What is the most representative way to measure the impact of a digital signage deployment?”
There are a few methods that pop up depending on the reasoning behind deploying the signage. If you are attempting to improve engagement, you’d probably want to complete surveys of groups impacted before and after the installation. Another method I’ve used for measurement is physically capturing the eyeballs that view the screen. If the signs are to sell, then you can probably use revenue of before and after for whatever product is being sold to determine possible success.