Complex sentence: Highway 90 passes through the county seats of all 15 counties on its course in Florida, not to mention being the address of almost every county courthouse, and is never more than six miles (10 km) from I-10 throughout the state.
Split into simpler sentences: Highway 90 passes through the county seats of all 15 counties on its course in Florida, and is also the road upon which many of the county courthouses are located. It is never more than six miles (10 km) from I-10 throughout the state.