Automobiles leave mountains of used tires: Two billion tires have accumulated in piles across the United States alone. Researchers at the Instituto de Acstica in Madrid, Spain, are using this waste to ameliorate another environmental impact of the automobile by packing crumbs of rubber from discarded tires into highway sound barriers.

Rubber turns out to have a broad sound absorption spectrum well-suited to traffic noise, and the tire crumbs stand up better to rain and dust than the glass and rock wool used in traditional sound barriers. Reverberation room tests have shown that the smaller the rubber crumbs are, the better they absorb sound. In June, the Madrid researchers tested a full-scale prototype barrier made with the recycled rubber.