When do traffic reports make traffic better?

We present a simple dynamic traffic model to study whether public information about road conditions increases or decreases travel times. We find that the effect depends on comparing (1) the increase in travel time when a road with a pre-existing delay becomes congested, and (2) the corresponding increase on a road with no pre-existing delay. Traffic reports are helpful when the first value is large relative to the second. Surprisingly, they are less useful when total road capacity is sufficient to potentially accommodate all drivers without congestion. In that case, traffic reports are always harmful in the nonatomic limit as individual drivers become negligible.