I've wondered about this. I typically rent from National with their Emerald Aisle service, which means I have a membership card and have signed a master rental agreement in advance. So, for most airports in the US, when I rent a car, not only do I not have to sign anything, I don't even get anything back from them with the rental specifics.

That is, I have nothing that has my name on it, the expected return date, or anything. I get the hang tag that was on the dash or rearview mirror back from the agent at the exit gate, but this just has the car's ID number written on it. There's a pre-printed "Notice to law enforcement officers" that explains that the car has been rented under a master rental agreement, and gives a toll-free number to call should the officer need to verify any of the particulars. I assume that since this is being done as standard procedure by a major national company that their legal department has determined that it is legal in any state where they actually offer it.

I've never had the occasion to find out what happens if you run into a situation where an officer asks for the contract and all you produce is this tag that states that there is no contract carried by the renter. I've only been pulled over once in a car rented under this program, but the officer didn't ask for the contract (I was driving a bit erratically, and once I explained the "it's a rental, and I'm not familiar with how it handles" and more importantly him determining that I wasn't drunk, all was well).

But I can't help but wonder that in certain circumstances, the officer would simply not believe or accept that it was proper to be driving a rental car without a contract with particulars being in the rental vehicle.