What is an inappropriate use of a vehicle's horn in traffic?

Using a vehicle's horn to hurry drivers at an intersection is inappropriate because it can create additional stress and confusion, potentially leading to unsafe driving behaviors. The horn is intended as a safety device to communicate essential information regarding potential hazards or to alert others to your presence, rather than as a means of expressing impatience or frustration. In many driving situations, especially at intersections, it is crucial to maintain a calm and controlled environment to ensure the safety of all road users. Using the horn in this manner does not contribute positively to road safety and can escalate tensions among drivers.

The other options represent more acceptable uses of the horn: alerting a friend can be seen as a friendly gesture when done respectfully, indicating a lane change communicates intent to other drivers for safer navigation, and warning another driver of an obstruction is a responsible and necessary safety measure.