* Sudden stop without warning: If the lead driver stopped suddenly and without any warning (e.g., brake lights malfunctioning, unexpected obstacle appearing very suddenly), the following driver might have a stronger argument against being at fault. However, even then, the following driver still needs to prove they were maintaining a safe following distance and were driving at a safe speed for conditions.
* Road hazard: If a road hazard (e.g., a large pothole or debris) caused the lead driver to suddenly brake, and this was unavoidable, the following driver might bear less responsibility. Again, demonstrating a safe following distance and speed is crucial.
* Mechanical failure in the lead vehicle: If a complete and sudden mechanical failure (e.g., brake failure) in the lead vehicle caused the stop, the lead driver might share some or all of the fault. This is very hard to prove, however.
In most cases, the driver who rear-ends another vehicle is presumed to have been following too closely, driving too fast for conditions, or otherwise not paying attention. The burden of proof to demonstrate otherwise usually rests with the at-fault driver. Even with an exception, the at-fault driver may still share some responsibility. Ultimately, fault is determined by investigating officers and potentially a court of law based on the specific circumstances of the accident.
