It is illegal to leave a car idling unattended for more than three minutes.

The Law Explained

During freezing New England winters, it is tempting to start your car and go back inside to let it warm up. However, state environmental and traffic laws prohibit leaving an unattended vehicle running for more than three minutes. Originally enacted to curb air pollution and prevent auto theft, it has resulted in frosty mornings and occasional fines for unwary commuters.