What does "severe" mean in this context?
From Goodyear: "The 3PMS designation is an indicator that a tire has a strong level of winter performance, per the U.S. Department of Transportation’s requirements. Several tiremakers, including Goodyear, are developing all-season tires with 3PMS designation. These are tires that carry the 3PMS symbol, but can be used year-round. These products offer more snow traction than a traditional all-season tire, and they are a great alternative for customers. However, for someone who drives in harsh and unpredictable winter conditions, such as during a winter travel advisory, Goodyear would still highly encourage motorists to consider a dedicated set of winter tires. To designate a tire with the 3PMS symbol, the tire must maintain standards, including a certain level of grip on a surface covered in medium-packed snow."
From Nexen: "A 3PMS marking does not represent maximum winter performance. It is more a minimum. A tire that just meets the 3PMS level is a good step up in winter performance from most all-season tires, but has nowhere near the traction and control of a dedicated winter tire."
There is a new ice traction test and symbol being introduced. The 3PMS symbol represents that the tire has at least 10% more straight line acceleration traction than the reference All Season tire on medium packed snow; braking and cornering are not tested. The new ice braking test will require at least 18% more braking traction on ice than the reference All Season tire.
So...the Severe Snow Service rating isn't so severe at all. And ice...including snow that has been packed and polished by many spinning tires at an intersection...isn't covered in the testing until the ice symbol gets here. All that said, tire companies design and test their tires to meet many parameters, including several that are at odds with other parameters. We get the compromise in performance that the tire companies choose to give us. For example traction and tread wear are at odds with each other...max traction = minimum tread life and vice versa.
The new ice traction symbol alongside the snow traction symbol: