It can also form when the temperature of the pavement or a bridge deck is the same as the dew point, said Jennifer Post, a spokeswoman for the New York State Department of Transportation.
“At freezing temperatures, dew will freeze, causing black ice,” she wrote in an email. “This can happen any time, but seems to be more prevalent at night, when there’s no solar heating.”
How do you know if it’s there?
Pavement with black ice will be slightly darker and duller than the rest of the road surface.
If the road is glistening and the tires of the car in front of you are spraying water, you have little to fear, but if the road looks wet and there’s no spray, beware, said Robert Sinclair Jr., a spokesman for AAA Northeast in Garden City, N.Y.
Drivers should pay close attention to the thermometer on their car, which measures the ambient temperature of the air, said Sam Schwartz, who was the traffic commissioner for New York City from 1982-86.
“The warning lights in your head should go off when you are at 34 degrees,” said Mr. Schwartz, who is also known as Gridlock Sam.
Vehicle thermometers, which were once subject to inaccurate readings depending on the placement of sensors, have improved over the years, he added.
Which areas are most susceptible?
Roads in heavily populated and traveled areas are less likely to have black ice compared with those in rural or suburban areas, Mr. Schwartz said.
He warned to be especially cautious on bridges, viaducts and overpasses because they are exposed on the top and bottom, allowing them to cool faster and create conditions conducive to black ice.
Black ice can also form at the bottoms of hills and in areas that are heavily shaded.
The State Department of Transportation relies on roadway sensors to help monitor conditions, Ms. Post wrote. Anti-icing materials, such as salt brine or salt, can be used to treat trouble spots in advance.
Here’s how to react if you get caught in black ice.
“You hit a patch of ice on a turn, that’s it,” Mr. Sinclair said. “It’s going to take some very capable driving skills, high-tech vehicular performance and nifty tires to avoid something bad happening.”
If your car does begin to slide, keep your foot off the gas, Mr. Schwartz said. Do not hit the brakes or turn the steering wheel, because that can only make matters worse. Drivers should also not be lulled into a false sense of security because their cars have anti-lock brakes, he added.
If you don’t want to end up like drivers in black ice car crash videos on the internet, the best thing to do is take it slow.
Mr. Schwartz warned against trying to pass a poky driver ahead of you. Accelerating in uncertain winter road conditions will reduce the coefficient of friction and could send your car spinning out of control.
“Just be patient,” he said. “You may think it’s a pain to be behind a slow driver, but just take it slow and you’ll get there alive.”