We’re well-practiced at carrying our driver’s licenses in the US, but it might surprise you that there are countries in Europe where it is illegal to venture out without your passport.
While it might be rare to be stopped and issued a ticket, Italian law mandates that all citizens and foreign visitors must carry a valid form of identification at all times. For non-EU citizens, this means a passport – the only official document that proves your nationality and immigration status. Your US driver’s license will not suffice.