New Hampshire's Ignition Interlock Program requires any person whose license has been revoked or suspended for an aggravated DWI offense, and those convicted of a second or subsequent DWI offense, to install an Ignition Interlock Device (IID) in any vehicle registered or used by the offender for a period of 12 months - 2 years. An IID is a handheld breath testing device that attaches to the ignition system of a vehicle. The device prevents the vehicle's ignition from starting unless a sober breath sample (typically under .02) is first provided. Random additional breath samples will be required while the car is in motion to prevent a friend from starting the vehicle.

Persons required to install an IID are prohibited from driving a vehicle that is not equipped with the device. It is illegal to tamper or attempt to circumvent an IID device. Breath samples above the allowed limit will be recorded and communicated to the proper authorities.