When you take out any car insurance policy, whether it has a black box or not, you’ll be asked to estimate your annual mileage. Driving less means lower premiums, while motorists who log lots of miles will pay more.
Technically, you’re not supposed to exceed your mileage estimate. Insurers will give you some wiggle room - unexpected trips and commuting changes happen - but wildly overshooting it can invalidate your insurance. That’s why insurers ask you to update them if your estimate turns out to be too low. They may then push up your premiums.
With black box policies, insurers have more immediate access to your mileage. Miles driven is one of the factors the telematics device records and factors into your driving score and premiums. Insurers can therefore see if you’ve exceeded your estimated annual mileage or if you’re on track to do so.
But they don’t use this information to fine you or prohibit you from driving. Rather, they’ll just adjust your annual mileage, which will push up your premiums.
They might do this if your mileage across a couple of months puts you on track to overshoot your estimate. Some insurers will have you monitor your own mileage and top-up your policy with additional miles, much like you would with a pay-as-you-go mobile phone.
Conversely, if your black box reveals you’re driving less than you estimated at the start of your policy, your insurer may discount your premiums.