Radio stations have a system for determining which commercials get played first during spot breaks. Some stations play the commercials with jingles and music first and save the voice-only spots for last. Others do it the other way around. Most play 60 second commercials first, saving the 30s for last. It's important for your commercial to be one of the first played.
Research shows that after the second or third commercial in a row, most radio listeners tune away.
That may have been more or less true in the past, but things are changing. Cable TV systems are dramatically increasing the number of channels they offer. New low-power TV stations are popping up everywhere. Soon, thanks to new digital technology, regular broadcast stations will be able to split their single channel into several. All these new channels need advertising to survive.