What is the most representative way to measure the impact of a digital signage deployment?”
I believe the best way to measure the impact of a digital signage deployment is to carve out 20 minutes of time, on several occasions at differing times, and actually OBSERVE how pedestrians interact with your deployment. Then, jot down your observations and thoughts about how they engage. Do they just glance at it? Do they pause to look? Do they have a reason to look at it? Is there an obvious attention grabber and call to action in the content design? Is it deployed in a logical venue? Is it deployed in the ideal spot of that venue? Does the placement and content meet your goals for the deployment? Is the content fresh and updated often enough to keep people looking?
If it is an interactive deployment, are you tracking touch button interactions? Which buttons are used most often? Which ones are used the least? Do you need to use that information to redesign the interactivity experience? Is there something missing that they would use more often if added?