I use panniers, but it's mostly empty. Some days I've got nothing on me but an ATM card to buy a sub at lunch with. Usually I brown bag it though, so there's lunch. And almost always I carry a phone (turned off, just for emergencies) and a camera. I don't carry a lock or anything like that. Sometimes I'll have some fresh clothes or something in there. Maybe even a whole new 8 pack of soda. So it can vary from a credit card to 30 pounds of stuff in 2 panniers. But typically it's one pannier half full, and I could see, especially in the summer, riding around with almost nothing.
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Work: the 8 hours that separates bike rides.