Tonight for awhile, I browsed over Massachusetts General Law 'Chapter 90 Motor Vehicles and Aircraft' to try locate the text of the laws from which charges against the driver were drawn from, and that the grand jury was presented with. From the article I provided the link to in the earlier comment, I looked for laws addressing: "...vehicle homicide by negligent operation, unsafe overtaking of a bicyclist and failing to take precautions for the safety of other travelers, ...".
Here's a link to the Mass page for Chapter 90:
http://www.malegislature.gov/Laws/Ge...er90/Section24. Scroll w-a-a-a-y...down, fairly close to the bottom of the page, and look for a section that's designated (2) (c). An excerpt from section's start to help find it:
"...(2) (a) Whoever upon any way or in any place to which the public has a right of access, or any place to which members of the public have access as invitees or licensees, operates a motor vehicle recklessly, or operates such a vehicle negligently so that the lives or safety of the public might be endangered, ..."
It's worth browsing over the whole thing to get an idea of what the legal people and jurists are dealing with.