overheating at redlights

First check the coolant level (well after the engine has cooled down). As the engine gets hot in normal traffic it relies on the electric cooling fan attached to the radiator to "switch on" allowing it to draw air through the radiator cooling the coolant in the engine where it circulates through the engine when the thermostat reaches a sufficient temperature at which it opens. In normal traffic the electric fan is responsible for keeping the engine cool. If the engine over heats in traffic, and the thermostat is working correctly (it opens at about 190 degrees Fahrenheit) and the water pump is capable of circulating coolant around the engine and radiator then perhaps the the fan, or fan thermo-switch is not working correctly. As a car is driven at freeway speeds "rush air" flowing through the radiator is sufficient to keep the coolant circulating through the radiator cool without the need for electric fan operation.