I'd argue that the correct directions, if you're coming from a northbound red line train, are to use the north head house to exit on the east side of Mass Ave, next to Harvard Yard, and just stay on the damn sidewalk. You have to trust the process, but it will deliver you unimpeded directly to the Science Center plaza.
And it has the advantage of being a path that works even with Harvard has locked the gates to Harvard Yard (e.g. around graduation time.)
@unlambda
The problem is it looks like it doesn't work. It curves around the wrong way, and all you can see is the road dropping down and forming a massive obstacle. There's no visual confirmation that the sidewalk crosses to the other side. So the obvious thought, looking at a map, is "Oh I suppose I have to cross the street here" – the street being four lanes of Mass Ave at its worst. And then you discover you have to cross back the other way on the far side of that mess.
@whitequark