You may want to try both main and side roads and see what your prefer. I'm a bit timid to ride 12 MPH on the 40 MPH main road portion of my commute so I go one block down where it's a residential neighborhood. The cool thing about that road is that it's really wide - wide enough to put 2 lanes per direction even. That way I can stay well away from parked cars and driveways and cars have ample space to pass me without giving me the squeeze. The down side is 4-stops every 500 to 700 ft.