It is certainly understandable why some owners may choose to leave a dog in a parked car occasionally. The errand may literally take only a few minutes to accomplish, for instance, or the establishment may not allow animals on the premises. Leaving a dog tied to a lamp post or bicycle rack could result in an unfortunate encounter with other animals or people. A dog left outside could also be stolen or be exposed to bad weather. Most experts suggest that it is a bad idea to leave a dog in a car for any longer than a few minutes, however.
When a dog is left in a parked car, the animal can become overheated extremely quickly, even in weather that's not exceptionally hot. An unventilated vehicle can become a death trap within minutes, so it is vital that owners provide proper ventilation by leaving the windows down while the dog is inside. Opening the windows just a crack does very little to relieve the heat. Parking in the shade usually does not make the temperature inside the car stay significantly cooler.
Another consideration is the physical comfort of the dog. A dog that is left in a car with no water and no opportunity to relieve itself can quickly become distressed. It may damage the upholstery in the car or urinate on it as a result. The owner can leave some fresh water in a travel bowl for the pet, although it's very likely to be splashed and spilled in the car. The dog should also be let out before leaving it in a parked car. Even when such precautions are taken, the dog may be uncomfortable and destructive.
While it might not be illegal to leave a dog in a parked car, an owner may want to consider alternatives before putting a pet in an uncomfortable and potentially dangerous situation. Incidental errands to and from a dog park or a veterinary visit may be okay, but a situation where time may expand, such as a grocery run, should be postponed until the dog can be left at home or in the care of a responsible caretaker. Many dogs enjoy riding in cars with their owners, but they may not enjoy long periods of waiting in an overheated car without water, food, or attention.
Leaving a dog in a parked car for a very short amount of time may not cause any harm, as long as the car windows are partially rolled down for ventilation and the pet cannot escape, but it's best avoided. It is not a good idea to ever leave an animal in a car for hours at a time, even if it has ventilation and water. Pets cannot take care of themselves, so it's the owners' responsibility to do everything they can to keep them safe and healthy. If a pet doesn't need to be in the vehicle in the first place, it should be left at home rather than stuck in a small, hot space with little stimulation.