In the alternative it shall be deemed a compliance with Sections 56-5-5090, 56-5-5100 or 56-5-5110 in the event three portable reflector units on standards of a type approved by the Department of Public Safety are displayed at the times and under the conditions specified in said sections, either during the daytime or at nighttime, and such portable reflector units shall be placed on the roadway in the locations as described with reference to the placing of electric lanterns and lighted flares.