As you suggested Wray, I went to the Nokian site. The text below is from their research and no doubt is where the position taken by the Washington State Patrol and my Volvo professional contacts comes from.

"Studded tyre offers superior ice-grip"

Numerous studies have shown that even the best studless winter tyres do not grip an icy surface as well as studded tyres, which have about a 20-30% better braking grip on mirror ice than studless winter tyres. The braking distance on slick ice at a speed of 90 km/h is about 300 meters with studded tyres and 400 meters with studless winter tyres. The studded tyre's lateral grip on ice and packed snow surfaces is also superior to that of the studless winter tyre's. Since the majority of winter driving accidents occur as a result of the loss of lateral grip (see Appendix III), good lateral grip becomes the most critical safety feature.

I continue learning from others

steve m