Black ice forms when moist air or rain freezes on frozen roads, it also forms at -18C and lower when car exhaust condenses and freezes and when road de-icer stops working.

High traffic areas are really dangerous in very cold weather as even the roads that look clear could be more suitable for shinny than for walking or riding.

Studded tyres and good riding skills solve most of that, riding through packed ice is a different beast as then you need more off camber studs to let your tyres crawl up and down and provide extra lateral stability and braking.