The rate of recycling has fallen steadily since 2013, checking in at 87.9% in 2015 and 81.4% in 2017, the association reported.
While there are more used tires to handle now than ever, that source of rubber is not ending up in illegal dumps. That's not always been the case.
Instead, tires are being directed to landfills when there is no recycling demand.
"We have not seen a corresponding increase in illegal stockpiles. In fact, stockpiles continue to come down," John Sheerin, USTMA's director of end-of-life tire programs, said.
The U.S. Environmental Protection Agency (EPA) breaks down municipal solid waste into a variety of categories to report on generation and recycling rates. That includes one category that includes both rubber and leather.
The latest EPA statistics, from 2018, show 9.2 million tons of rubber and leather was generated in municipal solid waste. That's 3.1% of the total waste stream.
Paper is essentially dissolved in a slurry as part of the recycling process that recaptures fibers and reconstitutes them into new products. Metals are melted down for reuse, an energy-intensive effort that's rather straightforward.
Plastics are commonly cleaned, melted and sometimes purified, in preparation for reuse.
But rubber, because it undergoes vulcanization, is especially challenging to recycle compared with its material cousins.
Tires are commonly shredded into small pieces at room temperature or frozen at temperatures of 100 degrees below zero or lower before they are broken into small pieces.
These are processes to simply break down the size of the tires, and because rubber maintains its physical properties there are fewer outlets for reuse. Crumb rubber, called that because the material is reduced to a small size, has found popularity for use on fields as a cushioning for athletes as they compete. Other applications include playgrounds and landscape projects.
Recycled rubber also can be made into products such as mats and flooring and be mixed in with asphalt in a process that backers say improves the life and performance of roads compared with traditional asphalt. Incineration also plays a role, commonly used as kiln fuel in cement factories.
And then there is pyrolysis, a process that super heats, but does not burn, materials in the absence of oxygen to break them down into reusable constituents such as oil, synthetic natural gas and carbon black char.
It's the application of crumb rubber in asphalt that's at the heart of a collaborative research project involving the USTMA and The Ray, a Georgia-based non-profit that calls itself "a proving ground for the evolving ideas and technologies that will transform the transportation infrastructure of the future."
Research on using ground tire rubber in asphalt will create what the collaborators are calling a "state of knowledge report" that will feature existing research but also identify gaps in data for the use of the material in rubber-modified asphalt.
Rubber-modified asphalt creates long-lasting roads that rut less than traditional asphalt roads, its proponents claim. Roads using recycled tires also are quieter and feature better vehicle grip and less spray in wet weather, a University of Arizona study indicates. These roads also reduce tire and road wear particles by half.
Including used tire material in roads allows asphalt to be recycled repeatedly.
"Recycling scrap tires to create rubber-modified asphalt appears to be a cost-effective way to reduce tire and road wear particles and advance the circular economy," USTMA CEO Anne Forristall Luke said.
"This study will allow us to share what we know about the technology and what additional research is needed to build more sustainable roads and infrastructure."
The Ray is an offshoot of the Ray C. Anderson Foundation. Mr. Anderson was the founder of Interface Inc., a carpet company. He created a name for himself by promoting environmentalism as part of his business strategy. Research will include laboratory and field data, including performance and lifecycle information of crumb rubber.
Established in 2014, The Ray has helped develop an 18-mile "living laboratory" along I-85 in western Georgia that incorporates sensors and the Internet of Things to collect data on a various aspects of transportation, with a goal of advancing safety and sustainability.
Among The Ray's resources is a tire-safety-monitoring system located at the Georgia Visitor Information Center near the Georgia-Alabama border. This WheelRight system can monitor tire inflation and tread depth and can notify drivers via text if it determines their vehicle has a potential problem.
The section of road was paved in 2018 with rubber-modified asphalt.
Asphalt contractors and mix designers also will be involved to help create a better understanding of the challenges involved in the wider adoption of crumb rubber in asphalt. They also will help with data collection and analysis.
The USTMA and The Ray plan to publish results of the work in 2021.