Yes, studs like Z-BO are legal, some of his "dates" maybe not so. Anyway, the WSDOT clears up any confusion:

WSDOT Rolls out Campaign for Winter Tire Options
Date: October 20, 2004

OLYMPIA - As the temperatures drop, the number of phone calls rises at the Washington State Department of Transportation. Already, the department’s fielded a number of calls from the public asking if studded tires are legal this year. The answer is yes; studded tires are legal from Nov. 1 to Mar. 31.

WSDOT is concerned about the damage studs do to Washington State roads and highways, which in turn create safety hazards. That’s why the department is working with the tire industry to remind motorists there is a variety of alternatives to studded tires available, tires that provide excellent traction in inclement weather and are legal for use year-round.

The campaign, featuring the “You’re in the Driver’s Seat. Know Your Tire Options” slogan was presented by WSDOT to the Transportation Commission at its monthly meeting in Olympia today.

“Information from the Northwest Tire Dealer’s Association shows us the number of studded tires being sold annually is decreasing,” said Chris Christopher, State Maintenance Engineer. That fact, coupled with a survey the department conducted last year that revealed a majority of motorists aren’t aware of all the different winter tire options, prompted WSDOT to launch a campaign that educates drivers about their winter tire options.

“The work WSDOT’s put in to working with the tire industry and develop this campaign is commendable,” said Dale Stedman, Transportation Commission Chair. “Let’s hope this strategy works and we’ll see less studded tires on the road.”