Urban ridings affect on average speed

I usually do not like doing hard effort training rides around the city if I can avoid it. I have always hated all the stop and go's of the traffic lights, and it never really lets me judge my previous performances against that day because the stop lights and slowing down, etc. throw off your stats.
I did a training ride yesterday and since it was late and I wanted a fairly flat route and it was dark out so the street lights can be nice at times. It seemed I couldn't get a break with all the stop lights. I decided to run some calculations of my ride because I have a device that records all of my riding data and this is possible (and yes, I can be this anal at times).
During yesterday's ride I had to come to a stop 7 times (which doesn't include all the times I had to slow down) and my average speed was 15.8. I knew while riding my average would be lower than it should be because of all the stopping and slowing down. The number of stops and slow downs was much more than I usually have on this ride.
Although my numbers are not scientific, they are thought out. Each time I come to a stop it takes a minimum of 60 seconds (usually closer to 70 seconds, but I am using 60 to keep from making myself look too good. :) )from the time I start slowing down to the time I am back up to speed. I then took the amount of time it would normally take me to cover the same amount of distance at normal speed as I took from slowing down to speeding up and it takes me 19 seconds to cover that distance at speed. This means I lose 41 seconds every time I come to a stop.
After making some corrections to the numbers in my route to figure out how well I actually did I was able to determine that had I not had those 7 stops (again, not factoring in any times I had to slow down, it is too hard to calculate) I would have averaged a speed of 17.5 mph vs. the 15.8 that was recorded.
As you can see, there is a big difference between riding in the suburbs and riding on farm roads and the effect on your average speeds.
I know many of you don't really care about this, but several of us cyclists do. So there! ;)