I have noticed my car's temperature gauge will go up when I am in traffic. Here are some examples.

Highway driving
- around 80, usually stable

Traffic driving
- would go above 100 and then fan kicks in to reduce it back to between 80 and 100

Parked
- would go above 100 and then fan kicks in

Any idea what is happening?
All other cars I have owned had a stable temperature reading..

Thanks