1. Why Roads That Freeze at Night Are Salted Before Ice Forms
In many regions, temperatures rise during the day, leaving water on the roads, which then freezes from night until dawn. In areas where this happens often, pre-treatment—spreading salt before freezing occurs—is commonly used [3]. The National Institute for Land and Infrastructure Management (NILIM) classifies spreading into two types: "pre-treatment," done before freezing, and "post-treatment," done after ice or snow accumulates [1]. Pre-treatment aims to lower the freezing point of water on the road surface, a process called freezing point depression. Post-treatment aims to melt existing ice or snow [1]. Although the same salt is used, the expected effect differs. So, what is the salt doing on the road surface before it freezes?
2. How Salt Lowers the Freezing Point of Water on Roads
Saltwater has a lower freezing point than pure water, and the higher the concentration, the lower the freezing point. However, there is a limit. For sodium chloride (common salt), the maximum concentration is 23.3%, beyond which no further change occurs [2]. This 23.3% is an upper limit; road water does not always reach this concentration. According to NILIM, if water is present on the road and no salt is spread, the water freezes completely at 0°C, causing tire friction to drop extremely [2]. If the road water becomes sufficiently concentrated saltwater, it is easier to prevent it from freezing all at once, maintaining friction similar to that of a wet road [2]. For example, a 10% saltwater solution does not freeze completely even when cooled to its lowest possible temperature (the eutectic point). Before freezing, it is enough to turn the road water into saltwater of a certain concentration. But what is different after it has already frozen?
3. Why Spreading Salt After Ice Has Formed Is Less Efficient
Once ice has formed, salt works to melt it. However, the amount of ice that can be melted depends heavily on the temperature. NILIM data shows that 1 kg of salt can melt 4.3 kg of ice in one hour at -3°C, but only 1.6 kg at -12°C [2]. At -12°C, even after one hour, only about twice the amount of ice as the salt spread can be melted [2]. A comparison at the test road in Tomakomai by the Cold Regions Research Institute found that pre-treatment generally results in higher slip friction coefficients and slip resistance values than post-treatment, suggesting pre-treatment is superior for preventing road freezing. However, the number of test days was limited [4]. A local survey in Shikoku (February–March of Heisei 26) also reported that frozen road surfaces appeared when pre-treatment was not done [7]. If we should spread salt before freezing, how much should we spread?
4. How Many Grams of Salt Should Be Spread per Square Meter
The Ministry of Land, Infrastructure, Transport and Tourism’s "Draft Standards for Maintenance and Management of Directly Managed National Highways" (March 2011) states that sodium chloride is the basic material, and the spreading amount should be around 20g per square meter, determined according to the conditions of the target section [7]. Numbers vary in other documents. In a 2022 test by the Cold Regions Research Institute in Tomakomai, 30g/m² was spread on artificially created ice films, assuming post-treatment. On the other hand, the standard amount for pre-treatment in the Hokuriku region is 20g/m² for dry salt only, and 15g/m² for wet method (mixing dry salt with brine), as cited in a Fukui Prefecture report [6]. One cannot simply say pre-treatment uses less and post-treatment uses more just by looking at the numbers, because standards vary by agency and conditions. So, are the set amounts actually spread as planned on site?
5. How Nagano Found Nights When Spreading Salt Was Not Needed
The Nagano National Highway Office used 20g/m² of sodium chloride as a baseline, adjusting amounts up or down based on workers' experience and intuition. Annual spreading was about 3,300 tons, costing about 50 million yen [8]. They then measured the salt concentration on the road surface. On one day, the road temperature was between -4.3°C and -0.1°C, and the salt concentration was between 3.25% and 18.75%. This state basically did not require spreading. Yet, workers spread 5–30g/m² intermittently simply because the temperature was below 0°C [8]. This is less about worker responsibility and more about the fact that intuitive judgments were not made visible. The report states that if these intuitive parts could be "visualized," spreading amounts could be reduced [8]. What other methods are there to reduce salt?
6. Other Ways to Reduce the Amount of Road Salt Used
The Hokuriku Technical Office’s draft guidelines reflect that in the past, extra salt was often spread for safety. The draft guidelines estimate the duration of salt effectiveness based on temperature, weather forecasts, and traffic conditions, aiming to spread before freezing if predicted. Standard amounts are 15g, 25g, and 35g per square meter depending on temperature ranges. However, 25g and 35g were determined by calculation, while only the wet method at 15g/m² was verified on site. This verification ensured salt concentration was maintained and daily spreading amounts decreased overall [9]. Wet spreading is said to be more effective and longer-lasting because the salt sticks to the road and is less likely to be blown away by wind or passing cars [6]. In a 2023 test by the Cold Regions Research Institute, wet salt, brine, and dry salt were compared at the same amount (15.4g/m²). On days when the friction coefficient without salt was below 0.3, sections with wet salt maintained a coefficient of 0.5 or higher [5]. Another reason for increased spreading is salt loss due to scattering or dragging. Spreading from 100m before the target area can maintain salt concentration in the target area through a dragging effect [9]. Why has research into reduction progressed? According to a paper by the Cold Regions Research Institute, spreading amounts increased after studded tires were banned in 1991. Chlorides negatively affect the durability of steel and concrete in bridges and roads [5]. SAFETY NOTE: The Hokuriku Technical Office guidelines emphasize estimating the duration of effectiveness based on temperature, weather forecasts, and traffic conditions. If freezing is a risk, they aim to spread before it occurs. Determine the time for re-spreading by referencing the time elapsed since the last spreading and the estimated duration of effectiveness.
7. How You Can Test Salt and Freezing at Home
For an experiment you can do at home, prepare three cups with the same amount of water. Make one with just water, one with 1 teaspoon of salt dissolved, and one with 2 tablespoons of salt dissolved. Put them in the freezer overnight and compare how much they have frozen in the morning. This shows the difference in freezing resistance based on concentration. When you go out, observe whether the road is wet or dry on a cold morning from the sidewalk. Do not go into areas where cars pass. For official information, local offices of the Ministry of Land, Infrastructure, Transport and Tourism and your prefecture’s road management pages have explanations about spreading "anti-freezing agents." Reports from the National Institute for Land and Infrastructure Management and the Cold Regions Research Institute are also publicly available.