The majority of members who voted for the change cited the fact that residents in that part of town are opposed to putting a roundabout at the intersection of Scottsdale Road and Dynamite Boulevard; the city does have roundabouts elsewhere.
Data generally show roundabouts are safer than other ways of managing intersections and reduce the number of serious crashes. But many drivers still don’t like them.
Tripp Shealy, an associate professor of civil engineering at Virginia Tech University, joined The Show to discuss.
Full conversation
MARK BRODIE: Have roundabouts in fact become more popular across the country?
TRIPP SHEALY: Roundabouts are a really good option for intersections. I, I think they do a lot of things well all at the same time, you know, they're aesthetically I think more appealing. They can reduce queuing time, more cars can go through that intersection much more quickly.
Generally speaking, they're safer than intersections, they can help reduce emissions, so there's a lot of positive benefits, and so I'm not surprised that, you know, more communities are adopting them or want to use them. I'm not sure if you're familiar with Carmel, Indiana, but they have over 140 roundabouts.
BRODIE: Why are they safer than either stop signs or stop lights or other forms of intersection management?
SHEALY: So they're safer for a couple reasons. One, they slow traffic down, the other reason is it reduces conflict points. You're only turning right, essentially you're not turning left, and so those are the two main reasons.
BRODIE: And what do the data say, generally speaking about how much safer they are, like, how, how much do they reduce crashes? Do they reduce the severity of crashes that do happen?
SHEALY: Well, the Carmel, Indiana, has reported 50% reductions in injury crashes, so quite a bit, and I think that's also why they're popular is because it helps slow down traffic, so the types of accidents are less severe because the speeds are much lower.
BRODIE: I'm wondering about people who are maybe not used to them because they can be, I think if you've never driven through one before, a little anxiety-inducing, and I wonder if there's maybe sort of a factor of people who don't really know what they're doing in a roundabout. Maybe, maybe for, for those drivers and the drivers around them, maybe it's not the safest environment.
SHEALY: There's certainly a learning curve, and if the community adopts roundabouts that doesn't have one already, there's a steeper learning curve if it's their, you know, first one. And, you know, I think most of the research has really moved past trying to understand driver behavior and roundabouts.
It's generally understood that while there is a learning curve, it's pretty quick. And most of the research now is trying to predict human behavior for autonomous vehicles that, in a roundabout.
BRODIE: Well, so AVs are obviously very common to see here in the Phoenix metro area, but I wonder like how roundabouts play into that. Like if autonomous vehicles don't have a problem with them, and I, I think that's what you're, what you're getting at that AVs are typically OK through roundabouts.
What does that tell us about that form of traffic calming as opposed to any other form that we might use at an intersection?
SHEALY: Well, the interaction between somebody driving a car and an autonomous vehicle, roundabouts are a tricky, a tricky spot in the road. Because roundabouts are somewhat unstructured, there is kind of that human-human interaction. “Do I split the gap? You know, do I, do I move into that intersection into the roundabout or not?” That autonomous vehicles have to kind of figure out and understand and predict kind of how the humans, other drivers are gonna behave in that situation and that unstructuredness makes it a challenge versus being queuing, stopping in a light. And then the light telling you to go or not to go.
BRODIE: I suspect there might be some number of human drivers who would say the same thing, right? Like they're not really sure what the people who are in the roundabout are going to do, which makes it difficult for them to decide, “do I enter, do I not enter, like which, which side do I enter, how fast do I need to enter?” That kind of thing.
SHEALY: Some, yeah, and especially if you're not used to using them, but, many studies have have kind of shown, you know, pre-installation of a roundabout, people are uncertain about them, months later after it's been installed, kind of coming back and doing the same survey and finding that people generally are accepting of it. And so generally drivers figure it out pretty quickly after they've been in that situation a few times.
BRODIE: So one of the reasons we reached out to you to talk about this is because of this situation in north Scottsdale where the city had planned to put in a roundabout at a particular intersection. There was an amount of money set aside to do it.
And residents came, came back and said, we don't really want this here. We'd prefer a more traditional form of intersection, you know, management there. Do you find this in other places that there's some number of people who just don't like roundabouts?
SHEALY: Yeah, and the, the problem there is not the engineering piece of it, you know, if Arizona DOT did the study and looked at it and felt that it was an acceptable solution to that intersection. Then the engineering piece of this is pretty straightforward. More cars can get through a roundabout than a traditional intersection.
Oftentimes you'll see roundabouts paired with road diets where they're able to reduce the number of lanes on the road because you don't need as much. Cars can get through that bottleneck point much quicker if you're not queuing and waiting for a left turn lane or right turn lane, and so the, the engineering is really there.
A lot of these things are behavioral and psychological, and we oftentimes kind of fixate on what we're losing and don't necessarily understand what we're gaining. So there's, there's a lot of behavioral, reasons for this. Sunk cost fallacy, you know, the intersection looks good. We've relatively invested in this, you know, relatively recently. Why are we gonna spend more money at this if it's perfectly fine? Present bias, it's really hard to imagine the future looking different.
Traffic's not a linear approach, and so this, you know, thinking that we can shrink the number of lanes or we're gonna reduce the number of lanes and shrink it into a single roundabout seems counterintuitive, but you know, the engineering behind them works. It, it shows that it's a much more efficient use of that, that space.
BRODIE: Well, so like, how do folks who do what you do, who clearly like roundabouts, like, how do you try to convince people who are skeptical about them or just flat out don't like them, that they might be things that people could get used to and and see benefit in?
SHEALY: Yeah, I mean, it helps when it comes from community members instead of an engineer standing up there. And so oftentimes what I tell my students is it's really important to get community groups that have both points of view to be able to stand up there versus just the engineer telling them, you know, what's best for their community.
So I think it, it is a community discussion about what's best. If there's other parts of the community or neighboring city or town that has implemented these, and you can show the positive benefits that have come from it, I think that helps.
BRODIE: Are there some intersections or some types of intersections where roundabouts are more appropriate than others?
SHEALY: Yes, it's not right essentially when there's an unbalanced road. So if you have one lane or one road that's very busy and another road that's not busy at all, then it doesn't really work. You want the roads to be relatively balanced, otherwise it would probably make more sense to have a, a queuing of the road that's less busy.
Topography, geometry of the space, the amount of space that you have in the, in the intersection, you don't want roundabout, generally that is you know, on a slope. And then a network of traffic, if there's a whole bunch of traffic lights in that network in which people are driving, you wouldn't want to just throw in a random roundabout and then have people queue a few blocks later in an intersection.
