What kind of charges can I expect for possession of marijuana?

Possession of 1 ounce or less of marijuana by a person 18 or over in Massachusetts is no longer a crime, provided that there is no intent to distribute the marijuana. That said, it is still considered a civil infraction subjecting the responsible party to civil fine of $100. So, if you were pulled over in Massachusetts, you could not be charged with a crime for merely possessing a roach. I assume, however, that because you say you were "on your way back" to Massachusetts, you were pulled over in another state.