The evidence that must be preserved is limited to evidence that would be both material and exculpatory. Evidence is material if it is relevant to an important issue in the case, and evidence is exculpatory if it supports a defense or tends to show that the defendant is not guilty of the crime.
This might not answer your question, but this was from the wedsite below......
I am answering this from experience. If you stop a car and you know that the person driving has a felony first of all a felon is not allowed to carry a weapon or have one in his or or presents at all times if they are cought with one they go to jail. Secondly if I pull over someone and they have a history of Violence with a deadly weapon or in general I will call for backup. You never go head on in a situation when you don't know if the suspect has a weapon inside of the car or on them. When backup arrives you start commanding the driver to step out of the car. If the driver does not come out of the car we walk up slowly but we make sure we have backup first. Never just walk up to a car with a driver who has a felony of assault with a deadly weapon because you have no idea what to expect. I hope that this has answered your question