the mid-longitude range. Our observations do not support
the idea that redshifts in the lower transition region (T $<\sim$ 0.1 MK) are
produced by field-aligned downflows as a result of impulsive heating and
warrant alternative interpretation, such as downflow of type-\rm{II} spicules
in the presence of a chromospheric wall created by cooler type-\rm{I} spicules.
