The excuse of "I didn't see them" doesn't apply.. that means they didn't look to begin with but just glanced for the outline of a car and that's about it.
While 100% I agree with Tom, the other danger itself is weekend bikers as well who are riding their bike just as a penis extension and because that's the "cool" thing to do.. you can usually tell real quick by how they stop for a red light etc.
So yes, ride as if everyone is out to get you; don't let weekend bikers stagger up with you, don't look for turn signals on cars, always assume they're not looking and are out to get you. Have an escape route handy at all times and if the escape route closes, switch lanes or do what you have to do to ride safe at the current moment and have an alternate escape route handy if **** does hit the fan at the same time.
Mark provided some very nice links for everyone to check out and as always.. practise practise practise, don't use your motorcycle to show off/penis extension/ego boost but as a bonafide method of transportation. Just because you can twist the throttle doesn't mean you know how to ride, e.g. you don't lean to take a curve; you counter steer. Learn how to completely stop your bike without putting your feet for 3 seconds and take off again and so forth.
I don't own a car and my motorcycle is my primary method of transportation; I've been riding for decades this way, snow/rain/sun doesn't matter and the stupidity/ignorance I still see today never ceases to "amaze" me.
Very sad to hear about this lady.. tragedy like this never seems to end.