Question:  What is silhouette lighting?  
Answer:  There are two approaches to streetlighting near crosswalks.  The IESNA RP-8 discusses negative contrast and positive contrast options.  Positive contrast occurs when an approaching driver sees the front of a pedestrian brightly lit and contrasted against a dark background.  Negative contract occurs when the approaching driver sees a dark silhouette against a bright background.  
When lighting a crosswalk at an intersection, negative contrast is often chosen.  In the judgment of this author, that is because the lighting must consider the style of clothing worn by pedestrians.  When dark blue denim or black fabrics are popular greater contrast can be achieved with negative contrast lighting.  
The figure below is an excerpt of the 2000 release of the "American National Standard Practice for Roadway
Lighting" (IESNA, RP-8-00).  It shows a lighting layout where, from the perspective of an approaching driver, the lighting illuminates the pavement behind the pedestrian.  The pedestrian is initially seen as a silhouette, or negative contrast, a dark silhouette against a bright background.  This type of lighting is effective with pedestrians wearing dark clothing.  

See http://www.eereblogs.energy.gov/tap/file.axd?file=2010%2F10%2FIESNA+RP-8-00.pdf for the full text of the RP-8 report.