Which of the following can indicate a vehicle is in trouble when seen at the roadside?

Flashing hazard lights are universally recognized as a signal for caution and indicate that a vehicle is experiencing trouble or requires immediate attention. When a vehicle's hazard lights are activated, it alerts other drivers that there is a potential hazard ahead, such as a disabled vehicle or an incident that necessitates extra care when passing. This warning is crucial for the safety of both the vehicle in distress and other road users, making it the clearest indicator of trouble when observed at the roadside.

A flat tire, while certainly a problem, does not in itself communicate to other drivers that the vehicle is in trouble unless the hazard lights are also activated. Parking on the shoulder could be routine for emergencies or temporary stops and doesn't inherently signify trouble unless combined with other indicators, such as lights on. Lastly, see-sawing between lanes may suggest a driver is distracted or intoxicated, but it is not a reliable sign of mechanical trouble and can actually pose a significant danger to other drivers.