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Corridor Optimization & Safety Considerations

Safe and efficient roadway corridors are essential in every modern community — creating the transportation networks that link people with their destinations. The first step in planning a new roadway or improving an existing corridor is a thorough inventory of existing infrastructure. A careful study of key aspects, such as traffic patterns and adjacent land use, helps our transportation planners and engineers develop an understanding of the issues affecting an area. The information gathered during this initial phase will greatly influence the roadway treatment options they recommend.

Listen as our traffic engineering experts Mark Perington, PE, PTOE, Brian Willham, PE, PTOE, PTP, Todd Knox PE, PTOE, and Nate Carhoff, PE, discuss the efficiency and safety factors they prioritize when designing and improving corridors.

Podcast Agenda

  • Identifying & Evaluating Corridor Deficiences (0:18)
  • Featured Corridor Safety Improvement Projects (4:28)
  • Safety & Operational Effects of Geometric Road Design Features (6:39)
  • Defining Correctable & Non-correctable Crashes (8:24)
  • Considerations to Mitigate Crash Potential (9:43)
  • Alternative Intersection/Interchanges Treatment Options (11:55)
  • Improving Roadway Treatments (12:22)
  • Safety for Different Modes of Traffic (17:19)
  • Infrastructure Prioritization & Budgeting (23:09)



Mark Perington, PE, PTOE

Traffic Group Leader

Mark Perington, PE, PTOE

Traffic Group Leader

Transportation planning, Traffic studies, Traffic impact studies, Conceptual intersection, Roadway design

Brian Willham, PE, PTOE, PTP

Transportation Engineer & Planner

Brian Willham, PE, PTOE, PTP

Transportation Engineer & Planner

Multi-modal traffic engineering, and transportation planning

professional headshot of traffic engineer, Todd Knox

Todd Knox, PE, PTOE

Traffic Engineer

Todd Knox, PE, PTOE

Traffic Engineer

Crash and traffic analyses, traffic signal system review, concept design, and operation timing

Nathan Carhoff, PE

Civil Engineer

Nathan Carhoff, PE

Civil Engineer

Highway and Street Design, Transportation Planning, Municipal Engineering, Funding Procurement, Trail Design, Pavement Management

Identifying & Evaluating Corridor Deficiences  

Mark Perington (00:19)
Hello, I’d like to welcome everybody listening to this podcast. This is Mark Perington with Snyder and Associates, I’m a Senior Traffic Engineer. I have with me Brian Willham, who has worked on a lot of traffic engineering and transportation planning corridor-type projects. Todd Knox, a Traffic Engineer, works on a lot of corridor elements traffic control, and systems. Nate Carhoff, Transportation Engineer, works on a lot of road design, everything from local collector roads and streets, to interstate highways. I’ve got a diverse group of people here to have a discussion about what we do in examining safety and considerations we make in corridor design. We try to correct and mitigate safety problems that we find in the field.

I’m going to open this up with Brian, if you would, maybe give some input on how everything varies and fluctuates when we’re dealing with the type of roadway we have and the land use that may be serving. There are a lot of ways we look at safety and a lot of issues out there. Could you comment on that a little bit, perhaps?

Brian Willham (1:23)
Yeah, sure. Thanks, Mark. As you mentioned, there are all types of streets and corridors out there that communities have and that we get the opportunity to work on; anything from local residential streets, all the way up to higher volume, higher speed arterial streets. When it comes to looking at crash issues or certain streets having higher crash rates than others, it comes down to a couple of things. One is conflict points and that could be vehicles conflicting with other vehicles, vehicles conflicting with bicyclists, pedestrians, or it could even be something like bicyclists conflicting with pedestrians. It depends on the context of the street that we’re looking at and when we think about safety and crash rates, it’s really, are we having issues with the points of conflict and that also gets into speed differentials between those various users.

Mark Perington (2:21)
Todd, could you expand on some things that you see in these different kinds of corridors? Brian was talking about what we may be focusing on when we start to drill down into the safety aspects of a corridor?

Todd Knox (2:31)
Sure, Mark. Some of the things that I tend to see expanding beyond the land uses are if we have too many driveways, we have people coming in and out of them, slowing down, stopping, blocking traffic, that kind of creates those rear-end crashes that you see all the time; then parking and somebody opens a door on you as you’re driving by and now you take out a door. Narrow lanes in an area where they might have tried to squeeze a four-lane roadway in basically a three-lane width corridor. Those are some of the things that I tend to see when we’re looking at crash concerns along some of these corridors.

Mark Perington (3:05)
Nate, you work on a lot of different design-type projects. As you go into everything from an intersection of a couple of city streets up to an interstate corridor, what type of things are going through your mind and what do you want to look into as we’re making sure that we consider safety on all of our designs?

Nate Carhoff (3:21)
When it comes to the design side of roadways, it usually boils down to the speed of the road and the anticipated traffic volumes. Those two tie into what we call a clear zone. When we design roadways, we try to first eliminate objects out of the clear zone. On a local street that’s generally speaking within 10 feet of the edge of the pavement. Higher speed roadways, you’re looking at 30 to 40 feet outside of that edge of the roadway. So we look at, first, trying to remove those objects that may become impacted if a vehicle does exit the roadway. Second of all, if we can’t eliminate them or move them, we look at protecting those objects with items such as guardrail barriers. Lastly, we could also look at replacing those objects with a breakaway style. For example, a lot of local streets have light poles that are within the clear zone. They are supposed to break away and not do as much damage. We always like to look at intersections as far as removing those turning vehicles from through traffic. The addition of turn lanes could help reduce the slowdown of traffic and those rear-end crashes for those turning vehicles.

Featured Corridor Safety Improvement Projects

Mark Perington (4:28)
At Snyder and Associates, we’ve been working on corridor safety studies going clear back to ‘92 if not a little before then, and we’ve done a lot of review over the years dealing with some of the initial four-lane to three-lane conversions here in the State of Iowa and many other types of corridor studies that truly were all about safety. Where there was a high crash rate and what can we do to change this? Todd, do you have a unique corridor that sticks out in your mind and how we looked at that, and what type of things we’re trying to address?

Safety & Lane Configuration

Todd Knox (4:59)
Yeah, one project that kind of comes to mind for me, was in Fort Dodge, we did a project around 10 years ago that was a four-lane undivided roadway, no turn lanes, lots of driveways, in an industrial area within the community. We started looking at the crashes and it’s like, there’s a lot of rear ends. There are a lot of overtaking type crashes. Just getting traffic into a single lane would help slow down traffic, help with the visibility of vehicles things like that gave the trucks coming through the corridor more opportunity to get out of the through lane or get more area to be able to turn. It eliminated some of the safety concerns by just changing it from a four-lane undivided to putting in a two-way left-turn lane, down the center, and just a single lane in each direction. That kind of project stood out to me as one that impacted safety along the corridor.

Redevelopment Plans & Corridor Studies

Mark Perington (5:51)
How about you, Brian?

Brian Willham (5:53)
There have been quite a few corridor studies that we’ve done recently, and a lot of times there may be a safety problem happening when vehicles can’t see around each other. When they’re trying to make left turns. It may be that a corridor is just to the age that it just needs completely reconstructed. There may be some redevelopment happening along the corridor too. That’s where we can have a good opportunity to come in and study these corridors and try to get a good feel for taking what’s been happening with the performance at some of the intersections or just the segments along these streets and come up with a good solution that’s going to serve what’s happening today, but as things are redeveloping, so that it’ll serve far into the future too.

Safety & Operational Effects of Geometric Road Design Features

Mark Perington (6:39)
Nate, could you comment a little further on some of the geometry features that are usually on the front of your mind to make sure, when we’re done with it, it’s as safe a corridor as possible?

Nate Carhoff (6:51)
Sure. On a design side, especially in intersections, one of the biggest factors is site distance. Whether it’s a signal-controlled, stop-controlled intersection, or yield-controlled, you want to have adequate site distance on the main road as well as the side road. So, make sure there aren’t obstructions in that site triangle. Say you’re on a side road approaching a major intersection, you’re going to want to make sure the traffic signal poles and fencing don’t obscure the view from vehicles entering that intersection. The same is true on the main road, you want to have the through traffic able to see cars approaching from a side road.

Mark Perington (7:33)
How about turn lanes and intersection layout, Nate? I mean, some further things on that related to what you focus on when you lay those out and create good site angles?

Nate Carhoff (7:43)
Yes, a lot of it has to do with being able to efficiently get into the turn lane. Opening up that turn lane with a proper taper ratio, that’ll allow vehicles to exit the through lane into the turn lane at speed, and then have proper deceleration distance within that turn lane so that you can exit the through lane at speed into the turn lane, and then begin your deceleration into your turn movement. The length of the turn lane dictates how efficient they are. Also, thinking about if you have a lot of traffic queuing, you not only want to have adequate deceleration distance, but you also want to have enough storage capacity to handle the number of vehicles turning.

Defining Correctable & Non-correctable Crashes

Mark Perington (8:24)
We hear it talked about, correctable crashes, and the types of crashes, and again, the thing of what we’re focusing on when we look at a corridor. Thoughts on that aspect of it?

Brian Willham (8:34)
Sometimes you’ll hear this term correctable crashes and really, probably one of the easier ways to think about that, is to imagine that you’ve got some sort of a four-way intersection. It might have stop signs, it might have a traffic signal but you’ll have someone either run a stop sign or run a red light and you could see a broadside crash. Some people refer to that as a T-bone. Those can be impactful crashes. A lot of times those do lead to injuries and maybe even sometimes worse.

When we think about this term correctable, if we have an intersection where we’re having that particular crash problem, we think about replacing that with a roundabout. Instead, there’s still the chance to have a crash, but if they do occur, they’re at a much lower speed and they’re more of a slight angle. That’s what we normally think about when we think about correctable.

There are non-correctable crashes out there too, like someone crossing a center line because they’re having a medical emergency. We can’t design infrastructure to address every single crash, but that’s kind of the difference between a correctable and a non-correctable crash.

Considerations to Mitigate Crash Potential

Mark Perington (9:43)
Todd, expand on that a little bit of how we think about things in the system and is a traffic signal or a roundabout the right solution? We have multiple in a corridor, what do we have to think about and how do we make them work together, to again, help mitigate any crash potential?

Todd Knox (9:58)
Yeah, everybody seems to think traffic signals are the answer to everything, but really, it just changes the manner of collisions. You try to eliminate the broadside crashes, however, you start creating some other types. The major through movement may have to stop so now you’re creating rear-end crashes. So you’re not eliminating crashes, you’re just changing the pattern of the collisions there.

Also, you were asking about the corridor type considerations. As you start getting multiple signals along a corridor. Now you’re potentially having them stop multiple times. We’ve been trying to work with communities to create coordination timings so traffic doesn’t stop up as much so they can flow smoother through the corridor, and not create the potential for right-angle crashes or rear-end crashes. That impacts the side street and it’s a whole balancing act that we have to do on the operation side.

Mark Perington (10:55)
I think that’s a good comment. Sometimes people think that we are trying to have everybody go faster when we do some of these things, and that’s not really what we’re trying to achieve necessarily. It’s more of a uniform flow. I would comment that that’s the thing we see a lot is if we can keep traffic moving more uniformly and we don’t have as many unexpected stops, rapid acceleration, rapid deceleration, which usually leads to a much safer corridor. I think that’s a great point, Todd, that many times it isn’t just about the delay, we’re trying to get a more efficient flow through a corridor so that we can try to help with safety and not create problems of people trying to drive fast to make up for lost time.

Todd Knox (11:39)
One thing we try to do is time the signal so that if you’re going at or near the speed limit, that’s the flow that we want you to be going from one signal to the next signal. We’re not encouraging people to go five, 10 miles an hour over the speed limit, because that creates a safety concern.

Alternative Intersection/Interchanges Treatment Options

Nate Carhoff (11:55)
A question for the group here, when we’re talking about corridors, especially with multi-lane maybe higher volume corridors where you have a side road that has an at-grade crossing. Could one of you speak to the non-traditional intersections, like a J turn? Where the vehicle on the side road isn’t permitted to turn left, they have to turn right, which is a safer movement. Then make a U-turn down the road. Have you guys had experience with those?

Mark Perington (12:22)
Yeah. I know that a lot of treatments we’re looking at deal with the four-lane or six-lane expressway style roadways that we see leading into metropolitan areas, or they can just be some of those four-lane divided that is out in a more rural setting. As Todd was alluding to earlier, sometimes a traffic signal out on a corridor like that that is designed for a higher degree of mobility over a greater distance or higher volume of traffic flow, isn’t the best answer for when you have a little bit of interference. Granted, trying to take that right-angle collision away. Sometimes introducing that red light out on the high-speed road, now means at some point in time, somebody’s going to stop.

That can be a difficult thing. We’re evolving here, I think, in the Midwest where we’re starting to see more of these types of treatments, as you mentioned, the R-CUT. Essentially you’re on the side road, you’re turning right, which is a simplified movement. You’re then making a U-turn, which again, we simplify that for you and give you fewer things to try to take in and deal with, and then re-enter the traffic stream. The goal is that we might be taking somebody about a thousand, 2000 feet out of their way, but the idea is if that one time we can eliminate that crossing attempt where someone, makes a mistake and is not double-checking for traffic in a certain direction. If we can take that away to where that can’t happen, we could be potentially getting rid of a fatality at an intersection with a high-speed, broadside crash.

If there’s one fatality removed out of an intersection that allows a family member, a friend, whoever, it is, to go back home to those people that are important to them. We think that’s a very, very important thing. So again, our approach is mitigating problems, eliminating these potential conflicts, which in turn tend to lead to severe crashes, injuries, and fatalities. I think that’s one of our big goals as designers is how do we do some of those things to get rid of those bad crashes?

Brian Willham (14:27)
Yeah, and I’ll just follow up with one additional comment on your question there, Nate. Sometimes we’ll do a traffic impact study for new development along the corridor. If we’re on a pretty busy street, we know that we’re probably going to have a problem if we allow a left turn out of new development across a busy street. Then there are opportunities to try to take a little bit of a proactive approach and turn that entrance drive into a three-quarters intersection. So, all of the movements are there except for the left turn out, across the major street. Then we can work to create a path for anyone who needs to take that route to come out of the development a different way onto a side street at the next major intersection down the road. That’s one way that we can try to be a little bit proactive with trying to head off future safety problems in some of those bigger corridors.

Mark Perington (15:23)
Brian and Todd, If we are seeing certain right angles or a lot of rear-end crashes, what might be things we’re looking for and how do we impact those? If you guys want to comment with maybe some more specifics as examples?

Brian Willham (15:35)
I think that one of the big opportunities we have with these four-lane to three-lane corridor studies is to improve the sightlines for left-turning traffic off of the mainline. We’re trying to do that in a couple of ways. One is if we can just align the two left-turn lanes across from each other, that helps give a driver the ability to see next to an opposing left-turning vehicle. Now, when we have the space, we try to even offset that more. We call it a positive offset, so you have a clear view of the oncoming through traffic you can much easier make your decision before you make the turn.

Todd Knox (16:15)
It’s not just for left-turn lanes either. We’ve been starting to do a little bit more positive offset or right turn lanes too, for just the traffic coming out of a driveway or public street. We’ve been shifting the right turn lanes out so that those people either turning right, turning left, or going through, can see around that vehicle slowing down to turn right. There are those left-turn lanes that Brian was talking about but also with signal control, we can do some things that can help mitigate some of these turning movements. We’ve seen the flashing yellow arrow pop up more and more and is getting to be pretty widespread across the state. That gives us the flexibility to, if we see by the time of day, that a left turn crash they can’t see around the vehicle opposing them. We can implement a protected-only left-turn movement for just those few hours that we deem as necessary. Then we can go back and release it to alright now will allow the protected left to go, but we’ll also follow it up with a permissive movement. There are a few things like that that we can do with the traffic signal.

Safety for Different Modes of Traffic

Mark Perington (17:19)
Nate, as we look at some of these corridors, we are serving a lot of different modes of traffic. It isn’t always just a low-speed street that may have pedestrians and bicyclists out there as well. We use these corridors to create movement for them as well, whether they’re recreational activities or commuting routes. Maybe comment on geometry as you look at those, what has to go through your mind as you approach intersections within the corridor or just any other design elements where you’re sharing these different types of modes of traffic?

Nate Carhoff (17:48)
Most times, we want to keep that far away from the through traffic movement, just for safety purposes. We want to keep pedestrians out of the clear zone as well. So a lot of times, in a four-lane facility, the sidewalk or the trail will be located at the edge of the right-of-way, or as far away from the vehicle travel way as possible. Like you mentioned at intersections though, we want to make sure that those crossings are as visible as possible and create as safe a crossing as we can. Some things to consider at the intersection is the location of the crossing, adding refuge islands in between the opposing traffic that provides you some sort of barrier so you can cross safely.

Sometimes the location at an intersection may not be best. We’ve seen some places where a mid-block crossing may work better typically on lower-speed roadways. Those can be made safer by the addition of raised pedestrian crossings, proper signing, or even pedestrian signals. There are specific situations where that could be advantageous for the safety versus locating them at an intersection. Of course, where applicable, we’d also look at a grade-separated facility where vehicles can cross a major roadway either going underneath that in a tunnel or up over top in a bridge just to eliminate that conflict. That is of course the safest option when it comes to pedestrian crossings.

Mark Perington (19:21)
Right, the vulnerability of bicyclists and pedestrians is key. We know from the pure physics of the outcomes when you have a conflict between these different modes car, truck, versus say bike, ped, and that’s something I think we all are very focused on when we deal with these corridors and what we examine. I think the other side to it is these corridor studies, and looking at safety within a corridor, sometimes it’s stepping back farther and looking at the intersection in its entirety that we see more trends. There might be this concern, this complaint, but if you get too zoomed on it, you maybe don’t notice what may be happening as far as severity with crashes and other issues of things we can correct.

We try to make sure that safety is an element of anything we’re doing, even when it’s as simple as we’re adding a turn lane or something. If we’ve taken a really good look at it to see, is there anything, while we’re touching this area in a corridor at an intersection that we can make better while that investment is being made? Because we’ve certainly emphasized it in a lot of our other discussions that budgets are so limited. You hate to more of a maintenance project, and then realize a little later that if we would’ve looked at safety a little more there are a few things we would’ve done while the contractor was out there doing the work and in the incremental budget of the whole thing. It could have been a very small difference to make a correction out there to, again, try to have a positive impact on the crash frequency or, mitigate things that have up and out there, reducing severity.

Nate Carhoff (20:53)
Could be as little as four feet that allows those bicyclists that are out in the more rural areas who are going to right on the road to give them a little bit of a shoulder to take them out of the through traffic, as much as possible. Could make a world of difference and at a very minimal incremental cost increase.

Mark Perington (21:15)
That little bit of extra pavement out there can end up having a lot of incremental difference on safety over time. Todd, did you have something you wanted to mention?

Todd Knox (21:23)
Yeah, these safety improvements that we’ve been discussing, some of them are not cheap. Yet, here in Iowa, we do have that transportation safety improvement program funding that does provide opportunities for agencies to obtain funding through the DOT to pay for some of these improvements. We’d like to address these safety problems as much as we can and this just provides a funding mechanism for that.

Nate Carhoff (21:32)
Guardrail isn’t a bad thing to have. Sometimes, we as designers, look at protecting stuff within the clear zone or try to eliminate objects out of the clear zone. If we can’t do that, we have to do that with a guardrail or some other sort of barrier. Sometimes it’s not necessarily in the clear zone that we’re trying to make safer. For example, on the interstate, the new standard for median width would lead a designer to believe that if a vehicle leaves the roadway towards the median side, there should be ample distance there to recover before creating a head-on collision on the interstate. We want to weigh the benefit versus the cost of what would happen if someone does leave the roadway. There have been such where there is ample median space. Yet, we still put the cable guardrail up because the worst thing that could happen is someone still does cross through the median and into a head-on collision at high speed. If you leave the roadway and you hit the guardrail, that prevented you from crossing the center line and most likely a fatal crash head-on.

Infrastructure Prioritization & Budgeting

Mark Perington (23:09)
Going back to the point for agencies of limited budgets, we don’t have unlimited resources to try to design everything. Many times we have to do these proven methods that tend to show a lower cost to implement but have a large benefit. The cable barrier rail that you’re referring to on interstates, is a great example of a fairly low maintenance item that they have to deal with.

Did it potentially save that vehicle though, from your point of crossing across the median? People with some body damage to vehicles is far less than if we have that one or two cars go across the median and end up in a head-on collision. That can end up resulting in fatalities, a lot of severe injuries, and just a very tragic thing. I think that’s what we’re looking for in the safety in these corridors is techniques and measures that, again, may have a different side consequence to it, but we help take away the severity of crashes, injuries, fatalities. These are things that we’re trying to look at these days and that can be applied at all levels of roadways and features we do with the infrastructure. Brian, with your city experience, you probably went through the process of various crashes. What are our priorities?

Brian Willham (24:30)
I mean, that’s always the struggle. It’s trying to step back and get a feel for where are the highest crash locations and then trying to build that into a Capital Improvement Plan so that you can address some of those. Todd’s point about some of the traffic safety funding, it’s trying to also identify some various outside funding programs to line up with what some of the local funding would be to try to do more of these safety projects faster. More of them, you know, as soon as possible.

Mark Perington (25:10)
At the end of the day, though, safety is such an important element of what we consider and it has to be. We hope through the processes and the approaches we take, we do our best to help agencies deliver and improve roadway networks, and make things as safe as possible for the traveling public. So thank you.

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