While I am a cat owner not a dog owner, I do have some cats that like to travel with me. I go with the theory that I don't leave my animals in a car any longer than I'D like to be left in a car.

I have taken Pooky and Dusty before with me and have had to stop for a very short stop (usually a very quick potty break) on the way in the summertime (2-5 minutes max, parked in the shade). I only crack the windows a very tiny bit (so not to let the cool air out) and I run my A/C on MAX up until I have to make my stop (to the point I'm almost freezing) so the car is good and cold and never take long enough at a stop that I'D be uncomfortable with staying in the vehicle myself.

If it's too hot for me to sit in the car without A/C for a few minutes, it's too hot for my pets!

The few stops I've had to make with the cats the car was not even stuffy when I got back in and I was perfectly comfortable checking on the cats and getting my things together before starting the car and getting the A/C going again without sweating.

While it's not possible for a common passerby to know that if they were to have walked by and saw my cats, it would be obvious from looking at them that they were not in any kind of distress or panting in any way. Even if they pried my doors open (which they wouldn't have time to do before I got back) they would find a comfortably cool car, cool fur and cool foot/nose pads.

I would never subject my cats to anything I wouldn't tolerate myself.