iTunes Radio will not automatically switch from a Wi-Fi connection to cellular. You will first need to give the app permission to use data on your plan before you can take it on the road. Once you opt in to let the app use cellular data on your device it will use around 28MB an hour on a 3G/4G data plan. This adds up to 840MB of data for 30 days if you use the service 1 hour a day.
How much data does iTunes Radio use?
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