It's not illegal but there are some states are quite liberal in their application of asset forfeiture law. In many cases it's "guilty until proven innocent" if they "think" the money may be involved in illegal activities. If you're driving cross country through states like Iowa I would avoid carrying large sums of cash. You catch an officer in a pissy mood you could be screwed. Chances are you're ok but if you can legally transfer the money electronically I'd go that way.