An engine that overheats in slow traffic but runs at normal temperature on the open road most likely has

Correct answer to question 30 (Auto & Shop Information): a failed cooling fan

Method

At speed, air is forced through the radiator by motion alone, so a dead fan goes unnoticed. In traffic there is no airflow and the fan is the only thing moving air, so the temperature climbs. A stuck thermostat would overheat the engine at any speed.