Boxing Day traffic will have the profile of a typical bank holiday and will be similar to a Saturday, in that the times of day / locations that are busiest on a Saturday will likely be busiest on Boxing Day.
So a comparison with Friday is likely to show both busier times such as mid morning and afternoon and quieter times for example a journey at normal morning peak time…
This pattern will also be true of the public transport that does operate, such as London Underground and long distance coach services.
This pattern will dominate the whole period between Christmas and New Year and will be somewhat similar on adjacent work days such as December 23rd and January 2nd. These days will have some ‘normal commuting’ traffic, but much reduced by those taking holiday or working from home.
Since 2009, the normal peak traffic on the ‘working days’ between Christmas and New Year has declined significantly. Looking at London Underground passenger numbers, commuter peak traffic was reduced compared to normal levels, but still evident in the early 2000s, but has disappeared in the years since, such that these days are now very similar to the adjacent bank holidays.