Intersection Safety Goals & Considerations
With all modes of traffic crossing paths at intersections, they are designed points of conflict. Safety and operations are critical components in limiting the number of these conflicts. Listen in to this second episode of a four-part series on transportation safety as our traffic engineering professionals Mark Perington, PE, PTOE, Toney Boes, PE, PTOE, and Justin Jackson, PE, discuss their process to improve operations and mobility, reduce conflict points, and design safer intersections.
- Key Components in Selecting Intersection Treatments (00:19)
- Benefits of Using Predictive Methods in Traffic Engineering (2:29)
- Population Density Effects Intersection Design (5:02)
- Maintaining Safety & Multi-Modal Accommodations with Private Development (7:46)
- Roundabouts are One of The Safest Intersections (10:13)
- Public Perception of Traffic Signals (13:02)
- Considerations when Signalizing Intersections to Improve Safety (13:58)
- Snyder & Associates’ Fundamental Approach to Intersection Design (17:33)
Key Components in Selecting Intersection Treatments
Mark Perington (0:19)
Good afternoon, thank you for joining us today. It’s Mark Perington, Senior Traffic Engineer with Snyder and Associates. Today I’ve got a couple of other folks from our traffic group. Tony Boes, Senior Engineer, and Justin Jackson, Traffic Engineer. We’re going to try to touch on some elements as it relates to intersections and how we deal with safety in relation to those intersections. I thought I would open the conversation with Tony, do you want to comment on some different intersection treatment types, and how we approach that as we look at where we’re working, what the issues might be, and what we think will help treat things?
Tony Boes (00:56)
The key factors in looking at an intersection treatment are considering the main elements of geometry and traffic control and how those work together to optimize operations and safety at the intersection. We need to understand the traffic demands for that intersection existing and probably future demands. As well as, the safety history, what’s the crash history at that intersection? Then based on that, along with other factors, such as, what kind of cost are we looking at? What are the potential impacts? What’s the surrounding area like? From that, we can choose some intersection ideas or concepts to explore, to improve safety and operations. Those might be at a traditional intersection with stop control or traffic signal control. It could be a roundabout, it could be a more innovative intersection, like a U-turn intersection such as an R-cut or a MUT. There’s a lot to consider. It really comes down to, looking at the basics of traffic and safety speeds, volumes, all those important things.
Mark Perington (2:02)
Justin, do you have some thoughts to add to that?
Justin Jackson (2:04)
Not only look at historical data that is easily accessible these days, but also looking at a forecast of predictive data that we have either forecasted traffic volumes or predictive crash modification factors that go into certain types of traffic control at intersections and utilizing all those things to get an overall view of all different types of treatments, controls, and types.
Benefits of Using Predictive Methods in Traffic Engineering
Mark Perington (2:29)
In the past, we had looked at crash history, as we could get it, versus what we have at our fingertips now. We can get to so quickly and more of this predictive type of crash relationship to designs we might consider.
Tony Boes (2:41)
Traditionally, we have been looking at the history of crashes and comparing that to some sort of baseline, like what’s the statewide average of crashes for that type of intersection or that volume of intersection? By using predictive methods, we can get an understanding of what would the normal crash rate be for this type of intersection and then what would be the crash rate with the proposed improvements to that intersection? That way we get a little bit more accurate understanding of what’s the benefit of that improvement.
Justin Jackson (3:11)
I also think with the collection of all the recent data within the five years of these newer types of intersection controls. You are able to put these models together before you could really compare apples to apples. Now, you’re able to compare apples to oranges and as Tony alluded to in these models that are put together. The regression will come back to a mean, even though there may be different characteristics of each intersection that you may not have been able to consider before in your modeling.
Mark Perington (3:44)
Could you go into just a little bit more about how safety plays into decisions? You could just give an example of a certain kind of corridor and why you might consider something like traffic signals or as you mentioned downstream U-turn movements, roundabouts?
Tony Boes (3:59)
Safety is a big part of why we do what we do. That needs to be a major consideration in looking at intersection and corridor improvements. Depending upon the severity of the issue, the solution might have to be unusual compared to what’s normally done, which might include, U-turn type intersections but it might be some typical things like pedestrians and bike access and crossings along the corridor that need to be addressed, which might influence intersection lane configurations, and traffic control and how we treat specific intersections along the corridor.
Mark Perington (4:32)
Anything to add, Justin?
Justin Jackson (4:34)
Safety is the number one factor when I’m looking at a project and there are other people within the team that are looking at other factors together, in accordance with the scope of the project. We may be able to provide the safest intersection treatment with also staying within the confines of some restraints that get neglected when you’re only just looking at strictly safety. You need to consider all within to make decisions in a project.
Population Density Effects Intersection Design
Mark Perington (5:02)
In many cases in the areas we work, we’re dealing with anything from a very urban situation in a downtown densely developed area, out to a very rural area. Could either of you comment on some of the ways in which safety really differs in your approach as you’re looking at intersections under those types of settings?
Tony Boes (5:20)
I think to me, the biggest difference really in looking at a rural area is you’re dealing with a lot higher speeds. The result is, there tends to be higher severity crashes. That’s an important consideration. That may affect the need for turn lanes to get turning traffic out of the way of through traffic. It might require, if it is a signalized intersection, to make sure our signal poles and obstructions are outside of the clear zone, so vehicles are less likely to hit obstacles if they run off the road.
Mark Perington (5:52)
Certainly, there’s a driver’s perception and what they should be experiencing, or even the size of the community population base they live in. Do you guys have some thoughts as it relates to those types of issues with what we’ll call rural and urban?
Tony Boes (6:07)
I think that’s a good point. In rural areas, there’s less expectation that there might be conflict. We need to consider that in a driver’s perception and reaction time to opposing traffic or side street traffic.
Mark Perington (6:20)
I would agree with your statements. We certainly have an expectation level, just a driver patience level of if they’re in an urban area versus what might be a less densely populated area. Where what they’re willing to wait through or what they are willing to drive through. In going through traffic control that we would add to an intersection on safety or even geometrically changing some things, which is posing a safety problem. Again, there can be quite different expectations between rural and urban areas as it relates to that type of thing. How about when they come to us with an idea of modifying an intersection, as it relates to new land use on one side of the road, or the thought that more traffic control is needed just more from a delay basis? How do you guys approach, thinking about it from the standpoint of, well, what about safety maybe that wasn’t even asked of you initially?
Tony Boes (7:16)
I think it’s important to bring that up, even if it’s not something that we’ve been asked to look at. It certainly could influence what goes into the design or the study or, some of the key factors that are being considered. Even if our scope doesn’t include it or a client doesn’t want to include it. I think it’s always a good idea, if possible, to look at crash history and see if there are any trends or unique things that might influence the need for something special or some other consideration to a certain type of crash that needs to be addressed.
Maintaining Traffic Safety and Multi-Modal Accommodations with Private Development
Mark Perington (7:46)
How about with new development, Justin?
Justin Jackson (7:48)
A new development is always a tough one too, because typically it’s driven by a private entity, and their main goal is to get the development up and going to make money. They don’t really want to spend a whole lot of money on the public improvements that are needed based on a private development. I think it goes back to looking at serving the community as a whole and making safety a highlighted item for this private development within the public right-of-way that’s adjacent to it. Because, within their own development, they’re always looking at onsite of having safety features of keeping the people within their property safe, but then it gets excluded once they cross that public right-of-way line, which it’s unfortunate. If we can keep that dialogue going with that private development. Let’s extend, you are providing a level of safety outside of your property line.
Mark Perington (8:36)
Yeah, I think that’s a really good point. I think at the end of the day, it’s the public and taxpayers driving around out there. Whether they’re going into a site, whether they’re driving down the road going by a site, it’s still an impact and a reality on all of them. It’s something important that we consider. Do you have any specific examples or thoughts as you consider the mix of traffic at an intersection of all kinds and all modes? How that can impact what you’re considering when you’re choosing a type of traffic control?
Justin Jackson (9:05)
Yeah, it definitely gets a context for the intersection and a feel for your typical users. Maybe if there was an upgrade to the intersection you may attract different types of users. Consider the most vulnerable users first and what we can do to that intersection to take that vulnerability out of those users.
Mark Perington (9:25)
The type of traffic control we have at an intersection kind of changes the driver and vehicle characteristics, stop signs versus signals, that type of thing. Maybe, you’d comment on that a little bit?
Tony Boes (9:36)
The volume of how much pedestrian or bike traffic we expect is important, but oftentimes we need to accommodate, even if there’s only a few a day for a trail crossing or a sidewalk crossing or bike lane. Part of it is, we need to make sure we have a safe facility, even if it’s only a few users for all modes of traffic. We also need to consider, for example, if we have a signalized intersection our timings at that intersection. Is there potentially a need for a pedestrian refuge area in the median? In order to split up the crossing and make it hopefully a little bit safer for peds and bikes to get across the roadway.
Roundabouts are one of the Safest Intersection Options
Mark Perington (10:13)
Roundabouts are more prevalent throughout the Midwest now, and we’re really seeing and learning the benefits of their characteristics and how they perform over time. When you look at quarter speeds and volumes, what are some things you consider in relation to those when designing a roundabout for high speeds, high volumes? It might be more than just kind of what some people think of with a simple single–lane neighborhood roundabout.
Tony Boes (10:41)
We still need to get speeds down to a low level as they enter the roundabout. If you have a high–speed corridor, say 45 miles per hour or higher, the idea is to slow them down before you get to the roundabout, so that they have lower entering speeds. We do that normally by either having some reverse curvature on those major approaches to gradually slow traffic down as they approach the roundabout, or the trend lately has been just to provide longer splitter islands approaching the roundabout, which could be up to 500 feet long. They get an understanding that there’s a need, to slow down, because of the feeling with these curbs along both sides of the roadway that something changing, and I need to slow down to get through this intersection.
Mark Perington (11:23)
Roundabouts are not always perfect for every situation. It isn’t one-size-fits-all. Things you might consider in relation to safety that might not be a roundabout, or a different kind of treatment is more what’s needed.
Tony Boes (11:35)
If you can fit a roundabout, that’s one of the safest options there is. There are obviously other factors to consider. The initial cost with the roundabout is typically a lot higher than other types of intersections. If you do a life cycle cost analysis, we might find that the overall cost when you consider crash history may be beneficial with the roundabout. There are other options for intersections that are not roundabouts that can be done certainly to improve safety. There still are people that have concerns or haven’t driven through a lot of roundabouts, and they think it’s a big spaghetti bowl mix of who knows what’s going on kind of thing. Pedestrian safety is a concern a lot of times with roundabouts. I think the issue there is really the difference between security versus safety.
At a signalized intersection, the pedestrian has a sense of security, because they have a signal that says it’s okay to walk. They still may not have a higher level of safety, because there are conflicting vehicles that may be turning left or turning right, crossing their crosswalk while they have the walk signal. Those vehicles are supposed to yield to the pedestrians, but they may not always do so. I guess it’s tough to get the public to understand that the safety aspects may not exactly be what they perceive as the safety issues or concerns at a particular intersection.
Public Perception of Traffic Signals
Mark Perington (13:02)
Justin kind of taking it the other direction with traffic signals. Do you have some thoughts with the public perception of those and then the reality of it is that what they need, or if you’re going to do it, what type of things you may need to do with the signal to make sure it’s as safe as possible?
Justin Jackson (13:18)
There’s a weird phenomenon of a traffic signal. It’s either, a cure-all if you’re on the side of the intersection where it’s needed to make your movement easier on you, or it’s the opposite of that. If you’re on the mainline and now you have to stop for this other person. It comes with a status symbol of a lot of towns, or areas within a city that’s prosperous and developing because we have enough traffic that now we need a signal. A signal is not the cure-all, maybe we should look at all these other alternatives to different intersection treatments or types.
Considerations when Singalizing Intersections to Improve Safety
Mark Perington (13:58)
As we go into traffic signals, there are so many things that go into a signal beyond just, oh put one up. Justin, do you have some thoughts on that? Some of the important things we must consider when we’re looking at a traffic signal at an intersection?
Justin Jackson (14:13)
Installing a signalized intersection designates right-of-way within that intersection and helps the drivers make better choices on when they can make a particular movement. With that, some of the things to consider when signalizing the intersection would be the vehicle speed, the amount of traffic on each approach, determining the correct amount of clearance times of the yellow clearance and red clearance to an intersection to get all the vehicles out of the intersection, before switching to the right-of-way to a different approach. I think determining the adequate amount of green time for each approach so that there’s no frustration felt on the drivers that they endure excessive delay. I think the frustration comes with not obeying the signal either in trying to press through a yellow light or go through a red light.
Mark Perington (15:09)
Tony, any thoughts to add to that kind of on some design features as well?
Tony Boes (15:13)
One thing that really needs to be considered is left-turn phasing. If we’ve got left-turn lanes, are we looking at protected only left-turn phasing due to speeds, crash history volumes, how many lanes we’re crossing that sort of thing? Do we feel that protected permissive or permissive only left-turn phasing would provide an adequate level of safety while potentially improving the capacity at that intersection? There are a lot of design details that we get into as we prepare a set of signal plans. Specifics of, where do we locate poles so they’re out of the clear zone? Push buttons need to be at a certain location so they’re accessible and within a reasonable distance from the roadway. We need to consider the visibility of signal heads so that drivers got a clear view of what that signal indication is. Those all need to be considered in the design.
Mark Perington (16:01)
I think our team, has dealt with so many different kinds of signal designs in so many conditions. A signal that may be out at what we consider a very rural high-speed intersection with hardly anything right adjacent to it. Clear to the densified location of a downtown setting where we hardly have room to put poles up. Yet in both conditions, we have a lot going on in the background, wide–open skies, trees, and things getting in the way. Buildings and other lighting, things that can be distractions to the driver, and just paying attention to the red, yellow, green, and what’s going on. I think at times there can be a misconception that we just put these lights up red or green, and there’s really a lot of little things that go into it.
I think Justin alluded to it quite a bit. What sets us apart is our ability to actually work in a signal cabinet and control how a signal works. Getting it designed and getting it constructed and put in the ground out at the intersection is one thing. Having it operate and move traffic effectively and efficiently, which will ultimately lead to how safe it operates, is something that we’ve done a lot of. It’s the second half of putting a signal in. The first half is design and somebody building it. The second half is making sure that it works right and properly. We try to mitigate any crash potential as best we can on what we’re doing. Thank you for those thoughts, guys
Snyder & Associates’ Fundamental Approach to Intersection Design
Mark Perington (17:38)
I think one of the things that’s important when we look at intersections. The fundamental thing in intersections is we’ve got two roads that meet out there. Bottom line, we’ve got a potential conflict between traffic flowing in a couple of different directions. The key is that at times it can’t just be about, well how many crashes are there an intersection? Where’s our biggest stack of crashes? It could relate to the type of crash you’re getting. Are they severe? Are people getting hurt, injured, or at worse fatalities? Versus when there’s not a lot of crashes or it’s just some fender bender type plastic thing occurring out there in the road. One of the things we try to do is take the right–sized approach to, what is the safety problem? Versus, what is the proper solution?
Tony, do you have some thoughts on that and, kind of stepwise, how you tend to look at things?
Tony Boes (18:21)
The solution needs to address the problem. We certainly have to understand what the problem is before we can identify the best solution for an intersection. That involves looking at not just how many crashes, but what types of crashes. What types of severity we’re having, and that may influence what type of solution we need to provide. Not every intersection necessarily needs to be a roundabout. That could be a solution where you’ve got a lot of crash history because we know roundabouts provide a significant reduction in crashes and even more significant reductions in injury crashes.
The step-by-step approach is, let’s look at the crash history, volumes, and details. Are a lot of crashes occurring at night? If so, is lighting an issue? Are there a lot of right–angle crashes? Maybe a traffic signal is the best solution? Then from that, we can refine, if a traditional intersection makes sense, what kind of details such as turn lanes are needed to get the turning traffic out of the way of through traffic to reduce the potential for rear–end crashes.
Justin Jackson (19:26)
Once we were able to identify specific causes of those crashes. We’re able to look into treatments and then with those treatments, also use a crash prediction module and safety performance functions of those treatments. To get an idea of the potential reduction in those specific crashes. Then also the potential reduction and overall crashes at the intersection. I think combining those two things of looking at the crash data history, and the predictive method is an extremely useful tool to come up with intersection treatment and types.
Mark Perington (20:03)
I think that gets us back to that right-size solution that we always try to aim for here. Knowing that the budgets for public agencies are tight as it is. They can’t fix everything everywhere right away. Helping them with, what’s the right size solution to fix what appears to be a certain safety problem at an intersection. Perhaps the dollars available for safety solutions for a public works agency might be able to spread over four or five intersections in a summer with some minor improvements or right–sized improvements versus an over-designed solution that might consume all their budget for one year, just for one fix. The incremental improvement in safety performance or mitigation may be minor. I think that is what is important about what both of you alluded to. How we dig down into the crashes. Is it about left–turn crashes? Is it about a speed–related crash? Does it seem to be crashes where somehow drivers are confused about where they’re supposed to be going? What they’re supposed to do? Those little particulars are important as we examine and understand, what is the right size solution?
I want to thank Tony and Justin for talking about intersection safety here today. There are a lot of elements that roll into it. At the end of the day, though, safety is such an important element of what we consider, and it must be. We very much subscribe to the Vision Zero Safety Goals, which is why we can’t get rid of all fatalities and all injuries and do our best to mitigate that. We hope through the processes and the approaches we take, we do our best to help agencies deliver and improve, roadway networks that will do just that. Do all we can to mitigate crashes and make things as safe as possible for the traveling public. Thank you.