Slow speed – less dust

There’s a fierce debate in Trondheim, NTNU’s home, as to whether the speed limit in the centre of the city should be dropped from 50 to 30 kilometres per hour. The arguments for lowering the speed limit are many – better air quality is just one of them. But until now, there hasn’t been any concrete information about the effect that lower speeds have on the amount of fine dust on the roads.

NTNU researcher Brynhild Snilsberg has examined the occurrence of fine dust in the summer and winter from winter tyres, summer tyres and studded tyres – and has measured the amount of dust associated with different speeds.

Her results show that the amount of road dust from studded tyres is halved when speeds drop from 50 to 30 km/hour. The dust particles are also less finely ground.

Fast studs, fine dust

“In general, it turns out that the amount of dust that is produced and kicked up increases proportionally with the speed, so that the amount increases from about 2.5 milligrams per cubic metre of air at speeds of 30 km/hr, to a little over 5, at 50 km/hr”, says Snilsberg.

“Also, the particles are on the whole much smaller with higher speeds. The increased speed enables the studs to grind the dust more finely”, explains Snislberg.

“That’s a strong argument for reducing the speed limit in the city, particularly in the winter months”, says the researcher.

Stronger than expected

Snilsberg says she wasn’t surprised to find the trend. “But I didn’t think it would be so strong”, she says.

Roughly three of 10 automobiles in Trondheim are outfitted with studded tyres. Consequently, a halving of the amount of fine dust caused by studded tyres will have a considerable effect on the total amount of dust in the city centre. The national average for the use of studded tyres is 45 per cent.

The problem with road dust from studded tyres is increasing, as both the amount of traffic and the demand for ice- and snow-free roads increase. That means that roads in residential areas outside of the city centre and the more built-up areas will also be affected by this nuisance.

A need for better measurements

The dust in question is called PM 10, particulate matter that is 10 micrometres or less in diameter. The current measurement requirements, which are EU certified, are based exclusively on weight. That isn’t a very adequate standard, Snilsberg believes.

“If you have one particle that’s one milligram on the one hand, and a thousand fine particles that together weigh the same on the other, there’s no doubt as to which is more harmful to your health. But we don’t have any better alternative when it comes to measuring and monitoring air quality in Norwegian cities”, she says.

Snilsberg took her PhD at the Department of Geology and Mineral Resources Engineering at NTNU, and conducted her research at the Norwegian Public Roads Administration.

By Tore Oksholen/Gemini