What is the primary factor in determining the traction grade of a tire?

The primary factor in determining the traction grade of a tire is its ability to stop on wet surfaces. Traction grades are part of the tire's performance indicators and are designed to evaluate how well a tire can grip the road, particularly in wet conditions. This wet traction assessment is crucial for safety, as it significantly influences a vehicle's stopping distance and overall handling in adverse weather conditions. High traction grades indicate that a tire provides better performance in these scenarios, which is essential for preventing skidding and maintaining control while driving.

Other factors, such as tire design and aesthetics, performance in tear tests, or the weight of the vehicle, do play roles in the overall performance and durability of a tire, but they do not specifically address the key concern of traction, especially on wet surfaces. Therefore, the most relevant and critical aspect regarding traction grading focuses on the tire's performance in wet conditions, making that the correct choice.