What's IRI Got to Do With It?

Improving Roughness to Meet FHWA Criteria

Sometimes, it’s easy to tell how rough a road is -- At least that’s what some of the driving public believes. But just taking a drive isn’t always the most accurate measurement of roughness. 

The International Roughness Index (IRI) is one of three metrics used to measure overall roadway health by the Federal Highway Administration. It’s not just a way to keep track of smooth rides – it can dictate federal funding for future projects. 

Measured by electronic vans equipped with lasers and a number of other instruments, IRI is scored based on the rideability of a specific road. The measurement is based on the concept of a 'golden car'  with known suspension properties.  IRI is then calculated by simulating the response of this 'golden car' to the road profile. After the data is collected, scores are delivered to yield a roughness index with units of slope (in/mi, m/km, etc.). For the Federal Highway Administration, IRI scores lower than a 95 usually meet criteria for funding. 

So, keeping track of your IRI is important. There are many ways to improve IRI, from rehabilitation treatments such as milling & micro milling to well-timed non-structural treatments.


Milling & Micro Milling

Milling and Micro Milling are commonly used tools to manage roughness. Over the years, milling equipment has been perfected to increase smoothness for new pavement within micrometers of spec.  By combining milling or micro milling with overlays, for example, road owners are removing most of the bumps that contribute to general roughness. 

These treatments weren’t always used in this way, though. Milling treatments used to be seen as a method to simply remove old pavement to make room for the new. With highly engineered drums and equipment, agencies are starting to use more specialized treatments to satisfy IRI needs. 

For Tom Chastain, an equipment manufacturer, the secret to successful smoothness is in the speed of the miller. 

“For so long, [the industry] milled poorly. There were a lot of production-oriented jobs. We lost focus on the quality of patterns we’re leaving behind,” says Chastain. “If we go slower, you can watch the quality go up.”

Milling & Micro Milling are not just used to make room for “tons of asphalt” anymore. It’s an important tool to keep roads in good shape and a cost-effective route to rehabilitate your pavement. 


Non-Structural Treatments

Although they haven’t been traditionally viewed as ways to smooth things out, surface treatments are also proving more effective than ever for IRI scores. 

Jerry Geib, at MNDOT partners with NCAT to evaluate Minnesota roads. After nearly three years, MNDOT and NCAT found, not only can non-structural treatments improve your IRI score in the short-term, they can have a long lasting impact on sustained IRI in the future.  

After implementing non-structural treatments, such as micro surfacing, slurry seal, scrub seal, chip seal, and milling combined with thin overlay, Geib has seen immediate and sustained success. All of the road sections measured post-treatment on the NCAT test sections exceed FHWA IRI criteria for success. 

“We’re getting the benefits of both improved ride and life extension-- meaning we’ve pushed off the cost of that more expensive fix,” said Jerry Geib, “It is now more years down the road.” 

These thin, non-structural treatments quickly improve IRI post-construction and maintain that level of IRI throughout the lifecycle of the treatment. 

“Until the underlying pavement deteriorates, we expect that [IRI] score to continue,” said Geib. 

When it comes to creating and maintaining better roads, that smooth ride is something that everyone can agree on. But it’s not just good for members of the community; A smoother ride usually means a better IRI score, which can help agencies maintain the funding they need to make the whole road network last for the next generation. 


Sometimes embracing new thinking is key to keeping pavement preserved for the miles ahead.