It's a waiting game to see whether alcohol interlock devices in cars are effective, as the number of alcohol-related crash deaths rise.

More and more of the devices are being installed, with more than 4000 ordered by the courts in just over a year.

They became mandatory for serious and recidivist offenders on July 1st, last year.

Smart Start Interlocks director Gavin Foster told Mike Hosking data showing whether the devices are making a difference is a year away.

He says it's common for interlock programmes to need tweaking after they've started.

But Foster is confident it will reduce recidivist drink driving rates.

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