ESPN was imploring Nielsen to measure OOH years ago and they did some pilot studies with questionnaires. The purchase of Arbitron in 2013, and its personal people meter (a pager looking device that listens for watermarks) set up the capability to do this better and fast enough to match the overnight tv ratings schedule.
http://www.bizjournals.com/losangeles/news/2017/04/10/espn-nielsen-out-of-home-ratings-service.html
http://www.nielsen.com/us/en/press-room/2013/nielsen-acquires-arbitron.html
