Don't forget, at highway speeds you have a lot of air passing through the rad so the fan is doing very little. When coming to a standstill all that heat generated while driving still has to be dealt with, that's when the fan will come into play to replicate the air passing through the rad needed to bring the coolant temp down. Normally when cold the fan and before the engine is started the fan will be stiff. As the engine starts to spin the fan will roar for a short while and will become loose on its clutch. Then when the engine gets hot enough the fan clutch will grip and the fan becomes more and more effective to pass heat through the rad.
A none sealing coolant system I. E the cap or tank in your case will allow the coolant to boil and spew like a fountain.