Drive time to radio is what prime time to television. Knowing that maximum traffic would be in drive time, the slot could be used to generate more revenue by charging premium prices.
It is usually the time between 6 am to 10 am and 3 pm to 7 pm.
This time slot is preferred by advertisers as a lot more potential customers are tuned-in to their radios. Also, being in traffic during office hours, ensure long durations during which the radio is on.