It is probably impractical to ask for all billboard owners to retrofit the lighting systems for bottom-lit signs. A first approach would be to ask for all new billboards to be top-lit. Older billboards could be left as they were for some time until they would ordinarily undergo major renovation due to their age. There is also the question of whether a billboard needs to be lit at all when the business being advertised is not open. These individuals probably do not realize that in addition to light pollution they are also contributing to air pollution and possibly global warming.
Most electricity comes from coal-burning power plants.
It does not take very much light to make a sign visible at night. The next time you are at an airport notice how dim the taxi and runway lights are. Nevertheless, they are easily seen. In your efforts to produce changes in the way signs are lit in your area, be reasonable and work with the sign owners and billboard companies. We know it can be done because we see good examples in many places.