I take my girls out frequently. They love car rides! I will leave them in the car/truck, for short periods of time if its cool out, with windows down of course. The one likes to sit up and check people out, the other passes out till my return. (I think she sleeps better in the car than the house) When the weather is decent I take them with me when I go to other farms to give riding lessons. They usually stay in the car, in the shade, while I teach. When it gets hot out they stay home..or if it is too cold.
What gets me is people taking their dogs out in the heat (like at a summer horse show) and leave them tied to a trailer for the day, or tied up where they cant get out of the heat/sun. A friend of mine had an outdoor event and someone brought their dog, left it tied for the day, by the time they realised the dog was distressed it was too late.
Personally with my dogs, I figure it I'm going to be hot and miserable, they would be too and they can stay home in the AC.