What could be a sign from a preceding vehicle indicating rough road conditions?

A vehicle's taillights bouncing up and down is a clear indicator that the vehicle is experiencing a rough road surface. When a vehicle travels over uneven terrain, bumps, or potholes, its suspension system can cause the taillights to visibly bounce. This movement serves as a visual warning to drivers behind that the road may be in poor condition and could require extra caution.

In contrast, other signs such as abrupt slowing, horn use, or sudden lane changes might not specifically relate to road conditions and can arise from various other driving scenarios. For instance, a driver may slow down for a different reason, sound the horn to signal a warning unrelated to the road, or change lanes to avoid an obstacle rather than due to rough roads. Hence, observing bouncing taillights provides a more direct and specific alert regarding potential hazards created by the road conditions ahead.