According to the Maryland Transportation Article 22-402, (3)
A motor vehicle engine may not be allowed to operate for more than 5 consecutive minutes when the vehicle is not in motion, except as follows:
(i) When a vehicle is forced to remain motionless because of traffic conditions or mechanical difficulties over which the operator has no control;
(ii) When it is necessary to operate heating and cooling or auxiliary equipment installed on the vehicle;
(iii) To bring the vehicle to the manufacturer's recommended operating temperature; or
(iv) When it is necessary to accomplish the intended use of the vehicle.
To report a suspected violation of this law, please contact the Montgomery County Police Non-emergency number at 301-279-8000.