Mass Ave is also known as Massachusetts Route 2A along much of its length.
Despite the fact that it is the only remotely direct surface route from Dorchester and Roxbury to Cambridge and points north, it is only one or two lanes in each direction for its entire length, making it a traffic nightmare for drivers and lethal for cyclists. In Cambridge, Mass Ave even has special booby traps laid out for unwary cyclists: they call them "bike lanes".
The bike lane runs in the door zone, which means any cyclist who falls for the trap and rides in the designated lane is going to get hit by a carelessly opened driver-side door sooner or later. (On 2 July 2002, Dana Laird died as a result of being doored into the path of a bus directly outside the Central Square office where I work, resulting in widespread anger and debate among cyclists but no real-world changes.) What with that, the potholes, the debris fields, the street-level railroad crossings, and the drivers who will scream at you and actually try to kill you on purpose, it's amazing I managed to survive biking to work for a year. It helps to wear army clothes and chains and bike like a paranoid maniac on speed, I guess.