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Ignition Interlocks

The ignition interlock system is an ingenious and sophisticated piece of technology now becoming widespread throughout the United States which prevents drivers from using their vehicles unless they are able to demonstrate a BAC (blood alcohol concentration) below a certain amount. Typically, this amount is much less than the maximum BAC of 0.08%, which has become a uniform figure in all states for adult, non commercial drivers.

Where are ignition interlock machines compulsory?

The interlock is now compulsory in almost half the 50 states for anybody who has been convicted of a DUI or DWI charge and had a blood alcohol concentration of around 0.15% at the time – the actual percentage does vary somewhat from state to state. The system is almost entirely foolproof, in the sense that drivers cannot by-pass it and get somebody else to drive or refuse to take a breath test before using the car. It has been so successful, in fact, that more and more states are introducing the system

The ignition interlock is usually prescribed by a court when a driver has had a BAC substantially higher than the maximum permissible level, or has refused a blood alcohol test or has been charged for a repeat offense. Some states insist that an interlock mechanism is fitted even if it is a first offense.


In some situations, if a driver is still allowed to drive on a restricted license, usually because of compelling work reasons, then the ignition interlock will be fitted as standard anyway.

How does the ignition interlock system work?

The ignition interlock, as the name suggests, is a device that shuts off – or locks – the ignition system unless it has been proved that the driver has an admissibly low BAC or none at all. The driver must blow into a type of in car breathalyzer which is linked to the interlock before being able to start the car. The interlock system shuts down if the driver’s BAC is above 0.02 to 0.04%. The actual cut off point is determined partly by the state law on drunk driving and the actual driver’s original offense

If the test is not passed, then the interlock prevents the starter motor from being engaged. The machine is sophisticated enough to be able to recognize the driver it is supposed to be monitoring and will shut down if somebody else, e.g. a sober friend or spouse, uses it instead.

Don’t try and fool an ignition interlock device!

It will also shut down if it is tampered with in any way. Some newer machines will actually take a photo of the driver as he or she blows into the test device and all of them will record information about it every time the machine is used or abused. If the driver tries to fool or tamper with the machine it will usually advertise the attempt with horn blaring and lights flashing!

Ignition interlocks may be used voluntarily by some large companies, especially if they want to ensure their drivers are sober when they drive a company vehicle, but normally their use is mandated by a court after a DUI conviction

The cost of the machine rental or purchase as well as the installation and data transfer must be born by the driver and is not cheap. The rental costs can go up to $100 a month and the installation costs may be as high as $200.









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