If you're not alone, you can still keep your IEMs in, keep the music off and talk with any passenger(s). The road noise will force the passengers to raise their voices loud enough to penetrate the IEMs. Conversation will not be a problem. If he/she/they fall asleep, turn the music on. Only turn the music loud enough to a comfortable volume. Road noise will make it impossible to turn the music loud enough to drown it out without reaching SPL levels that cause noise induced hearing loss.
Also, SPL that reaches your ears when you "crank your phones volume" < SPL that reaches your ears when you "crank your car speakers". This is because the passive isolation IEMs provide reduces (BUT NOT ELIMINATES) the amount of road noise reaching your ears. This allows you to set the music to a lower volume. Using IEMs will save your hearing.