Those tall fences beside snowy highways are not property fences; they catch blowing snow before it reaches the road, while plowing can cost 100 times more

Along many snowy highways, you might see a tall, slatted wall that looks like a farm fence. Its job isn't to mark anyone's land. According to the Federal Highway Administration (FHWA), it's a snow fence, built to stop blowing snow from reaching the road. The agency adds that research has found plowing snow costs 100 times more than trapping it with a fence.

Snow doesn't just fall; it moves

We tend to think snow just falls. However, wind moves it around a lot. Snow grains that are too heavy to stay suspended in the air bounce along the ground, according to Ronald D. Tabler in the Snow Fence Guide. Heavier ones roll or crawl. Trouble ensues once that snow hits a road. FHWA's technical brief notes that it can take crews hours to clear the drifts and open the road back up. Even if the road stays open, blowing snow can make travel dangerous, reducing visibility and even causing accidents.


A fence to break the wind

The snow fence does not block the snow as walls do. Instead, it slows down the wind. By doing this, the pressure exerted by the wind on the snow is reduced, and the moving snow grains stop. Most of the snow collects downwind of the fence. This creates a drift zone that keeps snow from blowing across the road. Design matters here as well. Per the guide, fences that are 40 to 50 percent open tend to work best, while solid fences don't collect snow efficiently. Moreover, Tabler recommends leaving a gap beneath the fence equal to about 10 to 12 percent of its overall height on flat ground.

Taller really is better

Height is a big factor. The taller the fence, the more effective it is, a point both FHWA and Tabler's guide make. Height is by far the most significant variable in fence design. That makes sense mathematically: a drift's cross-section grows roughly with the square of the fence's height, and deeper snow packs more densely, so an 8-foot fence stores 4.6 times as much snow as a 4-foot fence. That's why Tabler recommends a fence at least 8 feet high.

Distance is important too

The natural place you would think to place the fence would be next to the road. However, this is not the case. For level ground, the guide sets a minimum distance from fence to road of 35 times the fence's height. Thus, for an 8-foot fence, that works out to at least 280 feet. Tabler also says public rights-of-way are usually too narrow for effective fences, and easements on private land can be difficult to get, a challenge the guide devotes an entire chapter to.

Where the 100 times figure comes from

FHWA's brief says that research found that plowing costs 100 times more than trapping snow with a fence. Wyoming data in Tabler's guide backs this up. Snow storage with fences costs 3 cents a ton over the 25-year life of the fence, as opposed to $3 a ton to move the snow. The guide shares more results from Wyoming. In the 1970s, the state's Department of Transportation cut the costs of snow and ice removal by more than one-third on a 45-mile stretch of Interstate 80 where fencing had been installed. Accidents caused by poor visibility also fell by 70% along that road. However, it's worth keeping in mind that these figures are just from one state and decades old, so today's dollar amounts would differ.

Fences that grow, or get harvested

Not all snow fences are made of wood or plastic. According to the Minnesota Department of Transportation (MnDOT), its living snow fence program depends on farmers who leave standing corn rows, hay bales, or silage bags beside selected state highways. MnDOT applies the "living snow fence" label to all of these farm-based barriers, though the corn stalks, bales, and silage bags are temporary crop material rather than growing plants. Its permanent plantings use trees, shrubs, and grasses. During the winter of 2015-2016, the department paid an average of $4,800 per mile of protected highway. Tabler notes that it takes a few years for living fences to grow tall enough to catch snow.

The next time you see one

Those high fences are not there to keep people out. They are winter tools. However, they are not magic. Tabler's guide stresses that fences need to be properly designed and placed to work. It also notes a common mistake: not sizing a fence to hold the season's snow. Once a fence reaches capacity, it no longer affects blowing snow. So a well-planned arrangement of fences can really make winter travel safer. The next time you drive by one, it might be worth a second look to appreciate the work it quietly does.