IMO bad idea. These electronic billboards are much worse than traditional billboards in distracting already self-distracted drivers from doing what they are supposed to be doing... driving! Sounds like a huge step backwards in civic efforts at improving traffic safety.
After driving passed these electronic billboards (EB) a number of times since reading your comments and giving some thought and consideration to your them, I don't believe they are worse than the traditional billboards. Actually, a case can be made that EB's reduce the clutter and the number of distractions by reducing the amount of billboard distraction arising from traditional ones.
The operators of these electronic billboards could and should minimize the length of the message using the guidelines of "twitter" message while optimizing the visual portion of the message; 10 seconds (6/minute) should be sufficient time to instill a visual message aimed at the motorist. It also theoretically eliminates the need for five additional traditional billboards. The premise: "A picture is worth a thousand words" remains a very effective way to pitch a product to a consumer: if you show an image of a bottle of coca cola with the word "refreshing", you have made the only pitch that you probably need.