In the two and a half years since Nokian Tyres first invited motorists to drive into its SnapSkan machine and take photos of their tyres, a million cars have been through SnapSkan units in Finland and Norway. What Nokian has found upon reaching the seven-digit milestone in May is that a tenth of all drivers are taking a risk by driving on tyres with insufficient tread depth. The issue of insufficient tread depth is more pronounced for summer tyres, the tyre maker adds.