Is the 'tread alert' message finally getting through? According to a new survey it is - in one part of the country at least. The 2004 North West England Tyre Safety Campaign registered a four per cent fall in the number of cars with rubber below the legal minimum tread depth of 1.6mm. Only 12.2 per cent of vehicles tested by police during the month-long initiative were deemed illegal, down from 16.6 per cent the previous year.