They travel at 30mph often, but not over very long distances. Imagine your stereotypical cab driver, with excessive acceleration and lane changes, and insert a stop every two blocks to add/drop passengers.

Actual average speed depends a lot on the number of passengers. At rush hours, if you're walking at a fast pace you can outrace a bus. Only so much the driver can do when 20-30 people get on the bus at a stop.

The last figure I saw for the average speed of crosstown traffic was 6MPH.

The crosstown M23 bus is scheduled to travel the two miles between 11th Avenue and 1st Avenue in 24 minutes.

30mph - the cost of missing every stoplight.