Metro Phoenix freeways will be getting a new treatment in an effort to increase their longevity and decrease road noise.
It’s called diamond grinding, and regional transportation officials have been testing it on a few spots across the Valley over the past few years. They decided in 2023 that it would be the new plan going forward regionwide.
There are around 100 miles of freeway that already have this. Arminta Syed says the goal is to eventually have it on every mile of freeway in Maricopa County.
Syed is transportation policy and initiatives manager for the Maricopa Association of Governments. She joined The Show to talk about the project.
Full conversation
ARMINTA SYED: Yes. Diamond grinding is actually a treatment that’s given to roadway surfaces. So you have a concrete layer and then diamond grinding is when you take a big machine that’s got these, what they call diamond blades, and it implements these thin grooves into the roadway surface that run in the same direction as the tires. And so that’s all diamond grinding really is.
MARK BRODIE: So you’re taking something and going over the existing surface as opposed to putting a new surface on the roadway?
ARMINTA SYED: That’s exactly correct. That’s exactly correct.
MARK BRODIE: And what does that do? What are the benefits of doing that?
ARMINTA SYED: A few different things. So, one of the biggest things, of course, is that it helps alleviate a lot of traffic noise. So, a little bit of background, traffic noise, the way it work — what really drives it is that interaction between car tires when they hit the pavement.
And so what diamond grinding does, those thin grooves, is it takes those sound waves that are created at the tire-pavement interface and it basically kind of absorbs them so it doesn’t travel out and people can hear it.
MARK BRODIE: Is that assuming that the tires are in those grooves?
ARMINTA SYED: It is not, but it is assuming that basically as the sound waves travel out, they get captured by those thin grooves. Yes.
MARK BRODIE: OK. And what are some of the other benefits of doing that?
ARMINTA SYED: Sure, in addition, you always have to make sure that, you know, safety, friction, rideability, all of that is always taken into account. So it really helps maintain that. It provides great friction even under, you know, monsoon rainy conditions. It provides a smooth ride. And in addition, also continues to help lower that traffic noise.
MARK BRODIE: One of the things that’s so interesting about this is that when rubberized asphalt began to be used on the freeways, one of the big benefits was that it’s so much quieter than everything else. It’s interesting that now the conventional wisdom seems to be that this diamond grinding is maybe even more quiet and has other benefits over something like rubberized asphalt.
ARMINTA SYED: Yeah, it’s, it’s fascinating because — so to go back to even before rubberized asphalt overlay was on all of our freeways, what happened was in the late '90s, early 2000s, that’s when much of today’s freeways that you see here in our region came online.
They were open to traffic and at the same time, we were seeing a ton of population growth, kind of like we have been for the past few years. So all of a sudden you had all these new roads and a ... substantial increase in the amount of cars, and there was a noise crisis everywhere.
And so what happened was, at the time, ADOT, Arizona Department of Transportation, was constructing these freeways. They were concrete-based freeways and all of these major noise concerns were traveling up to the governor at the time. And so the governor, in response, said, "OK, I mandate that all of our freeways will just be covered with this layer of rubberized asphalt overlay."
And there was dedicated funding set aside for that overlay to be originally installed. But what there wasn’t was funding set aside to maintain that overlay or dedicated funding set aside for replacing that overlay when it got into a bad enough state that you needed to replace it.
Rubberized asphalt overlay was really, really great when it was originally installed and that’s still — that’s still correct today. But it very, very quickly starts to deteriorate. And because there wasn’t dedicated funding set aside for it, what you had was all across our region, you had that original overlay in failing conditions.
So what you have with diamond grinding is something that produces a lot of the similar benefits that rubberized asphalt did, but it’s also more durable than rubberized asphalt. And that is really where you have the benefits of diamond grinding way outshine the benefits of rubberized asphalt. There’s a lot less maintenance involved, it lasts a lot longer, it’s a lot cheaper. So from that perspective, it is worlds above rubberized asphalt.
MARK BRODIE: So let me ask you about the cost of this. How does it compare? And you mentioned that it’s more durable. Is it the kind of thing that you do it once and then ... does it last forever? Do we know?
ARMINTA SYED: Yeah, so diamond grinding is new to our region, so we’ll find out. But our elected officials who made this decision, they very much specified that when we’re doing this analysis, looking at how does diamond grinding compare against rubberized asphalt overlay, you need to do that comparison analysis over what they call the life cycle.
And what we found was, just looking at our peers, other states, what they’re doing, diamond grinding will at minimum last 20, 25 years if not 30. Rubberized asphalt, on average, would last about 10 years before you have to fully replace it. And even way before that, it starts to have those failing conditions, those potholes, those grooves, etc.
MARK BRODIE: So there are other places that are already using this, yeah?
ARMINTA SYED: Yeah. We’re actually very unique in our region in that we had a brand new concrete pavement and then we put brand new rubberized asphalt overlay on top of it. Most places that have that concrete pavement will do a similar type of treatment as diamond grinding where they’re installing grooves to help with rideability and noise.
MARK BRODIE: If you are on a freeway — I’m not advocating anybody walk around the freeway ... can you see grooves in the roadway ... where it’s been done?
ARMINTA SYED: Yeah, you can. And, you know, obviously depends on how much sunlight you’re seeing and that reflection, but you can. You can see absolutely those grooves.
MARK BRODIE: OK. Would this be suitable for surface streets also?
ARMINTA SYED: Yeah, it would be. I would say it depends a little bit on, again, if they have that concrete base already. So that was a key factor in all of our analysis. If we didn’t have such good, thick concrete-based freeway pavements, diamond grinding wouldn’t have been such a good option for us.
So, No. 1, it depends a little bit on that because ... if you have to add a lot more concrete, the more expensive everything’s going to get. And then it also depends, I think, on the amount of traffic that roadway gets. It is where there’s a lot of traffic, a lot of constant traffic, that’s really where it performs the best.
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