Driving around with hefty amounts in your car is not good. If the US is like my country there's nothing the police love doing more than pulling over innocent motorists. If you sweat on the big shit, this is what will get you caught.
Your country is a lot bigger than mine too... back in the day I would take a taxi. I've never had the police pull me over when I've been in a taxi. Rental cars are cool too, so long as you don't break the road laws the police can not have any reason to pull you over. They always check the plates before any pull, when it comes back as a rental they look for someone else.
For me though, taxis are the way to go.