Depends on how each station decides to configure their equipment.
If the engineering staff decides to allocate Constant Bit Rates (CBR)
to all sub-channels, then they don't "steal" bits from less busy programs.
In San Diego, this is typically used when there are none or only one
But if they configure it for Variable Bit Rates (VBR), busy programs
can "steal" bits from less busy programs.
You can look at the TSReader Output reports for US markets here:
In San Diego, KNSD and KSWB use Variable Bit Rate (VBR),
and the "unknown" report for KPBS appears to also be VBR.
Rather than reporting a Constant Bit Rate setting, when in VBR
mode, the TSR report shows the station setting a ridiculously
high bit rate (e.g. 90 Mbps!!!)....probably a maximum assigned value.
Statistical Multiplexers have been in use for many years on both Cable and OTA.
Here are some ATSC OTA experiences:
but KAXT is now down to 12 "SD" video and 3 audio channels:
[PS: Our cable system (QAM is twice the data rate) only carries 13 SD programs.]
A Harmonic White Paper from 2002:
Note how the MPEG2 coding efficiency has improved over the years,
making it possible to stack more and more programs:
The H.264 (MPEG4) coding efficiency curve has only just begun to come down
from 2:1 more efficient than MPEG2 to about 3:1 more efficient....so far....