On those boards, it would be very simple and easy to direct the light onto the board, instead of everywhere else including the eyes of the drivers.
Flagstaff Arizona is also rightly proud of its dark sky efforts -
http://www.azcentral.com/story/news/local/arizona-science/2017/01/11/satellite-images-show-flagstaffs-dark-sky-success-national-park-service-says/96460962/