Reframing Threshold Tolerance: A Global Contour Fidelity Metric for Visual Safety in Road and Airport Environments

Severe weather degrades visual information through attenuation, scattering, and wet‑surface reflections, affecting both human perception and automated systems in road and airport environments. Although adaptive local thresholds are widely used in image processing, no parameter captures the global loss of contour structure. This work introduces the Global Threshold Tolerance (TTg), a conceptual framework defining the point at which visual structure becomes perceptually and operationally unstable.The methodology comprises four components: global tonal assessment, large‑scale contour coherence, meteorological effects on visual clarity, and their synthesis into a single global parameter. The resulting indicator provides a holistic measure of visual stability, complementary to photometric metrics such as luminance, contrast, and RVR. Conceptual validation shows that TTg aligns with human perceptual mechanisms and accounts for phenomena such as glare, white‑out, loss of markings, and contour instability on wet surfaces.TTg has direct implications for operational safety, offering a unified framework for visual‑risk assessment, perception‑algorithm calibration, and future visibility standards. The work opens directions for numerical formulation, empirical validation on road–airport datasets, and integration into automated detection and decision‑support systems.