Display time for a billboard message is varied in proportion to
measured speed of one or more vehicles. This may include
varying not only the time a message is presented on the
billboard, but also varying the density of the message. An
overall message may be broken into segments displayed a
sequence of billboards depending on the speed, e.g., for a
faster moving vehicle the message has more segments. If the
speed of a vehicle is great enough, the message segments are
also reduced to fewer words or graphics, which may also be
larger. Also, clusters of vehicles are detected and a billboard
message display is varied in response to the clusters, which
may include display time or segmentation variation, so as to
more nearly optimize the number of persons who are able to
see the message. Further, a billboard is rotated per vehicle
speed, distance, length or clusters.