Cars from behind are absolutely NOT a non-issue when on the shoulder. Many cyclists have been killed by motorists drifting over the line, either from not paying attention or being drowsy (car tends to drift right) or by becoming target-fixated and drifting towards you. When on a shoulder you have to be keenly aware of what passing motorists are doing using preferably a mirror up by your eye, at all times.

I have ridden the shoulder of a 55 MPH 5-lane arterial/state highway a couple of times (try to avoid it if at all possible) and I always have to keep a sharp watch to make sure motorists aren't about to cross the line, and also that anyone at side streets or driveways doesn't pull out on me. At major intersections I will look for a break and move off the shoulder into the right thru-lane until passing the intersection.