Both of the above are really sound advice. I wouldn't ride on the highway without a really high quality hard case tire, due to road debris. In my experience, exist crossings can be hard due to the road conditions, not necesarily traffic. There are some I've ridden past that have prolonged rumble strips, which can't be ridden safely without a wider tire and the rumble strip takes the place of the shoulder, forcing you into the lane.
Anyway you could do this on a weekend and you know someone who can bail you out if there is a problem?
My current commute has some highway travel with a shoulder that's probably 8 feet and I still get buzzed from time to time by someone not paying attention that they're drifting out of their lane.