Driving south on I-75, I noticed big round spherical objects hung on wires wherever the hi tension transmission lines crossed the highway.
What were they and what is the purpose?
I think they’re there to make the wires visible to low-flying aircraft. A MedEvac helicopter, for instance, might need to land on a highway to get crach victims to the hospital, ASAP, and having some idea of where the high-tension wires are is of obvious benefit.
'Scusa da nitpick, but have you ever seen a spherical object that was not round? 
Loopydude is right about the aircraft. Here is a more lengthy answer from Google Answers.