Road Ice: A Basic Primer

Roadway ice is perhaps one of the most serious road conditions faced by many states, with road frost reaching even more temperate places like Florida from time to time. Sadly, every year, deaths and injuries related to road ice are in the hundreds of thousands, with a little over half a million crashes every year. In fact, icy pavements or icy roads account for 3% of vehicle crashes, or around 150,000 crashes, 2% of crash injuries, or 38,770 related injuries, and 2% of crash fatalities, or 559 people passing away due to road ice.

Road ice is sometimes called ‘black ice’, thanks to the optical illusion that makes it look black against the concrete. This makes it very dangerous for drivers both in the morning and at night, as the ice looks almost invisible. This is because road ice is often made of clear frozen water that makes it seem like it’s not there. Light hitting it at the right angle will make it sparkle, but an overcast day or under cover at night, many people won’t realize it’s there until it’s too late.

Some Good News, Though

The formation of road ice happens under meteorological conditions that are not only very well understood, they’re also often very predictable. It happens enough times across most communities that people have figured out ways to protect themselves and others against the dangers of road ice, and understand a bit more how road ice actually forms. Some general things to keep in mind:

  • Road ice can often appear when the temperature is in and around 35-40F, with clear or almost-clear skies.
  • Valleys and low areas are more susceptible to road ice, as cold air tends to gather in those places.
  • Heavy icing is a distinct possibility in areas that are experiencing heavy fog or are experiencing near-freezing temperatures

What Causes Icy Roads?

Ice needs two things for it to form: freezing temperatures and moisture. Our country’s climate varies between states, but cold snaps can happen most everywhere in the continental United States (lucky for Hawaii!), which can quickly bring the ambient temperature down below 32F, the freezing point.

Once the temperature is nice and chilly, all it needs is a bit of moisture on the road and you get road ice. This moisture can come from a wide range of sources:

  • Condensation that forms when slightly warmer fog passes over a cold road.
  • Groundwater that seeps through to the surface.
  • Freezing rain.
  • Melting snow

And many more. Take note, as well, that these icing factors can occur one at a time, or all at the same time, so long as freezing temperatures are present.

When is Road Ice Most Dangerous?

In general, ice will be most slippery when the temperature is near freezing. This means that ice must already be present on the road and the temperature has just started rising above 32F. It might seem counter-intuitive, but the colder the temperature, the less slippery ice becomes.

Why is this?

When temperatures reach below freezing, liquid water turns into solid ice. So long as temperatures remain constantly below freezing, this ice will stay in its solid form. Ice is more slippery than, say, concrete, because the former has more ‘loose’ water molecules on its surface that prevent other matter from adhering to it completely, like shoes or tires. However, the colder it is, the less ‘loose’ water molecules are present.

As the temperature goes up, the solid ice slowly starts melting, creating a very thin, almost-frozen layer of water on the surface, along with more of those ‘loose’ water molecules, creating a surface that is extremely slippery and a very dangerous road hazard.

What Dangers are Posed by Road Ice?

Road ice is the time when roads are most slippery. First and foremost, road ice can cause skidding. As mentioned earlier, road ice is often referred to as black ice, because it forms on black concrete without any kind of visual aids to identify it, making it practically invisible. When this happens, drivers will assume that the road is completely safe and fail to slow down. When their vehicle hits a patch of road ice, tires might slip, making the vehicle unstable. If the driver reacts poorly and turns the wheel, they can send their car into a tailspin, increasing their risk of injury and increasing the risk of causing an accident on the road.

But even if they react accordingly, road ice, as well as general freezing conditions, actually increase stopping distance by a factor of 10. Stopping distance is actually a solid, mathematical formula that measures the amount of time it takes for your vehicle to go into a complete stop as soon as you perceive a hazard on the road. The formula is:

Thinking distance + Braking distance = Stopping distance

Thinking Distance refers to the distance your car will travel for between you perceiving a hazard and you stepping on the brakes. Meanwhile, Braking Distance refers to the time it takes your car to stop as soon as you hit the brake pedal. This means that, on ice, your formula will have to be:

Thinking distance + Braking distance = Stopping Distance

Stopping Distance x 10 = Total Stopping Distance on Ice

For example, if you’re going at 50mph, with a Thinking Distance of 49 feet and a Braking Distance of 124 feet meters, then your normal Stopping Distance will be 173 feet. On ice, however, this is multiplied by 10, which means that your total Stopping Distance is a whopping 1,730 feet, or, one-third of a mile. To put it in perspective, that’s equivalent to almost 6 football fields from the time you perceive a danger and step on the brakes before coming to a complete stop.



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