But if the flow of traffic is under 40 mph, you are not required to use them; so if there is a big snow storm and everyone is going < 40 mph, then you are not required to use them.
http://www.nysthruway.gov/about/rulesandregs/p103.html#103.14
Flow of traffic is not the same as road conditions. It sounds like you don't need to use your flashers for a traffic jam, but you should for a storm. I think the distinction is he slowed down because of the weather, not the flow of traffic. If he was under 40, then it seems to me the law states he should have his hazard lights on.
