New Hampshire law requires some people who are convicted of driving under the influence of alcohol (DUI) to have an ignition interlock device installed on their vehicles. The device requires the driver to give a breath sample before starting the car and at random intervals while driving. If the sample tests positive for alcohol, the car will not start or will not keep running. But how reliable are these devices at measuring whether a driver has been drinking?
According to the U.S. National highway Traffic Safety Administration (NHTSA), ignition interlock devices should accurately detect a driver’s blood alcohol concentration, or BAC, 90 percent of the time. This means that under ideal conditions, your ignition interlock device will accurately measure and record your BAC – or lack thereof – every 9 out of 10 times you blow into it.
Several different conditions can cause the device to get its measurements wrong, however. If your breath sample is too small, for instance, the machine may not be able to get an accurate reading. Likewise, if you have used mouthwash or medication containing alcohol, the machine might detect this on your breath and refuse to start. Waiting 15 to 20 minutes after using these products before starting your car will allow any alcohol in your mouth to dissipate, increasing the chances of an accurate reading.