Traffic in the build up to Christmas - why?

This has never made any sense, but then traffic never does. Let me give you an overview. From January, every-so-slightly, day by day, week by week, the traffic improves. By Easter, it is fractionally better, with rush-hour journeys taking five or ten minutes less. Obviously, during the school holidays the traffic is much better. Then, as the nights grow lighter, the traffic continues to improve, presumably as the number of students commuting lessens (with exams looming). Finally, by the time the school summer holidays come, the roads are nice and empty. Relatively speaking.

Throughout the course of the six week break, the only nightmare traffic is during Fridays on the motorway network as families rush for the beaches or airports - it is particularly bad during the August Bank holiday, but otherwise, during rush hour, it is fine.

Then, September comes. For obvious reasons, the traffic worsens. Children are back at school, student back at college or university and people retrn from holiday. Then, as October arrives, the nights darken and people set out earlier for their journeys. I understand all that - makes sense. However, why, for pity's sake, does the traffic worsen, day-by-day in the build up to Christmas?

Are there more people on the road? Where do these people come from? Are they flown in especially to annoy me? Don't tell me that it is due to Christmas shopping, as how would this affect early morning rush hour traffic? Every year it is the same! Literally, every day it is bad bad bad until the point at which the schools break up a few days before Christmas. It has started already...

Answers on a postcard please...