What is the best time to cycle during the day (traffic)?
For the UK there are traffic statistics available. This graph from the 2015 Road Traffic Estimates shows the general trend for cars.



There are detailed tables of traffic flow available.

There is also geographical data available in the form an interactive map. This isn't broken down by time of day, but should give you an indication of which roads generally carry more traffic.

If you're interested in when is the safest time to use the roads, there are statistics on road casualties by hour of day in this report (page 153 - Reported casualties by day, road user type and hour of day).