Signal Ahead, Silhouette
The Advance Traffic Control symbol signs includes the Signal Ahead sign. These signs shall be installed on an approach to a primary traffic control device that is not visible for a sufficient distance to permit the road user to respond to the device. The visibility criteria for a traffic control signal shall be based on having a continuous view of at least two signal faces for the distance specified.
Federal Highway Administration Manual on Uniform Traffic Control Devices for Streets and Highways, 2003 Edition (including 2004 and 2007 revisions) (Washington, DC: United States Department of Transportation, 2007)