Mercer County Bike Map
Call for a copy of the Mercer County Bike Map. Info here:
Don't ride on Route 1. Take the D&R tow path, parallel to Route 1, and go in the back way.
I don't know where you live in Princeton Junction. You can cross at Washington Rd. (571), which is kind of scarey, continue towards Princeton, go left on the tow path, continue until you hit Province Line Rd. (after the golf course), take a left and then pick up Nassau Park Blvd after a few hundred yards.
There are other crossings, but they all have congestion issues, like Wash Rd. There is a sidewalk on the bridge over Route 1 on Alexander Rd., but crossing the on-off ramps is kind of scarey. It's hard to say whether it's worse than Wash Rd. Check them both out on a clear, sunny day and decide for yourself. If you cross at Alexander Rd., make a left onto Canal Point Road. Go almost all the way to the end, and make a right where the Theological Seminary apartments and the golf course meet. Take the bridge over the canal and make a left onto the tow path. Continue to Province Line Rd. . . . . .
I wouldn't recommend crossing Route 1 at night. In that case, I'd recommend riding into Princeton from Nassau Park, taking the shuttle train (the "Dinky") to PJ station and riding home from there.
BMS has a sustainability program, is bankrolling a path near its Hopewell and Lawrenceville facilities, etc. Part of this program probably includes direction recommendations to its facilities by non-car means. Look into it.
Good luck. Be safe.
PS. Look into princetonfreewheelers.com. Join. Share commuting info with riders.