From the 2009 National Household Travel Survey, the average speed of all trips was 31 mph. When restricted to just trips occurring in urban areas the number drops to a whopping 28 mph.

The data is freely available from the web. Search for NHTS2009.

P.S. Using the same basic information as the OP with the revised average speed yields a cost of slightly more than $10 per hour of operation... The lower the average speed the cheaper the per hour cost would be using the OP's methodology...