Safety advocates who worry about the dangers of distracted driving have a new concern beyond cellphones and gadget-laden dashboards: digital roadside billboards. These high-tech billboards marry the glow of Times Square with the immediacy of the Internet. Images change every six to eight seconds, so advertisers can flash timely messages — like the latest headlines, coffee deals at dawn, a cheeseburger at lunchtime or even the song playing on a radio station at that moment.