I wasn't suggesting that increasing the limit would be a solution to the issue, only that it could (mathmatically) increase the average speed.

I don't think anyone is suggesting that 20mph limits are solely to blame, and certainly not a "leading cause".

On a side note - Is 7.1mph average in a busy city even that slow for a bus? Considering they have to be stopped for several minutes to let people on/off and stop regularly for red lights.

I'd be interested to compare to the bus' average speed to a car taking the same route.