Road traffic noise has been measured for 40 years. Have roads become quieter?
Mark Bakermans, DGMR, commissioned by RIVM

Ever since the Noise Pollution Act (1979) was enacted, there has been a need for a method to calculate road traffic noise. After more than 40 years, have roads actually become quieter?

NO, the roads haven’t gotten quieter. This is mainly because the number of passenger cars on Dutch roads has skyrocketed during that period (in 1980, there were a total of 4.2 million passenger cars, and as of January 1, 2022, there were 8.9 million). The question, then, is whether the cars themselves have actually become quieter?

To find that out, we first need to take a closer look at a car's noise emissions.

For an average (gasoline-powered) passenger car, there are two sources of noise: the engine, transmission, and exhaust system, and the tires. At higher speeds, tire noise becomes more dominant than engine noise. After all, shifting up lowers the engine RPM, but increases the rotational speed of the tires.

The noise measurements conducted in the 1980s were repeated in 2021. To do so, locations were selected that had the same test characteristics as those used at the time: the type of road, the type of asphalt, the average traffic volume, and suitability for measurement.

In total, noise emissions from approximately 2,000 passenger cars and 800 heavy and medium-duty vehicles were measured at 14 locations in the Netherlands. These results can be plotted against vehicle speed. When this data is plotted on a graph alongside the results from 1980 (fig.), two things stand out.

Noise levels have decreased at lower speeds, while they have actually increased at higher speeds. On balance, therefore, noise levels have remained the same!

One possible explanation for the reduction in noise at lower speeds is improved engine technology. More powerful engines produce less noise at lower speeds, and tire noise is not a determining factor in this case.

That also explains the increase in noise at higher speeds. At those speeds, the tires play a greater role in noise production, and they are significantly wider than the tires of 40 years ago.

It might be interesting to repeat such a measurement using only electric cars. Given the above, this would mean that the influence of engine noise would be eliminated because only the tires would be producing noise. Tires produce less noise at lower speeds, but electric cars are much heavier, requiring wider tires that consequently produce more noise. On balance, the noise level will likely decrease only slightly. At higher speeds, noise levels will increase more rapidly, so the curve is expected to become steeper.

Source: https://dgmr.nl/kennis/publicaties/is-de-auto-stiller-geworden/

Mark Bakermans is a senior acoustics consultant at the consulting firm DGMR.