Not So Fast! Can driver behavior influence how speed limits are set?

Factors that determine speed limits on a given roadway have a lot to do with physical conditions along the route, but how fast drivers want to go figures into the equation, too.

Transcript

Bernie Fette: 0:14

Hey everyone. Welcome to thinking Transportation. Conversations about how we get ourselves and the stuff we need from one place to another. I'm Bernie Fette with the Texas A&M Transportation Institute. As operators of motor vehicles, we are required to comply with a variety of traffic laws -- which side of the street to drive on, when to stop, when to go. Of all the many rules of the road, however, only one -- the speed limit -- allows us a say as drivers. A chance to offer input. That's right. The process that determines what a speed limit will be takes into account what we would call "reasonable and prudent behavior" on the part of the drivers who will use a particular road. Here to help us understand that practice, and the other considerations that go into setting speed limits, is Marcus Brewer, a research engineer at TTI. Hey, Marcus, really glad you could join us because this is a topic I've been wanting to cover for a while.

Marcus Brewer: 1:20

Thanks, Bernie . Appreciate the invitation.

Bernie Fette: 1:23

All of us who drive know that traffic laws place restrictions on how fast we can drive, but what most of us don't know, however , is how those speed limits are determined, how they're set. And that's what I'm hoping we can unwrap today. Let's imagine a brand new road, and on the day it opens, drivers see that the speed limit is 50 miles an hour. Who decided that the speed limit was gonna be 50 miles an hour? Not 70, not 40. Who decided that?

Marcus Brewer: 1:55

Well, the road agency, whoever it is that's responsible for that road in general is who makes that decision. And there's a lot of details associated with that. But that decision then is based on how the road is designed in many cases, what the anticipated use of that road is gonna be, and therefore that's reflected in the design of that road, how many lanes it's gonna have, what kind of curves it's gonna have, hills, and how many driveways, intersections it's probably gonna have.

Bernie Fette: 2:25

Okay. You're getting into some of the features of that road now, which I think is really helpful . Mm-Hmm . <affirmative> . So without actually seeing that road, knowing only that the speed limit is 50, can you draw from your knowledge just to tell us what that road looks like? You know, the , you were about to get into number of lanes, whether or not the curves Mm-Hmm . <affirmative> . So what does a 50 mile an hour road look like?

Marcus Brewer: 2:47

Well, there's probably in , at least in this part of the country within Texas, there's probably gonna be a couple of different things that's gonna look like. But generally it's probably going to be in an area that we would call suburban or rural type of feel. Mm-Hmm. <affirmative> , it's probably going to be a roadway that doesn't have a lot of development, so not a lot of houses, businesses, et cetera . The , the number of driveways, intersections, is probably going to be low. Mm-Hmm . It's probably going to have either one or two lanes in each direction of travel, depending on what the expected volume is when it opens. And in the foreseeable future for what it's designed for. And probably with a speed limit of 50, it's gonna maybe have some curvature, but it's not gonna be very sharp. Obviously you wanna be able to have an alignment that people can drive at the speed that you're planning for. So some curvature, but not real sharp. No 90 degree turns, things like that. Yeah . If you're just designing a road from scratch like that.

Bernie Fette: 3:47

Alright . Then with those considerations in mind, can you walk us through the steps of how speed limits are determined?

Marcus Brewer: 3:54

Sure. And it's, that's a great question. Within Texas to start with, there's some legal basis for that. The Texas Transportation Code, the state law essentially provides that initial framework for how speed limits are determined. And there are some maximum numbers, some specific speed limit numbers based on certain types of roads and driving environments, particularly if they're located in an urban area or a street. If it's an alley or some kind of special access road, then that's even a lower speed limit. If it's a freeway or a numbered highway, then of course those numbers are higher to start with. And those are kind of the starting points based on state law. But then there's exceptions after that to determine what is most appropriate for that. And how we get into that is a study method that determines what either is anticipated for a road that's not built yet, or for a road that exists, if a speed limit change is explored, what are the conditions on that road and are they suitable for a change? And for a number of state and federal highways, et cetera , TxDOT, the Texas Department of Transportation is responsible for setting those in Texas. For city streets, then the municipal transportation department is responsible for that. County roads are managed by the respective counties. And so the city council would set an ordinance in a city, county commissioner court would establish a minute order or some sort of equivalent at the county level. Okay.

Bernie Fette: 5:24

There's one aspect of the speed studies that people in your line of work do that I was hoping you could explain for us, that I've heard over many years at TTI of the 85th percentile. As I interpret that, and I really want you to get into a boiled down version of this if you would, but as I consider that and asking you the question that I asked earlier about who decides what a speed limit is gonna be to at least a limited extent, people who are driving on that road have a say through their own actions of what that speed limit's gonna be. Is that fair?

Marcus Brewer: 5:56

Yeah, that's a good principle because that rule of thumb of 85th percentile essentially means that the posted speed limit is a number in miles per hour of what we estimate 85 percent of the drivers on that road are going to be traveling at or below. So it is reflective of the speeds that drivers are going for a road that's already in use and something that can be measured of the speeds that people are traveling. And in Texas, the Texas Administrative Code requires that speed limits be based on that as a starting point. And it's a rule of thumb for a wide variety of other roads, both within Texas and elsewhere with historic principle that many drivers are reasonable and prudent. They desire to avoid a crash, they desire to arrive at their destination in a reasonable amount of time. And yet there are always some number of drivers that are probably outside of that range, and so they're just gonna drive faster or whatever the case may be, whatever adjective we might wanna put on that. Okay. Relative to everybody else. And that's why that number is 85 and not a hundred, for example. Okay .

Bernie Fette: 7:04

Okay. And in some cases there are actually minimum speeds along with the maximum limits, isn't that right?

Marcus Brewer: 7:11

That's true. That's typically for freeways. And the idea is that because the maximum speed limit is at the high level that it is, we want to keep that speed differential relatively low. Because

Bernie Fette: 7:27

You mean the difference in the speeds that cars are driving when you say that "differential?"

Marcus Brewer: 7:31

Correct. So the fastest car versus the slowest car, you want to keep that at a minimum because while we've found that high speeds can affect, say the severity of a crash, so for example, if you're driving and you run into a fence and you're going 10 miles an hour, that's not all that severe probably. But if you run into it at 70, that's a different story. And the same way when cars run into each other, if they're going at vastly different speeds, the chances of that collision goes up. And so that's a different issue about crash severity versus crash frequency. So I would say in terms of a relationship between speeds and crashes, the severity can be related to the speed, but the frequency can be related to the differential. And if that's too technical, we might be able to reword that <laugh> . Yeah.

Bernie Fette: 8:26

Maybe, maybe we can get into that just a little bit more in a minute, but I wanted to stay on that minimum for just a moment if we could, because I'm going from my own memory over many years of driving on various streets and interstate highways, it seems that I don't see minimum limits posted as often as I used to. Am I imagining that, or is it a less common practice than it used to be?

Marcus Brewer: 8:51

Well, each state has its own characteristics of course. Okay . And And not necessarily every speed limit sign on a freeway is gonna have that minimum. A lot of times, for example, you see more of those signs at a state line, you're entering the state for the first time and there's lots of additional signs to tell you what state laws are. Right . And those are not necessarily repeated everywhere. It doesn't mean that the minimum doesn't apply on the freeways, but it may not be posted as often. Okay. But definitely having that minimum speed on a freeway is important to help kind of keep traffic moving generally at the same speeds and helps prevent other problems that could be caused by fast vehicles and slower vehicles in the same place at the same time.

Bernie Fette: 9:31

Are there any conditions or circumstances under which an established speed limit can be changed? Sure. When you tell us about that, please see if you can give us an example of.

Marcus Brewer: 9:43

Sure. There's examples of speed limits being changed in a lot of conditions, a lot of circumstances where individual roads are reviewed, usually at the request of someone who lives nearby or drives it often, or maybe the road agency itself. But there's also occasions where an entire area or an entire set of roads might be reviewed as one unit. So an example of a specific road could be there is a perceived or an actual change in how the roadway is functioning, either because there's a lot more traffic on the road than it used to be, or maybe there's been some crashes. Crash history or near misses. Correct. Exactly. Yeah . And so the road agency will look and say, our vehicles traveling, first of all, that speed study that we mentioned earlier, our vehicles really traveling near that posted speed limit in the first place. And if they're not, then that's one factor, obviously. But another is, let's say that the road we talked about at the top of the discussion that's 50 miles an hour and there aren't very many driveways on it. Well, as the neighboring city expands and now there's more driveways and there's more intersections and more people turning in and out, that causes more potential for conflicts. And so we may want to reduce the speed limit in that area because there's more cars and they're doing more things entering and leaving the roadway to where a speed limit of 50 isn't realistic anymore. Or it may not be safe, or it may not be the best operationally. And so those speed limit studies are done to determine should it be changed? And if so, maybe what is that new number?

Bernie Fette: 11:19

And it sounds like what you were saying about the addition of driveways as an example, that could be the new development that you mentioned, you know, commercial development, more businesses out along that roadside or more oil exploration, more energy exploration. Mm-Hmm . <affirmative> so that you've got cars slowing down a lot for those driveways. And those are just conditions that change Mm-Hmm . <affirmative> not because of any other reason than the, the circumstances or the conditions of that roadway changed.

Marcus Brewer: 11:48

Correct. And it's not even necessarily the roadway itself, it's everything that's kind of happening around the road in the, in the surrounding environment, the roadside and the neighboring development.

Bernie Fette: 11:58

Yeah. Does public sentiment ever influence decisions about speed limits?

Marcus Brewer: 12:05

It can. And the procedures that are on the books, both within, say Texas State Code and, and within municipalities and other agencies really do talk about following the procedures of the study and the protocols and so forth to arrive at the number based on what the data tell us. And that's the ideal solution. That's the way that we try to proceed as a profession when those happen. But yet we know that there's people that drive on these roads and they have perceptions and there's things that they're involved in and things that they observe that, you know, may be difficult to quantify <laugh> in a study. And those perceptions may or may not be the most accurate. And that's why we'd have the study in place to help quantify what we see. But yet it can happen where whether it's a few people that are influential or whether it's just a lot of people that have a particular issue with how a certain road is posted , uh, in its speed limit, they can bring that to bear. And that can be influenced by that kind of feedback from the public. Ideally, that should still be backed up by a study that says the data reflects what those revised conditions are. And part of the reason why that can happen is because even if the traffic engineers do their study and make their recommendations, ultimately the governmental agency is responsible for making that decision. So if a city council wants to set an ordinance to say we're gonna change the speed limit, it may be that the data don't quite ideally line up with that at the time. Maybe it's projected to in the future, but because the issue is here, now we go ahead and make that change because that's what's being brought to bear on, on the people ultimately responsible for deciding.

Bernie Fette: 13:44

Yeah. Interesting that you would mention the city council because I remember my days in city government years before I came to work for TTI, it, it was not unusual for a decision like you're talking about to be made based in part on just how many citizens showed up to the city council meeting to argue their case. Mm-Hmm.

Marcus Brewer: 14:06

<affirmative> . And I think it's that way in a lot of different areas of life, but this is definitely one example where a lot of times the people are satisfied with what's going on. You know, people don't tend to show up at the meetings to say, Hey, great , keep doing what you're doing. It's folks that have an issue or perceived problem and they want to hear about it. And understandably so then the council members or whoever it is on that board listen to those comments and that it can affect their outlook on things.

Bernie Fette: 14:33

Yeah. If we could get back to the relationship that we were talking about earlier between speed and crashes. Mm-Hmm. <affirmative> , uh, I'm gonna toss out something and ask you to correct me or maybe edit what I'm saying. I'm trying to follow what you were saying earlier about the speed differentials. Okay . Is it fair to say that higher speeds don't necessarily contribute to crash frequency, but they have a lot to do with crash severity?

Marcus Brewer: 15:01

Yeah, I think so. Let's talk about, I guess kind of those two concepts of crash . Yeah . Your turn . So maybe I can clarify <laugh> , maybe I can clarify what I , what I referred to. So there's crash frequency, which is how often do crashes occur? And that's a lot of times the numbers that we look at crashes per month or per year or whatever metric that we're using to, to count that number. And then we have crash severity, which is, you know, did we just have a fender bender or were people injured? And if they were injured, how severely were they injured? What was the extent of those injuries? What we found on crash frequency is that in studies that have been done in Texas and elsewhere, the results are mixed. So yeah, there's not a clear relationship between just a speed and the frequency of crashes. You can't just say, the faster we go, the more crashes we're gonna have. That's not borne out by the studies that have been done. But what we do find is that higher crashes do tend to lead to more severity because higher speeds the faster you go. Yes. Higher speeds, the faster you go when you hit something <laugh> , chances are it's gonna cause more damage to your vehicle and to yourself. But what we then do find about crash frequency is that speed differential. So the difference in speed between the two vehicles that collide makes a difference. You can have crashes between two vehicles going at relative close to the same speed and you can kind of get your fender bender , especially both of those vehicles are kind of slow anyway. But in the case of the freeway where if you've got someone traveling, we'll say 70 miles per hour and they're coming up on a vehicle that's only traveling 45, they rear end that vehicle, that's gonna be a pretty substantial event. So those types of things do factor in, which is one reason why I'll put in a small plug for roundabouts, because I like to do that. Roundabouts slow people down at intersections. And so it eliminates the opportunity for what we call those T-bone crashes. And those are high speeds because everyone has to slow down at a roundabout. And so the crashes that do occur tend to be at, you know , say 15 miles an hour. And they tend to be at angles that are fairly similar rather than people approaching at right angles and going 35, 40, 45 miles an hour. And so those crashes at roundabouts are much more survivable, injuries are fairly low and even so are the damages to the vehicles.

Bernie Fette: 17:29

And if we look back, staying on this part of the discussion for just another minute, if we look back at the COVID-19 experience, we had far fewer people out on the road due to the fact that we were having remote work, fewer shopping trips, et cetera. Mm-Hmm. <affirmative> , lots of us expected fewer people on the road to translate to fewer crashes. But that didn't happen. There was actually an increase in crashes, many of them pretty severe. That was apparently attributed largely to speeding. Can you elaborate on that instance a little?

Marcus Brewer: 18:05

Sure. And what I'm gonna say with this is gonna be a mixture of both things we've studied and probably just some personal and anecdotal observation, but ...

Bernie Fette: 18:14

That's fair.

Marcus Brewer: 18:14

As the amount of traffic went down, people stayed home for whatever reason, either the motivation or the requirement to be off the road, that opened up the roads for everybody else who traveled them. And so , you know, in the past where let's say you're driving to work at 7:45 in the morning and so are a lot of other people, and all of a sudden a lot of that traffic is gone and that happened throughout the day. You can pick a different time of day where if most people are staying at home, that opens up the road. And so you have a sense for a driver that's traveling during that time that hey, this is kind of my road and I can be a little more flexible in what I wanna do. And probably the perceived threat of enforcement might be a little lower, whether that's reality or not. And all those things kind of contributed to then a factor of, hey, I've got somewhere to be and I'm out on the road by myself. I'm just gonna drive fast or as fast as I kind of want to. But then you do get into that situation where when you do drive faster, this is where some of those studies that we talked about earlier kind of differ on frequency of crashes. There is definitely an influence on if you're driving faster, you have less time to react to something that's on the road, you have less ability to respond to things, especially if you're distracted in that car again, if you think you're on the road by yourself, so I'm gonna be on my phone while I'm driving other things that can be at play. Yeah, those have an effect. And so then the speed kind of exacerbates that situation. And especially again for the injuries because then as people go faster and they run off the road and they hit something or they hit another car, then that makes a difference. And so people kind of get used to that, having the road to myself and then they see another car or something else can cause a crash. And that does lead to an increase in crashes.

Bernie Fette: 20:03

So since you were talking about personal observations, I'll make one myself, it sounds like some of the people that we were talking about may have had a inflated sense of their driving skills.

Marcus Brewer: 20:14

Anecdotally, yes. <laugh>.

Bernie Fette: 20:16

Okay. But fair enough. A anecdotally, yes. I know you're a scientist, I'm not gonna try and push you too far in that direction. <laugh> You and your colleagues have been studying speed limits for a lot of years. I'm wondering what might be the next thing on your research agenda. If a sponsor, whether it's TxDOT or Federal Highway Administration or somebody else, and hands you a big blank check and you could study whatever you thought was important and you weren't gonna be influenced on the conduct of that research by the people who were funding it, you get to decide what's important. What are you gonna study?

Marcus Brewer: 20:54

Well , that's a great question and I definitely like the concept of a blank check. I think some of that's gonna relate to what we were just talking about, about why do people choose the speed that they drive. The roadway design is obviously a factor in that. So if you have a freeway, it's gonna have wide lanes, it's gonna have wide shoulders, there's not gonna be any driveways on it. The curvature is, is nice and easy. There's lots of things that promote a higher speed. And so the design of that roadway lends itself to drivers selecting a higher speed. In the urban areas, you've got narrow lanes, you've got on street parking, you've got sidewalks, you've got perhaps frequent traffic signals and driveways and things that slow people down. And people are kind of used to that kind . Where we're looking at in the profession right now is kind of that place in the middle of say that 50 or maybe a 40 mile per hour type of road that we've talked about before that maybe it started out as one thing and it's changed to something else over time. Or as we look at a roadway that may have been suitable for the conditions at the time. And as things change , how can we either design the roadway or maybe influence the roadway environment near it to help encourage people to pick a speed that's appropriate. And there's a lot of factors that go into that that aren't even necessarily within the purview of the roadway designer. I mean, there's certain things that the designer can do to say, we have X number of lanes and we have a shoulder, we have a curb, or we have a median, or whatever those things might be. And we have guidelines that direct those decisions and those are being reviewed. But even so, if we have to your hypothetical driver a moment ago that may or may not have an accurate sense of that driver's ability, what is it that motivates that driver to select the speed that the vehicle ultimately travels? And especially within Texas, as we do have higher speed limits than most other states and 75 mile per hour on even things that aren't freeway, they kinda look freeway, but they are not completely free of driveways and intersections. And even on rural two lane roads that may have a lot of driveways, <affirmative> , but yet we see people applying freeway speeds to non-free roadways because they get comfortable driving the speeds that are consistent with those freeways. And so I, that gets into the psychology and human factors and there's a lot of other things that aren't directly related to the roadway, but I think it gives us some insight into that speed. But then the resulting operation of the road and obviously the safety of the road and are we building roads and roadway environments that are consistent with how people are using them? And right now we actually do have an ongoing project with National Cooperative Highway Research Program, the N-C-H-R-P-A federally funded program that is looking at the concept of target speed. And it really is right now emphasizing this kind of middle ground of say 30 to 50, 30 to 40 range of posted speed limits to say what should our target be? And and that concept is even developing on what target speed means. But the idea is that if I'm designing a road, how fast do I think people should be traveling on it? And that's my target. And it's related to how the roadway is designed because we want to design a roadway that's consistent with how fast we think people should be using it, but how can we pull in some of those other things that aren't specifically number and width of lanes? You know , how can we pull in some of those other things into the overall design process about planning and roadside environment and development. And I think if we had a blank check, those are some concepts, some areas that we would really like to dig into further to say what's going on in the driver's head, what is the performance of the vehicle they're driving and what's going on in the roadside environment that helps to contribute to that driver's choice to decide, I'm gonna pick 50 or 55 or 70 or 30 on a road that may have a number on a sign that's different from whatever that is that they chose. And it's some fascinating stuff and there's a lot of people in the industry, not just at TTI, but elsewhere in the profession that are looking at that. And that kind of seems to be where we're headed and it's really interesting stuff that people want to know more about.

Bernie Fette: 25:32

Definitely. Sounds like you've got some job security there.

Marcus Brewer: 25:35

There's no shortage of people speeding, let me put it that way. <laugh>.

Bernie Fette: 25:39

Okay. Anything you'd like to add that I haven't asked you about?

Marcus Brewer: 25:42

Couple of things. One, you mentioned colleagues and they , some of the things that I've been talking about actually have been developed by others at TTI for TxDOT-sponsored projects. They've put together some discussion on how speed limits are set and and how they go. And Kay Fitzpatrick led that project among a number of other speed limit related items. Mike Pratt and Steve Venglar were part of that. And so I acknowledged their contributions and putting some of that material together more recently and they provided some resources that are out there for that. But also just, I would just say for the average driver, the person that's not necessarily in the transportation research realm, as you're driving down the road, think about what it is that motivates you to choose a speed. Some of it is, you know, I have a deadline to be somewhere and so I need to get there in a certain amount of time. And that's obviously a factor. But as we're driving somewhere, if I'm in town, am I looking for other vehicles? Am I looking for pedestrians or cyclists? Am I anticipating where that that next traffic signal or roundabout is? Um, if I'm out on the highway outside of town, am I just putting the gas pedal down and going, or am I thinking about what's gonna come up maybe around the next curve , over the next hill? And why do I drive as fast as I drive? And it'd interesting to know kind of what people think about that and maybe that helps motivate some people's driving decisions as they're going along about their daily lives.

Bernie Fette: 27:09

You mentioned what motivates driver decisions. So speaking of motivations, what is it that motivates you to show up to work every day?

Marcus Brewer: 27:20

That's a great question. I wanna be able to provide resources, information, et cetera, that help us be able to allow the public to get from place to place more safely, more efficiently, and hopefully even enjoy where they're going. And so if I'm able to do things in my work that give us a better transportation system so people can get from place to place and we can get things in the stores better and we can visit family and friends and enjoy things that we need to do and do it safely and efficiently, then I think I'm doing my job.

Bernie Fette: 28:00

We've been visiting with Marcus Brewer, a research engineer at TTI. Thanks so much for joining us, Marcus, and helping us understand at least some of the mysteries behind speed limits. We really appreciate it.

Marcus Brewer: 28:13

My pleasure. Thanks for the invitation.

Bernie Fette: 28:17

That road you travel most days -- the one that takes you to work, to school, to your favorite restaurant or shopping center -- what does that road look like? Four lanes or more? Traffic lights? Lots of businesses and driveways, or maybe just a few? All of those things are factored into deciding what that road's speed limit is going to be. Oh, and there's one more, the reasonable and prudent behavior of the people who use that road. So the next time you're driving along and wondering "who are the people who decided that this was going to be a 40 mile an hour road." Just remember -- it's possible that you are one of those people. Thanks for listening. Please take just a minute to give us a review, subscribe and share this episode, and please join us next time for another conversation about getting ourselves and the stuff we need from point A to point B. Thinking Transportation is a production of the Texas A&M Transportation Institute, a member of the Texas A&M University System. The show is edited and produced by Chris Pourteau. I'm your host, Bernie Fette. Thanks again for joining us. We'll see you next time.