Here in the States... and for sure in Iowa... the motor vehicle law states that hazard lights are required for any vehicle stopped on a highway or traveling on a fast highway but going much slower than the speed limit for any reason. Usually that happens while a traffic pile up is moving past an accident or continuing through fog or during a bad snow storm that prevents normal flow at speed for traffic. In those cases hazard lights are required... though many drivers neglect to use them properly.
there was the day that loaded truck in a grade was a common sight to see the emergency flashers going, but not so much with today's 1000hp turbodiesels, on the interstates if going under the minimum for whatever reason requires flashers
no requirement for flashers during snowstorms here in NH, but not a bad idea to use them if it's really coming down,, we do have a law that requires that you clean ALL the snow & ice from your vehicle, no scraping a donut hole for the driver to see out