Possible Clues:

• Letters sent abroad by a politician, way lighter
• Highway feature identified by arrangement of map letters
• Illumination on post
• Letters sent off to a politician in the post?
• It's a standard form of illumination, by the way
• The first electric one was used in Paris in the 1870s
• Letter maps out how to cast a shadow on the way
• Pavement illuminator
• Main liner
• Wayside light