YSK that when driving on wet winter roads, that when a car in front of you stops producing (or greatly reduces) its spray behind the rear tires that this means they are driving on ice.
from /r/YouShouldKnow
YSK说，当在潮湿的冬季道路上行驶时，当你前面的一辆车停止产生（或大大减少）其后轮胎后面的喷雾时，这意味着他们正在冰上行驶。