It's useful to consider that keeping the tank half empty will probably lose around 35-40lbs of weight which you lug around every day. Also, temperatures are beginning to drop which also affects mpg - as does very high ambient temps when you use A/C more often.My mpg is also around the 40 mpg UK range .
The problem is that my usual commute to work is only 2 miles .
I did a 120 mile trip just after the last fill up (mpg resets) and I got 68mpg.
However I have been doing only short trips after that And its slowly decreasing by around 1-2 mpg per day.
Next time I am going to fill only 10 litres or so as I think my fuel is getting stale (plus excess weight ) .
I have only done 850 miles since getting the car in June and have had only 2 fill ups.
The current tank is over 6 weeks old and has around 220 miles left and will last for another 2 weeks .
A 2 mile journey is never going to be good from a mpg point of view - you are likely to have the ICE running for a good proportion just to warm battery and car.
Doubt whether the fuel is going stale (it normally takes months, not weeks but you don't know how long the fuel is in the pump before it passes to your tank).
As for our OP, there's really no solution to sitting in traffic other than NOT sitting in traffic - but I guess all the `rat runs` are known and in full use like they are in the UK so there's little respite from sitting in another queue!