In New Hampshire if you are found guilty/plead guilty to aggravated DWI, or subsequent offense dwi, the interlock is mandatory. It is discretionary in other cases.
"About 1.2 million Americans were arrested for driving while under the influence in 2011, and drunk drivers cause roughly a third of the fatal accidents on U.S. roads every year."
All states have laws requiring some convicted drunk drivers to use ignition interlock devices, and in 14 states, you have to blow to drive after your first offense — and in some states, can be required even after an arrest that doesn't end in a DUI conviction. Such systems not only keep cars from starting if they sense alcohol, but can keep track of a driver's tests and be customized by authorities for different levels of warnings.
The devices don't just kick in at start-up: In most states, the systems force "rolling re-tests" — ensuring the driver didn't wait until the car was started before boozing by randomly alerting that the driver must blow again while on the road, up to four times in every hour of driving. And some require "arrival" checks, which force the driver to wait a minute or two after shutting the car off; if the driver simply walks away, the device records a violation."
One mother posted to say the rolling retest caused her daughter to total her car:
On her way to counseling she was on the highway and the system required her to do a rolling test. Keep in mind to her left was a cement barrier and to her right major traffic. She has a huge fear of being in trouble so she does the test but the cord gets caught on her emergency brake, and dont forget you have to breath into he device for 6 seconds. In the process she got in an accident. Her car is history and her airbag did not deploy. She calls me screaming...........worst noise in the world. The police officer doesn't believe that they make them do rolling test so she shows him the paperwork. He declares the interlock system the cause of the accident.
"Device errors appear common, due to bad calibration or over-sensitivity; there's lots of talk about not using alcohol-based mouthwashes, hair gels or even eating breads made with yeast, and having to test without the vehicle's heat on so that antifreeze or gasoline fumes don't register as a false positive — which can, if not contested, lead to having a license revoked or extending the time the interlock's required to be installed."
As noted in one of the comments to the post, ironically you need the interlock for a DWI drug case as well.
"My son was busted for DUI for pot. He doesn't drink at all but the state of Washington requires one of those things installed anyways. He blows his smoke into it and it still lets him drive."