Initially, I am unclear on what "DUI test" means (field sobriety tests, roadside preliminary breath test, evidentiary blood/breath test). That being said, I can answer your question without knowing exactly what "test" you were subjected to.....it will just take a little longer. Law enforcement cannot "seize" or pull a motorist over unless they have a "reasonable suspicion" that the motorist committed a traffic offense (or otherwise engaged in unlawful conduct). However, provided the initial stop is constitutional (i.e., reasonable suspicion of unlawful activity/traffic offense), Nevada law permits law enforcement to perform preliminary DUI investigations based solely upon a reasonable suspicion of a traffic infraction (meaning probable cause for formal arrest is not necessary). The foregoing is somewhat irrelevant to you as your encounter with law enforcement was predicated upon their response to an accident, not the officer's reasonable suspicion that you engaged in unlawful activity. In such cases, law enforcement is free to investigate accidents under what is called the "community caretaker" doctrine. The DUI "tests" that law enforcement are legally entitled to perform (based solely upon a reasonable suspicion of a traffic offense) are roadside "field sobriety tests" and a roadside "preliminary" breath test (administered on a less reliable device than the evidentiary breath test machines used to administer breath tests at the detention center). If they administered these tests upon you and not the other driver then it is because they suspected you of DUI and did not suspect the other driver of DUI. I assume that one of the two foregoing tests is the "DUI test" you are concerned about. Law enforcement is not entitled to perform evidentiary breath/alcohol testing on you unless they have probable cause to arrest you. These tests are generally performed at the detention center (after you have been handcuffed and transported away from the scene).

As for your specific questions. Law enforcement is not required to investigate everybody for DUI; just motorists that they have reasonable ground to believe committed DUI. They obviously suspected you for some reason and not her (may have been odor of alcohol, bloodshot eyes, lack of coordination, etc.) No, you were not investigated for DUI simply because you were at fault and the other driver was not cited. Again, they simply suspected you of DUI and not the other driver. I have several clients that were not cited for an accident (other driver was) but were ultimately arrested for DUI (rear-end victims).

No, they would only elevate a traffic accident investigation to a DUI investigation if they had independent grounds to suspect you of DUI. They must not have suspected the other driver of DUI or they would have investigated and potentially arrested him/her. I have represented many clients for DUI that were stopped at a red light and got rear-ended by another driver. You "got the DUI test" because law enforcement suspected you of DUI, not because you were suspected of causing an accident.

No, it's because cop felt you may have been driving. He needed "probable cause." if she didnt' appear to have been drinking, no reason to test her.

Left turn vehicle has duty to yield.

Unless she came through on a yellow light for her, I don't see her being at fault.