When do americans use the expression "Let's roll"?
I agree with Gillnetter. Most of the time "Let's roll!" means, "Get in the car and we'll go."
I think that all of these responses are correct. Ultimately, it means "let's go." Although the normal, daily use might mean "get in the car and go," "let's roll" also has a military meaning. To me, it has a more determined feeling than "let's go." In other words, "let's move no matter what the circumstances." It might also be a shortened "let's rock and roll," which also has a military and more aggressive/confident connotation (like its musical origin).