When your vehicle is on the motorway, the flow of air through the radiator is sufficient to keep the engine running at the correct temperature. However, when running in traffic or in town, then as the temperature rises it is the job of the cooling fan to come on. The fan will not always run just because the engine is running, it only comes on when the engine temp rises, an a electrical circuit connects and the fan operates. Once the temperature has come down, the electrical circuit disconnects and the fan stops. It may be that the sensor for your fan is not working correctly. We would suggest that you get it checked and replace if necessary.