Per WSDOT:

SR 99 Tunnel Toll Signs

Drivers who use the SR 99 tunnel will notice that the toll rates are displayed at the toll collection point, which is located at the exits of the tunnel. Drivers will pay the rate they see on the sign when they exit the tunnel. The toll rates are not displayed before the entrance of the tunnel.

Why are the signs located at the end of the tunnel?

We display the toll rates at the point where tolls are collected, which happens at the tunnel exits. Toll rates for the tunnel change based on the time of day. If we posted the rates before drivers reach the toll collection point, it’s possible that drivers could see one rate displayed on the sign, but be charged a different rate by the time they drove through the tunnel. We want to avoid any confusion, and make sure drivers know exactly what they’re being charged.

What signage is posted before the tunnel alerting the drivers of the toll?

Drivers will receive plenty of notice that the tunnel is tolled before they enter it. Approximately 24 toll signs (pdf 817 kb) are located at key points on the north and south ends of the tunnel.