Which of the following is considered mechanical evidence when investigating a motor vehicle crash?

Under-inflated tires are considered mechanical evidence in the context of investigating a motor vehicle crash because they directly relate to the vehicle's mechanical state and could have a tangible impact on the crash dynamics. The condition of tires is critical to vehicle performance, affecting factors such as traction, braking, and handling. If tires are found to be under-inflated, it may indicate a mechanical failure or a lack of proper maintenance, contributing to the causes of the crash.

Skid marks in the roadway, while definitely providing valuable information about the dynamics of the incident, are more about the physical evidence of the moments leading up to and during the crash rather than the mechanical state of a vehicle. Witness statements are subjective accounts of the event and don't directly pertain to the mechanical elements of the vehicles involved. Similarly, weather conditions are external factors that can influence a crash but do not constitute mechanical evidence in relation to the vehicle itself. Each of these other elements contributes important context to an investigation, but under-inflated tires are specifically indicative of mechanical issues related to the vehicle.