Signs Must Work with All Vehicle Types
Here is one more reason that brighter signs are better. More vehicles are SUV's and pickups. Higher off of the ground and with a greater distance between the headlights and the driver's eyes, the all-critical 'observation angle' for a sign is increased.
Observation Angle is defined as the angle between the source of the light (here the headlights), the sign itself, and the driver's eyes. The observation angle is larger with larger vehicles. To compensate, the sign must be Retroreflective and larger angles. They should be visible to every type of driver.