The Saturn's departure time is another subject of discussion here: https://www.reddit.com/r/MauraMurrayEvidence/comments/4yaimu/time_maura_left_amherst
However, under the best of circumstances, it is doubtful that the Saturn's driver could have made it to the accident site by 7:29 without taking Interstate 91, or a parallel road set like MA-63 N, to NH-10 to NH-112 E.
However, even if you were to say that she left right after leaving the liquor store at 3:43 PM - you have to admit she only could have traveled up Interstate 91, or a parallel road set like MA-63 N, to NH-10 to NH-112 E.
In fact I will say that it is impossible that she took I-93, simply because she would end up running into rush hour traffic near Boston.
I will also say that its not impossible to assume she took MA-63 N, to NH-10 to NH-112 E, because it leaves her 7 minutes to get gas, and she might have stopped more then once if her tank was low to start with.
Distance using I-91 North 152 miles (2 h 32min) at about 60 miles an hour average, the whole way.
Distance using MA-63/NH-10 and NH-112 148 miles (3h35 Min) at about 42 miles an hour average, the whole way. Route 91 doesn't have a major metropolitan area to go through, so this time is reasonable.
Distance using I-93 North 181 miles. Google maps says 3h 25 min, but that assumes about 53 miles an hour average, the whole way. Its unreasonable assume that 53 miles an hour will hold during rush hour in Boston. It would probably take 4 hours.
3:43 + 2:32 = 6:15 PM for route I-91 North
3:43 + 3:35 = 7:18 PM for MA-63/NH-10 and NH-112
3:43 + about 4:00 = about 7:43 PM for route I-93 North
This route and this route has been proposed to include I-93. Both would have Maura arrive at the accident site at 7:22, but requires that she drive an average of 57.53 miles per hour the entire way for the first, and 50 MPH for the second proposed route. However, her car had mechanical issues. For the first proposed route, if there was any congestion at Lowell at around 6:00 PM - that would make a difference. In both cases, this leaves little time for the Saturn driver to get gas - and it is generally believed that she had 3/4 of a tank at the accident site. These routes seem to go out of the way to get on I-93 just for the sake of argument, however.
This "Mixed route" - credit to avamonroe is also possible.