The park website at Grand Canyon National Park includes a warning about the potential for icy trails at the top of the home page, under a red and white banner titled "Park Alerts." I think it's safe to assume similar information is dispensed at visitor centers and other locations, so the photo below, posted on the park's Facebook page, helps illustrate why rangers get gray hair.

The full text of the warning reads: "Walking Trails at Grand Canyon May Be Slick Due to Snow & Ice - Especially in Shaded Areas. The park recommends slip-on type shoe traction devices, while walking on trails and pathways. Snow that melts during the day can freeze at night making pathways very slick in the morning. This can occur even if the pathway has been cleared."

Posted warnings or not, is it possible that some visitors just don't know "when to say when"?