Excerpt from the Washington State Traffic Manual

7.2 BANNERS
In accordance with RCW 47.24, district administrators may grant written approval for suspending banners above state highways without access control provided that the organizers or sponsors comply with the following criteria:

1. A vertical clearance of 20 feet to the bottom of the banner must be maintained above the pavement surface.
2. The banner is located so as not to interfere with, or obstruct the view of, any traffic control device.
3. The banner must be removed within three days after the event is over.
4. Banner messages are limited to name, date, and event sponsor. Failure to comply with these requirements may result in future request denials.

The content of the banner message must comply with the requirements of the Scenic Vistas Act, RCW 47.42. Banners to promote civic events are permitted only if the profits derived from the activity they promote go directly to the support of nonprofit organizations.