The science and research behind speed limits

Show Notes:

The guest: Dr. Timothy Gates, professor of highway design and traffic engineering at Michigan State University.

Topics include:

— Why we have speed limits and the history behind them in America.

— Some argue we could go without speed limits and drivers would self-regulate. Those people always cite the autobahn, including a lawmaker in California earlier this year.

— Speed limits on some Michigan freeways increased to 75 mph in 2017 based on requirements adopted by the Legislature. But some critics cite data that shows that the trend toward hiking speed limits has resulted in 37,000 additional deaths since 1993. Charles Farmer, vice president for research and statistical services for the Insurance Institute of Highway Safety (IIHS), says too many people think about the few minutes they're saving by speeding but not about the risk.

— Why officials in Oregon decided to roll back speed limits after some severe crashes in eastern Oregon.

— What research tells us about roundabouts, and why they make sense. Dr. Gates was part of a team studying how to educate the public on safely navigating roundabouts.



Narrator: It's time for Talking Michigan Transportation, a podcast devoted to the conversations with people at the forefront of the ongoing mobility revolution. In the state that put the world on wheels, here's your host, MDOT communications director Jeff Cranson.

Jeff Cranson: Hi, welcome to the Talking Michigan Transportation podcast. This week I have a very special guest: Dr. Timothy Gates, an engineering professor at Michigan State University, and one of the state's foremost experts, and at least in the research of speed limits - and he'll probably say that I'm overstating that, because he's a humble guy - but thanks for being here Dr. Gates.

Dr. Gates: Thank you, Jeff, and yes, you definitely are overstating that. We've got a great team here at MSU that's been studying speed limits for quite a long time, so I certainly can't take all the credit for that.

Jeff Cranson: Well, let's talk about some of the work that your team has done on this, and first give a little bit of your background so people understand why I've selected you to be the voice on this topic.

Dr. Gates: I led - or co-led - two studies, largely when I was at Wayne State back about - starting maybe 2012 that investigated studying speed limits. Actually, it first started out with the differential speed limit between cars and trucks on freeways, and then it evolved into looking - once the legislative discussion chatter started to ramp up with respect to raising the 70 mile per hour limit, actually, at that time, up to 80 miles per hour, we modified the scope of our current research project to also include looking at interstate and other freeway speed limits, and then that took another turn when the speed limit issue at the legislative level opened up a little bit more to all high speed roadways, so like, our two-lane trunk line roadways in rural areas, and so then we opened the project up just, really to basically all rural highways, so basically investigating what should be the appropriate speed limit on rural highways. Since then I've been involved with some work at the national level, and national cooperative highway research program project that's sponsored by the Transportation Research Board that's investigating speed limit guidelines, basically, is the crux of that project, development of speed limit guidelines for the United States, and so as a part of the projects I mentioned earlier, helped MDOT come up with an appropriate list of roadway segments in rural areas, both on freeways, and then non-freeway-type roadways, that could support, from a safety standpoint, that would be appropriate, or I guess highest priority candidates for higher speed limits. Acknowledging the fact that this might become state law, and we're going to have to, as a state, be ready to go with some highway segments, so that’s really, it's kind of a background.

Jeff Cranson: So, talk a little bit about the history of speed limits in the country. I know that from about the earliest days of the automobile, at the turn of the 19th to 20th century, that’s when discussions began, and I think from my research Connecticut was the first state to actually have speed limits, but can you talk about that history back in the early days of automobiles?

Dr. Gates: You know, speed limits weren’t a big issue because the roads were in pitiful shape, and the vehicles really didn't have much horsepower, so you couldn't really get up to speed, but it's really about - you know what, I don't know, the details are a little bit fuzzy, but you know, at some point vehicles and roadways became smooth enough, and there were enough roadways connecting cities, and the vehicle performance had improved, and these things are all interlinked, obviously people could actually drive fast, and then of course the proliferation of automobiles. So, once you started to get into like the 20s, 30s, 40s when there was more long-distance travel, now you started to see speed limits pop up, both in rural and in urban areas. Let’s fast forward, then, to about the 1970s when the Middle East oil embargo was in effect, and what happened then was because oil prices were so high, and there was a large fuel shortage, with the embargo, prices were high, and so the federal government basically mandated a national maximum speed limit for all types of roadways, including our interstate highways that we all know today are, in Michigan, are posted between 70 and 75 miles an hour, typically, in rural areas. Back in the ’70s, with those same designs, they were 55 miles per hour, but we found that people weren’t driving at 55 on these, especially on the limited access freeway system, and so it's difficult from a compliance standpoint. If we fast forward into the mid-90s the federal government relaxed that requirement. Obviously, the Mideast oil embargo was long gone at that point, but the national maximum speed was still in effect. They relaxed that on certain rural interstates, and then in Michigan you started to see some 65s pop up, like I-69 around Lansing, and then released it again fully, so basically turned speed limit setting authority back over to the states entirely on all types of roadways, and then we saw almost all of our freeways, within a few years, jump back up to 70 miles per hour in rural areas, and then of course that lasted until two years ago in 2017 when things went up on select freeways to 75 miles an hour.

Jeff Cranson: There's a lot to cover there, but speaking of - when we made that change in Michigan on some select freeways, jumping them to 75 a couple years ago, and some two lanes going to 65 - I know that the study is still out, the jury's still out on that, and that's still being studied to see what kind of a difference it's made in terms of crash rates, but do you have any sense of that, preliminarily?

Dr. Gates: I don’t have any sense of how the crash rates have changed, and it's, really, it’s a complicated issue. So is it crashes in total, is it injury crashes that'll change, is it fatal crashes that'll change? So, you really have to look at both the crash frequency, or crash occurrence, and also the injury severity of those crashes as well, and I really don't have anything to comment on at that point, because we only really have one year of after-data available at this point. I do know that speeds, the actual travel speeds, have certainly ticked upward slightly in those locations where the speed limit has jumped to either 75 on freeways or 65 on two-lane highways.

Jeff Cranson: But not necessarily proportionately, so in other words, if a lot of people were traveling, say, 78 when the speed limit was 70, it doesn't necessarily mean that everybody’s going 83 now that the speed limit is 75, right?

Dr. Gates: That's right. So what we’ve seen is that, in general, and these are kind of rule of thumbs, that about, so, on freeways that jumped up by 5 miles per hour from 70 to 75, we saw that travel speeds on average went up about 1 to 2 miles per hour, say, and then the two-lane roads where the speed limits went from 55 to 65 ,so a 10 mile per hour jump, we saw about a 3 to 4 mile per hour jump on average, in the average travel speed.

Jeff Cranson: It does feel a little like, you know, I liken it to the arms race, and that we keep building better bombs, and then we keep building better armament, and the cycle continues. It feels like that with cars where we're coming up with all kinds of driver assist options that make our cars safer. Whether it’s lane assist, or rear assist, or adaptive cruise control, auto braking, and we know that’s making a difference, but then we come up with more distractions, and we raise the speed limits. So, we potentially offset those gains. Without taking too strong of a stand on this, what do you think about the, kind of, libertarian point of view, like a bill introduced in California earlier this year, and others in the country, who say ‘you know, the freeways in the United States should all be like the Autobahn?’

Dr. Gates: Well, I think it really does come down to distractions. I mean, you're absolutely correct in terms of modern vehicles having so much more power, and you know, at very high speeds, now your car doesn’t rattle like it may have 20 years ago, or 30 years ago, whatever, but cars are built better, they have far more safety features, but we have that one human element that’s really becoming a problem, and that’s distractions, largely from smartphones, but from other things as well, and that’s really the problem here, and is a primary reason why we continue to advocate for reasonable speed limits, and reasonable travel speeds. It's like the vehicles are getting better, but the humans aren't, and that's part of the problem. Now, someday when we're in an automated system, or we have at least a mix of automated vehicles and human operated vehicles, we can start looking at the changes, and shifts in crash patterns and injury severity over time, but for now we're still in a human operated environment, and humans are more distracted than ever.

Jeff Cranson: The real problem is that most of us think that the other humans aren't very good drivers, but I certainly am.

Dr. Gates: Well sure, everybody likes to think of themselves as the best driver in the world, but we all make mistakes from time to time, and we all get distracted from time to time in one way or another, and you know, you never know if you're at the wrong place at the wrong time when you’re distracted. It might be a problem from that standpoint.

Jeff Cranson: That goes to why - and I know you're definitely onboard with this - why we've been pushing for a long time to call them “crashes,” and not “accidents,” because it is the human element that is a factor, and the estimates are always 94% to 95% of crashes are caused by human error. I actually think the numbers are higher than that.

Dr. Gates: Yeah, I would agree with that, there's very few crashes are caused by a vehicle defect or a road defect, a lot of it is really human error.

Jeff Cranson: And even then, that would be human error because some human designed the road, or the vehicle.

Dr. Gates: Oh sure, I’m talking about drivers though.

Jeff Cranson: Sure, I know, I understand. Well, I guess, where do you see this going? I mean, it seems like a national trend. There just seems to be a lot of pressure across the country on this issue, and it’s been this way for a few years now, and you know, you're called to testify, and you're called on as an expert, and you’re studying these things in an ongoing way, and you're trying not to be the sole arbiter, you know, you're not the judge and the jury on this, but I mean, do you think this is just going to be an ongoing issue, do you see any reason it would trail off?

Dr. Gates: Yeah, I mean, I do see it being an ongoing issue. At some point we’re going to reach our limit, I would like to think, in that. I mean, I think we all have our limit with our vehicle’s capabilities and just really kind of what we're comfortable with, and in many cases - I mean, for me personally, and not admitting to speeding at all, but that really is in the upper 70s, around 80, where even on a straight, flat, perfectly designed section of roadway, it’s getting to be a little bit uncomfortable at that point. Everybody has that limit, so if we had, like let’s say, an 85 or 90 mile per hour speed limit on an interstate, I'm not sure if I would drive that, and I've driven out west on roads that are posted at 80, and sometimes I'm driving a little bit under the speed limit in those cases. Especially at night, I just don’t feel comfortable driving that fast for, you know, a deer it's going to runout and in front of me

Jeff Cranson: Or a buffalo.

Dr. Gates: Sure, out in Wyoming, a buffalo, but - so I do think there is a limit, and you know, the limit wasn't 55. The 55 mile per hour limit back until the mid-90s on our interstates, that was unrealistically low speeds. The roadways were designed for higher speeds than that, but I really do - we’re kind of approaching what I would call a “design limit,” at some point, and maybe that’s 75, maybe it's 80, I'm not sure, but at some point I think we're going to be reaching the driver comfort limit.

Jeff Cranson: Well, talk a little bit about that, because I think that the layman doesn’t begin to know what goes into this. They think that changing speed limits is just about going out and changing the sign, and don't understand the geometrics involved, and you know, banked curves, and all the things that go into road design factor into that.

Dr. Gates: Exactly, and so and I do teach a highway design class here at MSU and I taught it at Wayne State as well but the primary um guiding principle for designing a roadway, especially in a rural area at higher speeds it's really the geometry. so, we're talking about like the curves, horizontal curves, and also the hills and valleys, and we call those the vertical curves, and those are designed for a specific speed that should be at or above the speed limit. so, the design speed should meet or exceed whatever the posted speed limit is, just from a safety standpoint. and so, these design principles when we talk about things like site distances a comfortable curvature, I mean it really comes down to and I just said it comes down to designing for a driver comfort. we don't want to have drivers driving with let's say sight distances or sight lines that are kind of right at the emergency threshold, so you’d have to like you know slam on your brakes in order to stop, we don't want to, that’s an uncomfortable driving situation, so we provide, we design around a little bit of a buffer to give drivers a bit more of a comfortable driving environment.

Jeff Cranson: So, you know, nobody - most people, I should say - if you weren’t a traffic safety engineer, or perhaps a police officer, even knew what the 85th percentile was until a few years ago, and then when this legislation became debated in Michigan, and reporters started asking about it, and you see it in stories now, and you hear people talking about it - explain what the 85th percentile is, and why or why not it should be the sole criteria for setting speed limits.

Dr. Gates: Sure, so the use of percentile speed is, as the name would imply, is the 85th, if you were to rank, order sample of speeds, let's say a hundred speeds, you rank them from lowest to highest, you'd pick the 85th highest, and that would be the 85th percentile speed. So, that means that 84% of the motorists, or I guess you could say 85%, are going at or below that speed, right? So, only 15% are going above that speed, and the idea was if we set our speed limit at that level, if we go out and collect a lot of data, and we find what the 85th percentile was, if we set our speed limit at that level that it would be reasonable and realistic from an enforcement standpoint, so that law enforcement could focus on those that are above that 85th percentile limit, and everybody else would be traveling at or below, and the idea was that, in terms of setting speed limits, again, if we designed a road for a certain speed, that we should be using the 85th percentile as one of many factors to set a speed limit, and again, this is probably a little bit more of a rural highway thing versus an urban sense where you have traffic signals, and stop signs, and pedestrians, and bicyclists to worry about. It's a little bit different in an urban area, but on a rural highway the 85th is a very good tool to use at least as guidance, or a kind of a starting point for studying speed limits. Now, there's other risk factors that would go into that, you know, the number of curves per mile, hills, if you’ve got a school on this segment, or if you've got speed reduction zones, certainly work zones, construction work areas, interchange ramps, those sorts of things, driveways, cross roads - those are all factors that come into play as well, but if you just took a garden-variety highway segment that was straight and flat and had none of those features on it, then the 85th percentile speed would be definitely your primary factor in deciding the speed limit.

Jeff Cranson: So, I guess that’s a good way to say it, primary factor, but never really should be the sole factor?

Dr. Gates: No, it shouldn't just be the sole factor. There are always other factors, or other conditions, that should play a role, and we have to think, too, about, like, weather conditions, and darkness, and deer in the state, of course. I mean, there’s always these unpredictable situations, weather and deer probably being the most unpredictable part of that that we really can't control for, that go into this, that we need to consider from a safety standpoint that's out of our control. So, we want to always consider these things, well, maybe we don't want to push that speed higher if it were kind of in a snow band area, or something like that. You know, these things always play a role.

Jeff Cranson: Are animal crossings something you’ve been involved in studying, because I know in Alberta and Western Canada, they've done some creative things that seem like they've had some success, with tunnels underneath the freeway that, you know, they direct elk.

Dr. Gates: Yeah, I’m aware of them. I haven't really done anything from a design standpoint, or from recommending where they might go. You know, the old joke goes it's not like the deer can read the deer crossing sign.

Jeff Cranson: Right, so how do they get those elk to use those? I mean they’ve got to base them on natural migratory patterns and figure it out.

Dr. Gates: Sure, they're definitely based on migration patterns, and I think the DNR would be, in these states, are probably called upon to assist, but they're definitely based on some sort of migratory routes. You wouldn’t necessarily just put them anywhere. In Michigan, with just a standard deer population they're everywhere, right? I mean, it would be impossible to really identify, in most cases, I shouldn’t say all cases, but it'd be very difficult to identify specific crossing routes. I mean you have them everywhere, right?

Jeff Cranson: They’re everywhere, but there are fewer.

Dr. Gates: Yeah, I think it’s something, maybe, the state has looked at, or would desire to look at, but personally, I haven't been involved in those sorts of studies for animal crossings.

Jeff Cranson: This could be a whole other podcast; I could talk to somebody at the DNR about it.

Dr. Gates: Right, but deer should be taken into the equation, right, because when we talk about - remember earlier, I said that we are seeing speeds pick up a little bit with these increased limits, you know, by about 2 miles an hour on interstates and other freeways, and then maybe 3 to 4 miles per hour on these 65s. I mean, when you talk about physics or energy, I mean, you're increasing the energy by the square of that speed difference. So you know, there is definitely - when you have higher energy and impact, and it has to go somewhere, so I mean, your likelihood for an injury in a higher speed crash is always going to be higher, if everything else stays the same, all the other factors stay the same, your likelihood of an injury, and that includes with collisions with a deer, obviously collisions with another vehicle, or a fixed object. So, if you remember back, early in this podcast, I did mention it might not just be the pure number of crashes or crash occurrence rates, but also are those crashes that are occurring becoming more severe? So, what used to be just a property damage crash, maybe now it's an injury crash at that higher level of speed.

Jeff Cranson: That’s definitely, that's played out as people have raised speed limits, and that’s why, you know, I talked to my kids when they were in driver’s training about living in the city versus living out in a rural area, that you’re less likely to be in a crash on the interstate, but if you are, it's more likely to be pretty severe.

Dr. Gates: Right, yep, and you know, one other thing that kind of gets back to a question you'd asked earlier about are we just going to be in this perpetual increasing speed limit mode - I do want to comment on the state of Oregon, and the state of Oregon actually did monitor two rural highways where they had increased speed limits, and they had some catastrophic fatalities on those highways, and they actually rolled the speed limits back to their original level, and that's one of the very few cases, recently, that we’ve seen, where a state has actually rolled the speed limits back based on evidence of a safety problem.

Jeff Cranson: Yeah, I do remember reading about that, that’s interesting. One other thing I wanted to touch on, because you've participated in some research in this, and this is always a hot topic, is the use of roundabouts. You know, where I live in Grand Rapids they’ve proliferated, and I think with success, although there are still critics and detractors, and there are still people that stop, and don't go by the rule that I heard about in Boston a long time ago, which has had rotaries for years, as you probably know, and they always said the best thing to do is not make eye contact, but what’s your research tell you about roundabouts?

Dr. Gates: Well, I can say a few things about roundabouts. One is that the more complex the roundabout gets, the more likely you are to see some crash or safety issues, for example, like comparing just a single lane on the inside, a single circulating lane versus a 2 or 3 lane inner circle, and some of these double roundabouts, I mean those are always going to be more complicated than just your single lane roundabouts, and are going to lead to a higher likelihood for crashes. Single lane roundabouts generally operate pretty well, but there's such a variety in the - I would say that the multi-lane roundabouts are much less ‘off-the-shelf,’ if that makes sense, or they're quite a bit more customized from roundabout to roundabout, and we see a whole variety of safety performance issues at those types of roundabouts, you know, we could have great safety performance, but we could also have some that aren't doing quite so well.

Jeff Cranson: For the most part.

Dr. Gates: Right, for the most part, though, we just, in total, for the most part, the science will tell you that roundabouts do reduce injury crashes. They might not reduce all crashes, but they generally reduce injury crashes, because you don’t have vehicles colliding at a right angle anymore, like a T-bone type crash that you get like when somebody runs a red light at a signalized intersection.

Jeff Cranson: Yeah, that's a good reason, and a good secondary reason is that they do a lot to eliminate emissions and reduce emissions.

Dr. Gates: Sure, they definitely would do that as well. I personally haven’t studied that aspect, but I can tell on the safety side they definitely reduce a right angle. So, instead of having a 90 degree collision, you're at a glance angle, or sideswipe, and those generally are much lower, and there are lower speeds as well, but those generally have a lower severity than a right angle crash, but it really is about that speed reduction on the entry to a roundabout. You don’t see these super high speeds coming, you’re just, you don't, because you’re forced to, in order to negotiate around the circle, you're forced to a lower speed.

Jeff Cranson: Yeah, well, that's very helpful. This is good stuff, and I think we're going to have to keep talking about speed limits and roundabouts for a while. I know that our social media feeds at MDOT are always good for a couple questions a weekend on roundabouts.

Dr. Gates: Yeah, great.

Jeff Cranson: But thanks for taking the time to do this, Dr. Gates, I appreciate it, and thanks for doing important work to try to make us all safer.

Dr. Gates: You're welcome and thank you for the opportunity to participate in this podcast.

Narrator: That's a wrap for this edition of Talking Michigan Transportation. Checkout show notes and more on Soundcloud, or by subscribing on Apple podcast.