There is currently no fee for a dog to ride in your car in Massachusetts. (Yay! But read on.)
If you are wondering whether or not a large dog in the passenger seat can be used to get you free passage in car pool lanes, consider that the only car pool lanes in Massachusetts are free anyway. They are only created temporarily during rush hours to relieve traffic flow. You can read about high-occupancy vehicle lanes (HOV lanes) at the Massachusetts Department of Transportation, Highway Division website.
However, the Highway Division defines an "occupant" of a vehicle as a human being. A dog, even a large dog, does not qualify for the purpose of carpooling.
If you are planning to put your dog in the back of your pickup truck, you should know the law about the Safe Transportation of Animals.
"No person shall transport an animal in the back of a motor vehicle in a space intended for a load on the vehicle on a public way unless such space is enclosed or has side and tail racks to a height of at least 46 inches extending vertically from the floor, the animal is cross tethered to the vehicle, the animal is protected by a secured container or cage or the animal is otherwise protected in a manner which will prevent the animal from being thrown or from falling or jumping from the vehicle. Whoever violates the provisions of this section shall be punished by a fine of not less than $50. "
Cruelty to animals includes the act of anyone who "carries it or causes it to be carried in or upon a vehicle, or otherwise, in an unnecessarily cruel or inhuman manner or in a way and manner which might endanger the animal carried thereon..."
Still, dogs may have more freedom in motor vehicles than people do. Adults and children, with some exceptions, must wear seat belts or, if under age 8, child passenger restraints. You can read those laws here (for adults) and here (for children under 8). Dogs are exempt from this.
So go ahead, pet your dog while you drive. It may do you both some good. (As long as you drive carefully.)