Approximate speed that a car would need to be traveling in order for the passenger to sustain injury

It depends. As a matter of fact, there are accidents that happen even when the car is moving slow. Whenever you meet an accident, be reminded of the steps that should be done. accidentattorneyhelp.com provides easy step guide when meeting an accident. It's not about the speed, but it's about how the individual is doing his duty of care.