Route 9 SUCKS! You do not want to ride on it. You are going to be overwhelmed by cars. Some places have sidewalks, some don't. Some places have wide shoulders, some have none. It is not a road that is built for bikes. I've seen some commuters using it, but this is really early and in the summer when there is significantly less traffic, both due to the time of day and summer traffic is always less than normal traffic in Boston. There is also a lot of construction there lately with new paving and sidewalk building, so it might be nice when it's done, but I don't know when that will be.
I highly advise you to find roads that parallel it, even if you have to zig zag a bit. They will be safer, prettier, and you will enjoy the ride much more.