Dynamic message signs are made up of thousands of "Light-Emitting Diodes" (LEDs).  The intensity of light emitted by each light bulb on the sign depends on the viewing angle.  The intensity is greatest when viewed directly on axis with the light bulb.  As you move off that axis the intensity diminishes.  As drivers move away from the maximum viewing angle, the sign will eventually appear dark.

Understanding the cone of vision of LEDs should clarify the need to properly aim an LED technology sign toward the desired viewing locations.  A sign mounted over the roadway, provides the maximum viewing intensity to drivers in all three lanes. The cone of vision is significantly reduced if there is a curve in the road approaching the sign or when the sign is mounted to the side of the road and not directly over a lane of traffic.