In order to analyze the detection ranges of infrared guidance systems for freeway traffic under different visibility conditions, various influence factors from atmospheric attenuation on infrared radiation in heavy fog are analyzed according to theory models and calculation methods of infrared atmospheric transmission. Based on it, a research method of detection ability of infrared guidance system in fog is proposed. The infrared radiation parameters of objects can be measured through simulating the meteorological conditions in natural fog in a laboratory. And the detection ability of the system in natural fog is evaluated through theoretical analyzing and practical validating. Experimental results show that the infrared guidance system for freeway traffic has better ability to detect objects in fog.