Cracks that start within 2–3 inches of the edge of the windshield spread the fastest because the perimeter carries the most structural tension. Critical Repair vs. Replacement Thresholds What Causes Windshields to Crack? - Auto Glass Now
This is the #1 cause of spreading. Glass expands in heat and contracts in cold. Rapid changes—such as blasting your defroster on a frozen morning or using the A/C on a scorching afternoon—create massive internal stress that pushes the crack outward. windshield crack spreading