Car accident - who is at fault?

The scenerio:

Main road direction is north and south. It has no stop sign and speed limit is 30 mph.

Side road comes into main road at 90 degrees (going east and west) and has a stop sign where it meets the main road.

Car A is traveling south on main road at just about 30 mph.

Car B is on side road approaching main road traveling east. They stop at the stop sign and then pull out to cross the main road. The driver of car B says they did not see car A approaching from the north. Car A attempts to avoid the collision by swerving to the right to get behind car B and brake hard but they hit the back of car B with the left side of car A.

Who is at fault in this accident?

 

Arg. And nothing would have occurred had car B obeyed the law and yielded to the right of way!! Apparently it is now going to some sort of arbitration or something between the two insurance companies. I'm just glad we got the car fixed through our insurance company even though we are right now out the $500 deductible.

I think the problem is that car B's damage was just the driver's side rear bumper had a dent and car A's damage was the front bumper, driver's side headlight and fender. It made no sense to me at first why car A had damage on the driver's side if car B came from the right but once it was explained that car A tried to turn right to avoid car B and so didn't hit at a 90 degree angle, it makes a little more sense.