With the first warm and sunny days of spring, the question once again arises: why are our city streets so dusty? Warmer temperatures and sunshine dry out the dirt and dust that have accumulated on the streets, and stronger gusts of wind whip it into the air. As a result, it ends up in the mouths and eyes of pedestrians, cyclists and users of light vehicles, causing frustration.

Outdoor air quality can be described by the concentration of solid and liquid particles in the air with a diameter of less than 10 micrometers (0.01 millimeters). This size is designated by the combination of letters and numbers PM10. The permissible 24-hour average concentration of PM10 in the air is up to 50 µg/m³.

In many parts of Estonia, those interested can assess local air quality through the monitoring network of the Estonian Environmental Research Center. In the context of Tallinn, the monitoring station on Liivalaia tänav is of particular interest — it has been measuring air quality since 2005. Looking at the PM10 indicator at the Liivalaia station from March 1 to March 25, the PM10 concentration exceeded the 50 µg/m³ threshold on ten days. This suggests the problem is very real.

If we also consider air quality monitoring data from previous years, a clear trend emerges. Air quality in Tallinn is at its worst in the spring. This means the largest amount of dust accumulates on the streets in the spring and during the preceding season — winter.

The origins of dust

According to the air monitoring website of the Estonian Environmental Research Center (EKUK), PM10 particles enter the outdoor air from road surfaces, combustion processes and other sources. Depending on how they are formed, these particles have different chemical compositions and affect human health in various ways. Fine particles in the range of 5-10 µg/m³ typically originate from pollution sources outside urban areas.

Although no known studies have been conducted in Estonia on the origins of street dust in the spring, a comprehensive study was carried out about a decade ago in our northern neighbor's capital, Helsinki. That study found that the majority of street dust came from road surface wear and from the sand or crushed stone used for traction control. Since vehicle fleets and road surface materials are similar on both sides of the Gulf of Finland, it is reasonable to assume that the composition of Tallinn's street dust is roughly comparable to what was found in Helsinki.

In Estonia, crushed stone is only used for traction control on sidewalks. If we subtract the 25 percent of dust attributed to crushed stone in the Finnish study and recalculate the remaining sources, we arrive at the conclusion that 64 percent of street dust comes from road surfaces.

Whether the share is 64 or 75 percent, in any case, the main source of dust is the road surface.

How do road surfaced turn into dust?

The answer is very simple: studded tires. According to the most recent study, slightly over 60 percent of passenger cars in Estonia use studded tires during the winter. Our road surfaces are primarily asphalt, which consists of about 95 percent crushed stone and 5 percent petroleum-based bitumen binder. Because the studs in studded tires are made of metal, the contact point between each stud and the road wears down the weaker material — asphalt — as the wheel turns. In this way, each stud grinds off a small amount of road surface, creating dust.

Researchers at the Swedish National Road and Transport Research Institute (VTI) published a road surface wear model in 2007 that has been validated with measurements taken on real, trafficked road segments. As in Finland, Estonia and Sweden have similar road surface compositions.

According to the model's calculations, a single passenger car traveling at city speeds wears down about three grams of road surface per kilometer. On highways, where speeds are higher, the amount of material worn from the surface roughly doubles — to about six grams per kilometer.

Estonia's road network stretches nearly 90,000 kilometers in total. Focusing only on the main highways, which span approximately 1,600 kilometers, the most recent data show that these roads see an average daily traffic volume of 5,878 vehicles. About 90 percent of those — 5,290 vehicles — are passenger cars, and of those, around 60 percent (or 3,174 cars) use studded tires.

This means that daily road surface wear on main highways alone amounts to
3,174 × 6 × 1,600 = 30,471,552 grams, or just over 30 metric tons. Given that studded tires are permitted in Estonia for 165 days each year (October 15 to March 31), the total annual wear exceeds 5,000 metric tons.

To make this more tangible, each user of studded tires can imagine, for instance, a drive from Tallinn to Tartu. Over that trip, a single passenger car equipped with studded tires grinds off roughly one kilogram of road surface.

How much does it all cost?

The share of vehicles using studded tires has a significant impact on how long the road surface layer lasts. On high-traffic road sections, the top layer may need to be replaced every five to seven years; on lower-traffic roads, less frequently.

On Tartu maantee in front of Ülemiste Center, one of the most wear-resistant types of synthetic aggregate has been used in the asphalt. However, since the aggregate and composition of the asphalt layer beneath the top coating are weaker, further wear becomes progressive.

If the share of studded tires were reduced by half — from 60 percent to 30 percent — the lifespan of the top layer would double. Understandably, the less frequently we have to replace it, the less of a burden it is on taxpayers' wallets in the long run.

The asphalt mix used for the top layer costs, on average, €75 per ton. This price refers to the mix at the plant and does not include transport or installation. Returning to the example of Estonia's main highways, based on this figure, every year about €375,000 worth of asphalt mix ends up as dust along the roadside
(5,000 tons × €75 = €375,000).

That number might not seem especially large on its own. However, one needs to understand road repair technologies. Especially on major highways like the Tallinn-Tartu route, resurfacing typically involves milling off the old top layer and replacing it with a new one.

To illustrate the cost, here's a simplified example. Road surfaces are usually designed to last 20 years. Milling off and replacing the top layer of a one-kilometer, two-lane (1+1) road costs around €100,000. Assuming the road between Tallinn and Tartu is a two-lane highway, the total resurfacing cost for that stretch would be 180 × €100,000 = €18 million.

Now, if that road section needs resurfacing every ten years, then over a 20-year period, an additional €18 million would be needed on top of the original construction cost. And realistically, that number would be even higher when accounting for inflation.

If studded tires weren't used at all, there would be no need to repair that surface due to wear before the 20-year mark. In other words, the use of studded tires generates an additional annual cost of €900,000 just for the Tallinn-Tartu highway section. Across Estonia's entire road network, studded tire-related repair costs amount to several million euros per year — possibly even exceeding €10 million annually.

The most recently built four-lane (2+2) highway section in Estonia cost around €3.5 million per kilometer. For €10 million, we could have built about three kilometers of new four-lane highway, eliminated numerous dangerous traffic spots or invested that money in something more useful.

That said, it's important to note that studded tires aren't all bad — they help roughen the road surface. This is especially beneficial on compacted snow or ice, but also on asphalt. It improves traction between the tire and the road. However, that effect can still be achieved with a lower share of vehicles using studded tires.

Solutions

It's clear that, given Estonia's conditions, a blanket ban on studded tires isn't a practical solution. It must be acknowledged that on lower-traffic roads, especially in rural areas where distances are longer and road maintenance standards are lower, the use of studded tires can be entirely justified due to a higher risk of icy conditions.

However, those who drive the majority of their mileage on well-maintained main roads or in urban areas should consider whether they truly need studded tires. If someone only occasionally drives on lower-standard roads, it is always possible to adjust one's speed to match the road conditions.

Nordic non-studded winter tires are actually very capable, and around 40 percent of Estonian drivers already manage perfectly well with them. Several scientific studies also emphasize that today's advanced driver assistance systems can largely compensate for the advantages that studded tires offer.

The Transport Administration and larger municipalities should certainly invest in more thorough public outreach on this issue — Tallinn University of Technology would gladly support such efforts. One positive example is the city of Tallinn's campaign, "Dust Kills," which encouraged drivers to choose non-studded winter meant for Nordic conditions. Unfortunately, its impact has not been very noticeable.

If larger cities want to reduce the use of studded tires, they can look to neighboring countries, where restrictions on certain streets have yielded good results. In such cases, however, an alternative route must remain accessible to vehicles with studded tires. If drivers want to take the more direct route, they must forgo studs.

Another option would be to implement a fee for entering a city with studded tires. Of course, such measures come with additional costs, since someone — or something — has to enforce the restrictions.

Beyond reducing dust, broader use of non-studded winter tires would also lower noise levels in cities. If you've never done it, stand near a road and you'll quickly notice which cars are using studded tires and which are not — studded tires are noticeably louder. Drivers would benefit, too, as non-studded tires transmit significantly less noise into the vehicle cabin compared to studded ones.

We recognize that a complete ban on studded tires is not realistic in Estonia. Still, the main goal of this article is to prompt drivers to consider whether they truly need to use studded tires in winter, or if non-studded tires might be sufficient. There is much to be gained from a higher share of non-studded tire use, as it reduces both dust and noise in our environment.

On top of that, more than €10 million of taxpayers' money is currently spent each year in Estonia on repairing road surface wear caused by studded tires. Every set of studded tires replaced with non-studded ones helps reduce that cost.

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