What is the most representative way to measure the impact of a digital signage deployment?”
This is a difficult question to answer. Even I struggle to qualify each decision I make around digital signage. We currently lack a convenient way to retrieve metrics. Sure we could install facial recognition software and cameras at each screen to determine how many impressions we receive but that is costly and perhaps impressions are not your end goal. I believe you have to trust your instincts, think about things that impress you, things that you remember, and try to emulate that in your digital signage deployment. The best things in life are not always the most expensive and not always the latest technology, however they are impactful and often in an indescribable way. So I have nothing concrete to offer but to know that the confusion and uncertainty you might be feeling is normal. Trust yourself and if you believe that your deployment is creative and impactful, then so will others.