There seems to be a rule of thumb for black ice strength: kilograms = centimeters squared times 5. Seems to jibe with recommendations for a 80 kg walker (4 cm),

There seems to be a rule of thumb for black ice strength: kilograms = centimeters squared times 5. Seems to jibe with recommendations for a 80 kg walker (4 cm), 2000 kg car (20 cm) or big 48 ton truck (1 meter).