First of all, utilization is counted in DC RUM in context of network links. While in general it is possible to create alert on this metric, I will focus on Network performance metric in my answer. Assuming that we want to trigger an alert when Network performance metric for any Site drops below, say 60%, alert's definition should look like this:
This will handle case with any site having serious network issues, but what about sites which are down ? In such case we can't see traffic from such site at all. That's why it will be good idea to select this option on Propagation settings tab:
This option will cause our alert to be triggered when conditions shown on 1st screenshot are met (Site has network problems case) or when there is no traffic at all from the site (Site is down case).