DEN: There seems to be no traffic on the tollway going south from DEN. Pena blvd is also fairly empty. (This driving at morning and evening rush hour.) However, just because you get to the airport doesn't mean you are there. At MDW you can return your rental car and be at the security checkpoint in a minute, and at your gate a minute after clearing security. At DEN, you return the car, then 15 minutes later you finally make it to the terminal (wishing you would have said you were flying Fronteir so he would have stopped at the first terminal...) and the checkpoint. Then you ride a train and walk to your gate. Even being close, it is far away. Even behemouths like ATL and ORD are faster in the 'airport arrival to boarding gate time.' And once Denver starts to develop out towards the airport, disaster is just waiting to happen... And ORD and ATL both have trains going right to the airport. Even DFW, LAX, and SJC have some form of train service.