Who is at fault in a left-turn accident?

A driver turning left across traffic generally must yield to oncoming vehicles close enough to constitute a hazard. That duty makes the turning driver the presumptive at-fault party.

Where it shifts. If the oncoming driver was speeding substantially, ran a red light, was driving without headlights at night, or was otherwise not lawfully in the intersection, fault can shift or be shared.

The recurring dispute is the light. Both drivers say theirs was green, or that the turning driver had a protected arrow. Independent evidence decides it — witnesses, nearby cameras, and sometimes signal timing data from the traffic agency, which is obtainable.

Speed matters more here than people expect. A turning driver who reasonably judged the gap based on a normal approach speed has an argument when the oncoming vehicle was traveling far above the limit. That's a reconstruction question, and vehicle damage and crush patterns bear on it.