Is it possible to predict when roads will have less traffic?
Generally speaking, Saturdays and Sundays are quieter than weekdays.

The Department of Transport collects various statistics on road use. This table (TRA0307) shows average traffic by time and day of the week.



There's also this table (TRA0306) which shows average traffic by day for different road types and different vehicles.



Finally, there are statistics for average annual traffic on every major road in the UK. There's an interactive map here.