Such a comparision makes not much sense to me. A smartphone screen takes pretty much all your focus, the amount of distraction is of totally different levels than what a radio provides.

For example, try at same time reading (and properly understanding) a mail and have a conversation with somebody. Then try having a conversation with somebody while music is playing, no matter how loud. Ok it gets harder if you actively try to follow spoken words on the radio, but you get my point.