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Hi James...,
Well, my comment was somewhat simplified. What happens when one slows down from highway speed to standing at a light is that the thermal gradient across the thickness of the metal from the inside of the combusting chamber, to the water jacket side. Running down the highway, the inside surface of the metal is quite warm. The water flowing in the water jacket caries the heat away as the pump is going quite fast, and the water flows is large. Now come to a stop, and the water flow is very much lower. But there is still heat stored in the inner layers of the engine metal. That heat transfers to the coolant, but because of the slow pump, not to the air. This causes the coolant temperature and pressure to increase a whole bunch. To improve the heat flow out of the coolant, the fans come on to incease the mass of air per unit time that flows over the radiator. Which caries away proportionally more heat.
Some sorta fan mechanism is needed. Now the Prius and other engine stopping vehicles still have the radiator fans. The Prius has two actualy. Although, I think I have only heard them come on once or twice in the 2 1/2 years I have had the car. The Prius has an electric coolant pump for the inverter coolant circuit. I imagine its variable speed.
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