There was a recent case in Raleigh, N.C. where a woman was stopped by a man impersonating a police officer. He was in plain clothes, which isn’t completely uncommon, but after asking her a series of suspicious questions, the woman asked to see some ID. At that point the man got flustered and took off. This sort of thing happens more often than you might think. And it’s one reason why police departments take their ID card programs very seriously.

Because police officers have a degree of power when interacting with citizens, it’s important that both officers in uniform and undercover agents are able to verify their identities when asked. Certainly it’s not every day that you’ll feel the need to question an officer’s identity, but when personal safety is a concern, asking for further identification is not considered out-of-line by any stretch.
Most traffic stops involve officers in uniform with clearly marked cars, which is usually enough to indicate the stop is legitimate. But in cases where there might be suspicion, an officer will generally present his or her police ID card when approaching. If proper identification isn’t presented, and there’s any level of doubt, an officer will certainly understand if the driver asks to see ID.