WSDOT provides this info on traffic numbers for the first day of the SR99 tunnel. There were a total of 22,145 trips on Monday, a low number because of the snow and ice.
Just how low is that 22k figure for opening day of the #99tunnel? Well it's about 20% of what WSDOT's predicted average traffic level pre-tolling. Once weather is more favorable, we'll see if their prediction of significantly higher SR-99 traffic than before comes to pass.