This isn’t a problem I’ve encountered before, so perhaps this contextualises my response, but I don’t think it’s a great solution. When I book an Uber, I mentally note the registration of the car, and then use my phone to make a call, send a message etc. When the cab arrives I usually continue this activity as I get in. Breaking this to hold my phone as a sign (which would have a social stigma attached to it) isn’t an attractive solution. I’d prefer to upload my picture to the Uber app, and then for the driver to be able to identify me that way. Identification the other way doesn’t need fixing, as the reg works just fine.

This is a survey of one however, so I may be the outlier 😉