Given how long Hwy 99 is, using that as an intermediate destination without using an address on 99 seems like a recipe for problems. Your search result was probably the geographic center of the highway in Seattle, which is a lat/long and not attached to the roadway itself. Also, I think Waze is designed to not end routing in a tunnel. So if that geographic center happened to be in the tunnel (pretty sure it is), Waze would route to the surface.

In the future I'd suggest starting by entering your primary destination and then using the 'routes' option to see if Waze is avoiding the accident or problem area in question. If not, then "add a destination" and search for an actual place or address you want to route past.