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How to Keep Roads Snow-Free Without Pickling the Environment

January 16, 2015

When on a snowy night we half wake from the rumble of the plow and see the yellow flashers reflected on the ceiling, we go back to sleep with the conviction that the mayor and the local public works crew are doing a great job keeping our streets clear.  The politicians know, if voters don't see snow and ice combated military style, their re-election is in jeopardy.
Salt in abundance



But the thing that should be keeping us awake is the ugly flip side of the battle for clear roadways. Recent news highlighted the increasing salinity of our rivers and streams, and points the finger at one chief culprit: the exponentially growing consumption of salt, sodium-chloride, to fight icy roads, a practice that started in the US quite a bit after the automobile, in 1938.

Imagine this: US road salt consumption jumped in 25 years from 9.5 million metric tons annually (1980) to 19.6 million tons in 2006 (in 2013: 22 million tons).  Knowing that one truck holds about 7 tons of salt, that is a fleet of over 3 million trucks that dumps its entire cargo into the environment.  That many salt trucks, lined up bumper to bumper, would stretch over 20,000 miles or more than 4/5 of the way around the earth. That is to say, 22 million tons is a lot of salt! 
The common and rather simple method of salt application
on roadways via tilted truck bed and spinner

Especially if one considers that that sodium chloride is a stable compound (or chloride a conservative ion) which doesn't dissolve but only dilutes.  In other words, each ton of salt brought out onto the roadways and ditches will stay in the environment forever. While road salt application is measured in pounds per lane mile, salt in the water is measured in milligrams per liter. Highway folk easily bring out 400-500 pounds of salt per road mile. Environmental agencies set the toxic threshold for salty water at around 600 mg/liter, but aquatic life is affected on anything north of 250mg (also the threshold for drinking water). Values found in many streams around the nation are much higher, and increase in winter, a clear indicator of the cause.
Increased salt usage 1972-2005 in Madison Wisc.


Just what we need, another environmental scare! Do we really have to worry about road salt in addition to the salt in our diet?

Environmentalists and those who watch and guard our water supplies certainly think so. Rivers, streams and roadside plants are just the first victims, shallow groundwater and wells are next. 

Time is money and ice free roads are a basic requirement for safety, so what can one do other than shrug and move on? Salty rivers, how bad can it be?  Just remember last year's Snowmaggedon in Atlanta (Jon Stewart on the event) and the estimated billion dollar loss from it, all because road crews had not been out there fighting the ice and snow.

Because salt used to be cheap and abundant, users were extremely wasteful. The sharp increase in usage can be attributed to three major factors: 
- Decreased tolerance for impeded mobility due to more vehicle miles traveled, just in time delivery models and a general acceleration of daily life. - Increased inclination to consider snow removal as a proxy for the effectiveness of local government.  - Continued sprawl presenting a larger and larger road network to be treated (imagine that Virginia DOT alone is responsible for 150,000 lane miles to treat, that all US roadways together are 4.1 million center line miles (17 times to the distance to the moon) plus about 13,500 lane miles of roadway a year (average between 2000 and 2012).
Before it is possible to see if this salt splurge could be curbed for a less salt rich diet it helps to understand a bit better what salt does and how it is applied to the roads today.
Salt is a freezing point suppressor that alters the phase
transitions of water (source: Salt Institute)


First, salt gets mined and even the product itself varies between countries. In Europe finer salt allows more accurate dosage and distribution, in the US chunky rock salt makes exact distribution much harder. Then the commodity gets transported via ships, trucks or rail to ports or shipping terminals before getting delivered to the salt stock-piles of highway administrations or local public works departments. We will skip here straight to the end user and address only practices that are directly in the control of those agencies that put the salt on the road.

The first big best practice element deals with how salt is stored and whether it is well-contained during storage.
Eventually, salt will get loaded into dump trucks which then fan out to treat the roads and streets. At this point practices and equipment vary widely: Typically, the more northerly a state, the better the practices. Does the truck drive into a covered and contained facility or does the salt get loaded out in a yard? Does the yard have containment facilities and how sloppy is the loading? Do the trucks have regular loading beds (those that can be tilted up to make the load slide back) or do they have special cone shaped salt containers which bring the load to a controlled discharge point by gravity? Or do they even have a mixer installed that mixes dry salt from the container with water to make sticky wet salt or a slurry?  Finally the spreader, is it a rotating disk (called a “spinner”) that turns at a constant speed or is it tied to the speed of the truck? Is it a simple disk or a device scientifically shaped to create a layered and effective spread, or is there no disc at all and the salt just dribbles off the edge of the loading bed? Does salt get recovered by sweeping or loading snow mounds? Is salt mixed with abrasives or other components designed to make ice and snow less slippery? Or does the agency use tankers with liquid brine? 
Salt truck with special salt holding container and  mixing
apparatus in back for wetting salt and speed sensitive
spreader technology


Yes, there exists research on the topic. There is even a Salt Institute with its own "Snowfighter's Handbook." One of the independent researchers is Wilfried Nixon, a professor of the University of Iowa who has written papers for the Iowa Highway Research Board and is kind of a salt guru advising state and local governments across the country how to use less salt. He spoke at the 2015 Transportation Research Board Conference in DC about how salt is really not melting the snow as much as depressing the freezing point, how the goal is to depress the bond between snow and ice and asphalt and in which situations the use of salt is completely useless. 

The good news is that there is lots of room for improvement on salt application practices before much effect would be noticeable regarding road safety. Salt usage could be brought down sharply while still having clear roads, stopping or even slowly reversing the increase in salinity of rivers.. The following are some of the possible measures, all reasonably simple to implement:

- Keep all salt storage covered so rain cannot dissolve salt and wash it away while being stored.  - Capture all run off around salt storage and loading yards and filter it. Recycle captured salt.  - Do not salt at temperatures when it is too cold to be effective, i.e. below 5F (-15C) pavement temperature (not air temp!).  - Pre-wet the salt so it sticks and doesn't bounce as much on the road surface - Use up to date spreader technology that is efficient and speed correlated. Tilting the truck bed back and letting the salt run out doesn't not qualify as an efficient spreading method.  - Adjust the pounds per lane mile (or grams per square meter) to what road conditions and snow fall suggest using the guide tables which provide a range from 100 to 500 pounds per mile.  - Change to liquid brine application. This probably is the least wasteful chloride application method. 
Brine application prior to snowfall as a pre-emptive method 
 This blog usually investigates cities, transportation and the environment. It turns out, salt, its use on streets, and its presence in freshwater as well as its impact on animals and vegetation is related to all of these topics. Ironically, the cost of road salt is not even limited to the environment but also includes the accelerated deterioration of our infrastructure, roads and bridges, and destructive corrosion on utilities and equipment. Maybe most ironically, it turns the very cars it is supposed to protect from crashing into rust piles.  Road salt and its cost burden in environmental and financial terms is one of the ugly sisters of our automotive mobility society which we like to hide in the closet. 

Needless to say, not all mobility requires salt, certainly not rail transport and even aviation is rather harmless in terms of salt with its well-controlled settings. Certainly there is enough cost to seriously think about best practices and alternatives in cities, in the suburbs and in the open landscape.

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