Because it contains relatively little entrapped air in the form of bubbles, black ice is transparent and thus very difficult to see (as compared to snow, frozen slush). In addition, it often is interleaved with wet road, which is identical in appearance. For this reason it is especially hazardous when driving or walking because it is both hard to see and unexpectedly slick.
Bridges and overpasses can be especially dangerous. Black ice forms first on bridges and overpasses because air can circulate both above and below the surface of the elevated roadway, causing the pavement temperature to drop more rapidly. This is often indicated with "Bridge May Be Icy" warning signs.
Black ice may form even when the ambient temperature is several degrees above the NTP freezing point of water (0°C) if the air warms suddenly after a prolonged cold spell that leaves the surface of the roadway well below the freezing point temperature.
The term black ice is sometimes used to describe any type of ice that forms on roadways, even when standing water on roads turns to ice as the temperature falls below freezing. However, this use of the term black ice is not included in the American Meteorological Society Glossary of Meteorology.
A similar hazardous condition can also occur when diesel fuel spills onto a road surface because the lighter fractions evaporate quickly to leave a greasy slick which is difficult for oncoming drivers to spot in time to prevent skidding.
In New England, "black ice" refers to a clear type of pond ice that forms in very cold weather. Black ice has the appearance of thick, slightly cracked glass laid on the water, and its transparency reveals the darkness of the pond beneath: hence the name. Black ice is very hard and smooth, making gliding easier, but stopping slower and more difficult.