Metrological Characterization of Pavement Friction Measurements for High-Friction Surface Treatments: SI Traceability, Uncertainty Evaluation, and Adhesion–Hysteresis Separation via Water–Soap BPT Protocol

Laboratory friction measurements are central to material screening for High-Friction Surface Treatments (HFST), yet metrological aspects including a proposed metrological traceability framework or uncertainty, and reproducibility are consistently underreported. This study applies a metrology-aligned framework to British Pendulum Tester (BPT) measurements performed in three states: dry, wet (water), and water–soap to assess operational adhesion and hysteresis components, document traceability to the International System of Units (SI), and report GUM-style uncertainty with covariance for the adhesion difference. Measurements were obtained for calcined bauxite (CB) and rhyolite (Rhy) in HFST and Coarse gradations across seven polishing protocols (baseline; LAA-1000/2000; MDA-105/180; PSV-10 h/20 h), using n = 3 replicates per Treatment × Material × State. Replicate-based Type A uncertainties were combined with instrument/system Type B components geometry, slider hardness, temperature, soap film consistency, and calibration to yield combined uc and expanded uncertainty U(k=2). The Wet–Soap cross treatment physical correlation ellipses demonstrate strong positive correlations (r ≈ 0.88–0.98; p < 0.001) between wet and soap states, supporting the interpretation that wet friction is governed primarily by hysteresis, with adhesion acting as a small offset under BPT kinematics. The slider hardness and temperature typically control the wet state uncertainty budget; including measured Wet–Soap covariance reduces adhesion U(k=2) by up to ~6.5%, consistent with GUM’s law of uncertainty propagation. Together, these measurement science practices enhance road safety by making laboratory friction data traceable, comparable, and decision-ready.