AJ, rather than address the individual comments in your posts, I just want to point something out.
Firstly, motor vehicle accidents that go to trial are usually heard in civil courts and relate to damages. The greatest part of damages is what's called "general damages" and this relates to loss of income. A seriously wounded person may never work again and they'll claim $X per week for the balance of their working life. Then there are "special damages" which covers all the medical bills, physio, medical care at home etc, past, present and future caused as a direct result of the accident. Negligent or reckless driving causing injury is the most common reason for legal proceedings.
I think you may be confusing the above form of negligence with "criminal negligence".
Sometimes there has already been a criminal conviction based on the same conduct related to the crash, and a civil court will accept a certified conviction as sufficient proof that the defendant is responsible for the damages. The standard of proof in a civil case for car crash injuries/wrongful death is lower than in a criminal case, so a conviction in criminal court is more than enough evidence”.
http://blogs.findlaw.com/injured/2013/10/can-criminal-charges-affect-car-crash-lawsuits.html
If someone is charged and convicted of a criminal offense that relates to fault in a car accident -- either by recklessness, negligence, or bad intent -- then there is sufficient proof that the convicted driver was responsible for the injuries.
Convictions for DUI can also prove useful in civil court, as violating laws which are meant to protect driver safety can be considered negligence per se.
I'll give you an example to try and explain the difference. Car A is driving along the highway at the prescribed speed limit. Car B which is travelling behind him is also driving within the limit. Car C is travelling along the highway 70 ks over the limit when it swerves into B. Car B is then forced into the car in front of him (Car A) causing damage. The police are called to the scene. They charge the driver of Car B with negligent driving because they say he was driving too close to the car in front of him (Car A). The driver of Car C is charged with criminal negligence because when the police investigate and see the skid marks, speed cameras etc. not only do they establish negligence but they also discover he was speeding away from a store he'd just robbed.
Driver A sues drivers B and C. The court establishes that while driver B was driving too close to A, they only apportion, say, 20% of the negligence to him, i.e. 20% of the damages. Driver C however will face paying 80% of the damages because he committed a robbery and was speeding away from the scene of the crime when he had the crash.
Driver B, on the advice of his lawyer declines to make a statement. Driver C also declines.
The ballgame changes from negligence (even if someone dies) to criminal negligence when there is a criminal element. This is when the police will caution him (read the Miranda rights or equivalent) and interrogate him. This is in no way unfair because he still has the right to arrange for a lawyer before he answers any questions, and the lawyer may well tell the police that his client chooses not to answer. Without the requisite proof, a person cannot be found guilty by virtue of the fact that he chose to remain silent and/or not to provide a statement. The prosecution still needs to prove its case.