Take down the car’s color, model, make, and license plate number. Have the owner paged in the nearest buildings. Dial 311 (non-emergency) for local humane authorities if the dog doesn’t appear to be distressed. Call 911 if dog is suffering. Have someone keep an eye on the dog. Don’t leave the scene until the situation has been resolved.

If authorities are unresponsive or too slow and the dog’s life appears to be in IMMINENT DANGER, find a witness (or several) who will back up your assessment, take steps to remove the suffering animal from the car, and then wait for authorities to arrive.