If it is still heating up in traffic with the fans working properly then it would probably be a flow problem. Either the radiator is restricting the coolant flow through a partial bloackage which may be the case or the thermsotat is not fully opening on the car.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record