Even more realistic (because if you are driving, you have to at least show a driver's license).
Lets say you are in South Florida and you are on a Greyhound Bus and you are Hispanic but you don't have any ID on you.
Then what happens?
Are we really ok with people being detained inside the US because they cannot provide proof on their person that they are a US citizen?
When I pass the customs and Border patrol checks in California and Florida, they simply as me my nationality, and I tell them USA. I don't have to prove it, because that is not a requirement in the constitution or any law.