[hammock]

I've seen this done in airports before, only using a plain laminated card that you carry through the line - the low-tech solution. The card was used by the TSA to internally track wait times

[evan_]

They used to do this at theme parks as well

[tdfw]

Disneyland in Anaheim still does. They use it to update the wait time signs at the entrance to each ride and, just recently, their official wait time mobile app.