Why does my AC get warm in traffic?
This is the same class of failure as 'warm at idle' — the condenser needs airflow the moving car normally provides. In stop-and-go, the electric condenser fan is the only thing pulling air through the front of the radiator stack. If it's slow, seized, or the relay is intermittent, cabin temps climb the moment you stop. It can also be a system that's marginally low on refrigerant: it works when it's easy (60 mph) and fails when the pressure climbs (idle). Have the fan and refrigerant pressures checked together — that's a 15-minute shop diagnostic.
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