The State of Vermont's school bus idling law, enacted in 2007:
ACT 48. AN ACT RELATING TO THE IDLING OF MOTOR VEHICLE ENGINES ON SCHOOL PROPERTY. (S.13)
It is hereby enacted by the General Assembly of the State of Vermont:
Sec. 1. 23 V.S.A. § 1282(f) is added to read:
(f) Subject to state board of education rules, which may provide for limited idling, the operator of a school bus shall not idle the engine while waiting for children to board or to exit the vehicle at a school and shall not start the engine until ready to leave the school premises. The board, in consultation with the agency of natural resources, the department of health, and the department of motor vehicles, shall adopt rules to implement this subsection. The rules shall set forth periods or circumstances that reasonably require the idling of the engine, including periods when it is necessary to operate defrosting, heating, or cooling equipment to ensure the health or safety of the driver or passengers or to operate auxiliary equipment; and periods when the engine is undergoing maintenance or inspection.
Sec. 2. SCHOOL BOARDS; VEHICULAR IDLING
Nothing in this act or in department of education rules shall prevent a school board from adopting idling policies for motor vehicles other than school buses when present on school premises. By January 1, 2008, the department of education shall develop a model policy relating to idling of vehicles other than school buses to be distributed to schools for their use.
Sec. 3. EFFECTIVE DATE
This act shall take effect on passage. The rules required by this act shall take effect during the 2007–2008 academic year.
Approved: May 25, 2007 

Vermont's Unattended Motor Vehicles law is mainly designed to discourage vehicle theft (no insurance coverage for stolen vehicles resulting from running unattended), but can also be applied toward idling enforcement. Here's the statute:
Title 23: Motor Vehicles
Chapter 13: OPERATION OF VEHICLES
23 V.S.A. § 1111. Unattended motor vehicle
§ 1111. Unattended motor vehicle
No person shall permit a motor vehicle to stand unattended without first stopping the engine, locking the ignition, removing the key from the ignition and effectively setting the brake, air temperatures permitting, and, when the vehicle is standing upon a grade, turning the front of the front wheels toward the curb or side of the highway. This section does not apply to authorized emergency vehicles. (Added 1971, No. 258 (Adj. Sess.), § 3, eff. March 1, 1973.)

Burlington Code of Ordinances, Sec. 20-55
(e) No person shall leave idling for more than five (5) minutes any motor vehicle in any area of the city during the period from April 1 of every year to November 1 of the same year, except in the following instances:
(1) Motors used to run refrigeration units may be left idling to permit uninterrupted refrigeration;
(2) A motor vehicle may be left idling if necessary for the repair of that vehicle;
(3) This provision shall not apply to motor vehicles which must be kept idling in order to install, maintain or repair equipment or infrastructure.
(4) This provision shall not apply in any situation in which the health or safety of a driver or passenger requires the idling of the vehicle.

Massachusetts General Law ch. 90
§ 16A. Unnecessary Operation of Engine of Stopped Motor Vehicle Prohibited; Exceptions; Penalty. No person shall cause, suffer, allow or permit the unnecessary operation of the engine of a motor vehicle while said vehicle is stopped for a foreseeable period of time in excess of five minutes. This section shall not apply to (a) vehicles being serviced, provided that operation of the engine is essential to the proper repair thereof, or (b) vehicles engaged in the delivery or acceptance of goods, wares, or merchandise for which engine assisted power is necessary and substitute alternate means cannot be made available, or (c) vehicles engaged in an operation for which the engine power is necessary for an associate power need other than movement and substitute alternate power means cannot be made available provided that such operation does not cause or contribute to a condition of air pollution. Whoever violates any provision of this section shall be punished by a fine of not more than one hundred dollars for the first offense, nor more than five hundred dollars for each succeeding offense.