Archive for the ‘Uncategorized’ Category

I was grooving on this almost Ed Ruscha-style illustration (”27 Onramp Configurations”?) in a new paper from David Levinson and Lei Zhang, “Ramp Metering and Freeway Bottleneck Capacity,” in Transportation Research: A Policy and Practice 44(4), May 2010, Pages 218-235.

The findings were sanguine on ramp metering:

Traffic flow characteristics at twenty-seven active freeway bottlenecks in the Twin Cities are studied for seven weeks without ramp metering and seven weeks with ramp metering. A series of hypotheses regarding the relationship between ramp metering and the capacity of active bottlenecks are developed and tested against empirical traffic data. The results demonstrate with strong evidence that ramp metering can increase bottleneck capacity. It achieves that by:

(3) and increasing queue discharge flow rates after breakdown – the average queue discharge flow rate is 3 percent higher with metering than without (with a 3% standard deviation).
Therefore, ramp meters can reduce freeway delays through not only increased capacity at segments upstream of bottlenecks (type I capacity increase), but also increased capacity at bottlenecks themselves (type II capacity increase). Previously, ramp metering is considered to be effective only when freeway traffic is successfully restricted in uncongested states. The existence of type II capacity increase suggests there are benefits to meter entrance ramps even after breakdown has occurred. This study focuses on the impacts of ramp metering on freeway bottleneck capacity. The causes of such impacts should be more thoroughly examined by future studies, so that the findings can provide more guidance to the development of ramp control strategies. It should also be noted that both types of capacity increases on the freeway mainline are at the expense of degraded conditions at the on-ramps and possibly arterial network. Therefore, without more comprehensive system-wide analysis, the findings of this paper, though in favor of ramp metering, do not necessarily justify its deployment.

Final nail.

Photo by hurryqi/Flickr

Photo by seamoor/Flickr

We’ve seen that slime mold can function as an incipient urban planner, but leaves offer lessons too, notes the Economist:

Traditionally, biologists have celebrated the trunk, branch and twig system of a tree as no accident. Many mathematical formulas have suggested it is the best, least wasteful way to design a distribution network. But the very end of such a network, the leaf, has a different architecture. Unlike the xylem and phloem, the veins in a leaf cross-link and loop. Francis Corson of Rockefeller University in New York used computer models to examine why these loops exist.

From an evolutionary point of view, loops seem inefficient because of the redundancy inherent in a looped network. Dr Corson’s models show, however, that this inefficiency is true only if demand for water and the nutrients it contains is constant. By studying fluctuations in demand he discovered one purpose of the loops: they allow for a more nuanced delivery system. Flows can be rerouted through the network in response to local pressures in the environment, such as different evaporation rates in different parts of a leaf.

It’s interesting to think of this configuration vis a vis urban/suburban street networks, when less permeable systems push traffic to larger arterial systems — a benefit for those living in the less permeable areas (say, the second photo above, which I believe comes from a stalled subdivision in Florida), until of course there’s some traffic issue on the main line and less opportunity for rerouting flows. The leaf has no cul-de-sacs, no dead-ends.

You may have heard the news: Cars that accelerate inappropriately down local streets, veer out of control on rural highways late at night, fail to brake in time to strike a pedestrian, follow lead vehicles too closely to stop in the event of an emergency, and so on. There was a technical problem in all these cases, but one that, I’m afraid, is difficult to fix with a factory recall, for I’m talking about the human decision-making apparatus. Towards this end Leonard Evans provides some much-needed perspective about the Toyota recalls:

Consider: According to various reports, 19 deaths have been associated with Toyota’s gas pedal problem over the past decade. But over the same decade, a total of 21,110 people have been killed in Toyota vehicles, with an additional 1,261 killed in Lexus cars (based on analyzing 1999-2008 fatality data from National Highway Traffic Safety Administration). Almost none of these deaths had anything to do with technology, faulty or otherwise. Almost all of them were the result of driver behavior.

So if the prospect of a sticky gas pedal alarms you, just slow down a little. The result will be that you are safer with the defect than you were without it.

Obviously, deaths linked to faulty cars are a serious problem, and it’s also clear that if attention is not paid, the safety problems could grow much worse. And still, however, I am struck by the sheer volume of the coverage about Toyota — almost verging on a panic — given the comparative risk posed in the numbers above. The study of risk perception is instructive here: Risks seem to loom larger in our imagination when they are novel, and when they are seemingly out of our control, among a host of other factors. Toyota is certainly novel, and the idea that an accelerator might suddenly activate on its own fills us with much more dread than the calculated decision to drive very fast down a street — itself a risk for the drivers and others but seemingly under one’s own control.

There’s a larger story here too, of course, which I was talking about last week with a writer for the Globe and Mail; i.e., the kind of shattering (or cracking) of a mantle of sheer confidence in not just the Toyota brand but the idea of the modern automobile as more or less infallible. When I think of my MacBook Pro or iPhone, I think of wonderful devices that are also prone to bugs (the later device had to be swapped out three times). But thinking about my Subaru, another incredibly complex device, I basically expect that as long as I take it in for its regular maintenance plateaus, I do not expect to encounter any difficulty on the road (needless to say, the experiences at the Genius Bar and Subaru dealership are distinct; one is tense anticipation as I wait to hear the diagnosis, the other is simply showing up to check off the list). Like many other drivers (or at least I suspect), I barely cracked the owner’s manual (this was studied Talmudically in my father’s era) when I bought the car, and certainly didn’t spend much time under the hood because, quite simply, I wouldn’t have understood much of what I was looking at (nor, for the record, do I take apart the MacBook). One still sees articles in the AAA magazines and the like with “driving checklists,” a tally of things you should do before setting out, but I would guess that very few of us do this, for a very simple reason: It has become an article of faith that the car will perform. This contrasts with the situation when I drove used American cars of 1970s vintage as a teenager, during which I experienced all kind of random breakdowns, faulty gas gauges, blinking ‘check engine’ lights that seemed to come on, as if by a law, late at night far from an open service station.

It’s hard to quantify, but I imagine this sense of the machine’s infallibility has changed the way we operate it. It is known that average speeds and following distances changed over time on certain highways, causing engineers to rework their models, and one of the reasons given is, inevitability: Superior handling and performance of the modern car. In this respect, all the coverage given to Toyota is a good thing — if it serves as a reminder of the risks of the road. If it merely shifts further focus away from driver behavior and onto a large, litigable car-maker, this won’t mean much in the overall picture of traffic safety.

Writing that “previous design guidance was influenced by documents such as the AASHTO Green Book, which is inappropriate for urban streets where modes of transport other than the automobile are present,” Nelson/Nygaard has made available its Abu Dhabi Street Design Manual, which provides guidance to “design streets that create a safe environment for all users; transition from a vehicle-trip based society to a multimodal society; introduce fine-grained street networks into the existing super-block pattern.”

It is, they suggest, “perhaps one of the most progressive in the world.”

Judge for yourself here.

Photo by Pat Linsley/Flickr

The town of Ashford in the U.K. was one of the highest profile experiments in ’shared space’ when it launched over a year ago. The changes, meant to reconnect the center (severed by a hostile ring road) and make the town feel more ‘town-like,’ were quite radical — removing signals, blurring formal notation of right of way — as well as drawn from more traditional traffic-calming approaches (special pavement treatments). The reaction ranged from skeptical to hostile (Jeremy Clarkson, whose opinion on anything but the braking ability of an E-Class Mercedes should be heeded with a yellow flag, predicted ‘millions’ would die).

Ben Hamilton-Baillie, one of the leads on the project (he appears in Traffic), has sent along a Q&A he did with a local paper on the status of the project after one year. Though such things need continual monitoring, the early prognosis is positive and a sign that fresh thinking in terms of the built environment and accommodating traffic can bring good results.

You can read what Ben had to say after the jump. (more…)

Driving home from the Yale event last night (which was packed, and filled with all kinds of interesting traffic types, ranging from Norman Garrick to Anne Lutz Fernandez), as I was listening to various renditions of La Boheme on Doug Fox’s wonderful program (Mr. Fox, I didn’t catch the details on that second act), which I discovered for the first time, a warm presence amidst the eerie fog-tinged, arc-lighted Stygian gloom of I-95, I was thinking back to Donald Shoup’s reply to a question I had posed to him, which itself was related to Brian Pijanowski’s study of parking-lot sprawl in Indiana. Despite a huge and quantifiable overabundance of parking in the county he studied, he was interested to note that people still complained “there wasn’t enough parking.”

I asked Shoup, who of course from the groves of academe has helped ignite a quiet but fomenting revolution in parking policy, to what extent this question of perception in the parking equation had been studied or quantified — keeping in mind that perception is a crucial, if often under-appreciated part of the traffic/planning nexus (e.g., commute times, etc.). One part of Shoup’s answer stuck with me: He talked of studying a parking garage in West Hollywood. On the bottom floors, there were cars, and in the empty spaces, plenty of oil stains to indicate past users. On the upper floors, he noted, it looked as if the spaces had never been graced by a single car. And yet the word from drivers was that there was ‘nowhere to park.’ But the problem, Shoup noted, is that drivers’ perception parking supply is informed by the parking spaces they can actually see. Call it “parking availability bias” (ode to Tversky and Kahneman). And the spaces that are most easily seen, of course, are curb spaces, hence the importance of rational market pricing policies to ensure turnover and vacancy. A few empty spaces (15%) can go a long way.

This perception is a powerful force and leads cities into all kinds of policies that turn out to be misguided and rife with unintended consequences; take the “free holiday parking” approach. Towns hoping to lure shoppers downtown, away from the big boxes, offer up free parking. But beware the power of incentives: Given that many of the best parking spaces in front of local businesses are often occupied (it happens right here in Brooklyn) by the store keepers themselves, the free parking bonanza ends up actually enticing local employees (who would have parked elsewhere or not driven) to grab some free real estate for the day — leaving would-be shoppers with the perception (all-too-real in this case) that there’s ‘nowhere to park.’ Here’s how it went down in Providence.

This is a case where ITS may prove quite useful: Let the algorithms, not fallible human perception, guide the driver to the (properly priced) parking. In the meantime planners and politicians should take parking complaints with a healthy dose of salt.

Photo by misskyra/Flickr

In my drive-through piece a while back, I speculated the numbers about drive-throughs somehow being environmentally superior to parking lots might be off for a number of reasons, including the idea that some drivers use the drive through and then park. In any case, I came across this curious sign that seems to connote just that behavior, rather paradoxically.

Calling New Haven-ites (New Havinians?): I will be appearing in a talk with the estimable Donald Shoup next week at Yale University.

As I note in Traffic, there has been some research into parking and gender, ranging from the indefatigable John Trinkaus’ “informal looks” at “No Parking — Fire Zone” violations at a shopping mall (women driving SUVs were the leading offenders), to suggestions that female drivers spent more time searching for the “best” parking spot (to which the above cartoon alludes).

Now, Claudia C. Wolf and colleagues at Germany’s Ruhr University-Bochum have explored the idea of parking ability, in a new paper in Psychological Research titled “Sex differences in parking are affected by biological and social factors.”

As the authors note, some previous work has found men to have a slight edge on certain cognitive tasks involving spatiality, in particular the “Mental Rotation Test,” while women have, in some cases, outperformed men on more language oriented spheres, like the “phonological retrieval in the letter fluency task.” But real-world equivalents for things like mental rotation have not been in abundance. Which is why the authors headed to the parking lot.

During everyday life—and obviously especially during parking—individuals are required to imagine themselves from different perspectives, which involves mental rotation. A driver who steers towards a parking space must predict the outcome of spatial relationships between objects (including own car, parking space, further cars, and kerb) after changes in viewpoint, which arise from the car’ s—and thus the driver’ s motion.

But curiously, they note, “the cognitive mechanisms involved in parking have never been investigated.” Of course, it’s not just innate spatial ability that’s involved; confidence in one’s ability to do the task matters as well. This belief is “domain specific,” and can socially conditioned by stereotypes, etc.

For the test, the authors asked subjects, divided into similar levels of driving experience, to park an Audi A6 in various ways (back in, parallel, etc.) in a closed-off multi-story car park. The result? “The present data revealed a sex difference in parking performance in driving beginners as well as in more experienced drivers.” Women took longer to park the car, which might be seen as an offshoot of lesser risk-taking behavior by females in driving, but interestingly, even though men parked more quickly, they also parked more accurately, as measured by distance to neighboring cars.

Before we get into a whole “are men or women better drivers” argument, let’s remember that men also come out on top on another variable of driving performance: The tendency to get oneself killed or injured. And this was a relatively small sample. And it’s just parking, after all. And then there’s that question, raised by the authors themselves, of how much this is in any sense innate — always a dangerous word — and how much is generated by social expectations or other feedback loops along the way:

Additionally, unequal base levels of parking performance — which could be due to unequal spatial skills in unexperienced drivers — may have resulted in differential feedback during training of parking skills, leading to a change in self-assessment and thus differential behaviour and achievement… [I]n a recent driving simulator study, it was found that women, whose self-concept was manipulated by confronting them with the stereotype that females are poor drivers, were twice as likely to collide with pedestrians as women who were not reminded of this stereotype.

Strangely, just after reading this paper early yesterday, I came across an item in the BBC about new ‘car parks for women’ in China.

The women-only car park in Shijiazhuang city is also painted in pink and light purple to appeal to female tastes.

The parking bays are one metre (3ft) wider than normal spaces, he said.

Were the Chinese government ministers reading Wolf, et al’s paper? And are they taking a potentially biologically innate, and perhaps marginal, difference, and whipping it up into an ever-perpetuating, and debilitating, social construct of drivers with particular, gendered needs? (not to mention the environmental impact of all that extra asphalt — three feet times the soon to be many, many millions of Chinese women drivers).

(thanks Peter)

For those looking to explain why the U.K. has made comparatively greater advances in traffic safety than the U.S. over the last few decades, urban speed zones are one good place to look.

An article recently published in the British Medical Journal, “Effect of 20 mph traffic speed zones on road injuries in London, 1986-2006: controlled interrupted time series analysis,” by Chris Grundy, et al., notes that “the introduction of 20 mph zones was associated with a 41.9% (95% confidence interval 36.0% to 47.8%) reduction in road casualties, after adjustment for underlying time trends.”

The reduction, they also note, was greatest for young children — which brings up the point that it’s not merely children’s risk-taking behavior responsible for their deaths as pedestrians, that addressing driver’s behavior can make a difference — and mattered more for KIAs (killed or serious injuries) than for minor injuries. They also report that “there was no evidence of casualty migration to areas adjacent to 20 mph zones, where casualties also fell slightly by an average of 8.0% (4.4% to 11.5%).” Perhaps driving more slowly on one set of streets even had a carry-over effect. The reductions are impressive and seem beyond what might be explained by some other factor, such as a reduction in pedestrian volumes over that same time period (although other factors, like the presence of enforcement cameras, need to be kept in mind).

About now is where someone usually complains that putting up 20 mph signs is ineffective and won’t change driver behavior. But we’re not talking about mere signage here, we’re talking “self-enforcing roads,” with a variety of engineering and design measures, and as the authors write, some evidence “suggests that the self enforcing 20 mph zones are effective in reducing traffic speeds to an average of 17 mph, an average reduction of 9 mph.”

The benefit wasn’t merely for pedestrians. “A somewhat counterintuitive observation,” they write, “is the apparently large reduction in injuries to car occupants.”

And not surprisingly, given their findings, the authors argue for extending, where justified, the 20 mph zone throughout London, and other metropolitan regions. Which isn’t necessarily an easy task, as Shanthi Ameratunga notes in a reply, also worth reading. “Giving provincial or local agencies the authority to reduce national speed limits is an important step in achieving this vision. Yet the 2009 global survey on road safety reported that only 29% of 174 participating countries set speed limits of 50 km an hour or below on urban roads and allowed local authorities to reduce national speed limits. These findings probably reflect both the lack of evidence on cost effective speed management strategies in low income and middle income countries, and the reticence of most governments to enforce laws that limit driving speeds, possibly because of perceived public opposition.”

But progress is being made, at least in the U.K.

Some interesting parking-related figures I came across today, in a forthcoming paper in the journal Land Use Policy, “The environmental and economic costs of sprawling parking lots in the United States,” by Amélie Y. Davis, Bryan C. Pijanowski, Kimberly Robinson and Bernard Engel:

A large proportion, over 6.5%, of the urban footprint, is allocated to parking lots in Tippecanoe County, Indiana. We estimated that this is the same size as 1075 American football fields. In our mall study area, we found that parking lots exceeded the footprint of buildings they service by 20%.

If the percentage of parking lot area in the county (0.44%) is scaled to the area occupied by the conterminous United States, the entire states of Connecticut, and Massachusetts (12,550 + 20,305 = 32,855 km2 ) would be paved over with parking lots.

If only Fermat had lived to the age of the automobile, he too might have grappled thusly:

How much extra length (above the length of your car) do you need to parallel park?

Maths (as they say in the U.K.) professor Simon Blackburn, working on behest of Vauxhall, has cracked the code (study can be downloaded at his page). Though much of it was beyond me — I suffer from horrible innumeracy — I was happy to learn about such things as “The Ackermann Linkage” (Ludlum-esque, that!). The footnotes also reveal that Blackburn is not the first to take a calculator to the curb — or kerb.

(Horn honk to Nathan)

Photo by Pedals/Flickr

In my latest Slate column, I consider the drive-through.

One thing that struck me was the historical novelty of the form; McDonald’s didn’t begin to unroll them until the mid-1970s, and they now, rather shockingly, account for the majority of their restaurant business. It’s a subtle, yet indicative, symbol of how much American society has changed, driving-wise, in a few decades. At one moment, most children, like me, were walking to school, and while we may have driven to McDonald’s, we actually got out of the car to eat our meal (and something like McDonald’s, pre-drive-through, was then an occasional novelty, at least for me).

Photo by Tom Vanderbilt

I flew in L.A. traffic reporter Mike Nolan’s Cessna as part of the research for Traffic. The piece of paper taped to the stick above, by the way, has the text of commercials he read as part of his report.

These days, as this AP dispatch notes, he’s no longer airborne.

Airwatch, a subsidiary of radio giant Clear Channel Communications Inc., has 60 reporters and producers working around the clock to provide traffic updates to more than 40 Southern California stations. They sit side by side in a small studio overlooking an Orange County freeway, staring at computer monitors and TV screens as they speak into the microphones, sometimes talking over each other as they file live reports.

Nolan was one of them. He took a substantial pay cut to work from the ground. He chose to work from home rather than commute 40 miles roundtrip to the Airwatch studio in Santa Ana.

He now takes a few steps from his bedroom to his study to start his split shifts, from 5 to 9 a.m. then 3 to 7 p.m. He puts on a headset, turns on the stopwatch application on his iPhone, and pulls up a half-dozen Web pages to gather traffic information.

When it’s his turn to come on at the top of the hour, 20 minutes past and bottom of the hour on KFI-AM and twice per hour on KOST-FM, Nolan rattles off a list of congested freeways in 40 second to one minute bursts.

Growing up in the San Fernando Valley in the early 1960s, Nolan saw freeways expand deeper into suburbs. Flying over Southern California day in and day out gave him an understanding of traffic patterns that enhance his reports from the ground.

When he reads traffic maps on the computer, he can picture every tunnel, hill and curve and knows when drivers should be slowing down. He can suggest alternate routes and knows what type of incident is likely to cause more misery.

He said that kind of knowledge can’t be replaced by GPS-equipped gadgets.

“The radio reporter is going to tell you what’s going on where you’re going to be in addition to where you are,” Nolan said.

Seeing Traffic positioned on a reading list recommended by Foreign Policy’s “Top 100″ thinkers had me in mind of book lists, and so I thought I’d round up the transportation-related books (or at least marginally so) that have crossed my desk this year and would make good holiday purchases for your mobility-minded friends (or yourself).

In no particular order:

1.) Joe Moran, On Roads. I’ve noted my interest in this book before, but suffice it to say it’s cracking cultural history of the U.K. motorway system, a must-buy for bitumen boffins everywhere.

2.) Ted Conover, The Routes of Man. OK, this one’s not out until February, but the galleys of this book accompanied me on a cross-country flight, and I was hooked. A far-flung, elegiac, honest examination of roads and their impact on us and society, Conover’s book ranges from the tangled “go slows” of Lagos, Nigeria to an (illicit) “capitalist road” trip in China.

3. The Yugo: The and Fall of the Worst Car in History, by Jason Vinc. If you’re old enough to remember actually riding in one of these things, and enough of an automotive-cultural obsessive to remember, say, the Yugo’s appearance in the plot-line of Moonlighting, then this tale of geo-political commerce is for you. And as Vinc reminds us, the Yugo was the “fastest-selling first-year European import in American history.”

4. Carjacked, by Catherine Lutz and Anne Lutz Fernandez.
OK, this is turning into next year’s list — this one’s not out until early January — but in Carjacked, an anthropologist and writer delve into American car culture — the romance that longed ago turned into marriage — and offer a thorough, gimlet-eyed assessment. Sample quote: “In the period from 1979 to 2002, the period in which seat belts, air bags and other improvements in vehicle crashworthiness were installed, U.S. crash deaths declined by just 16 percent, while those in Great Britain declined by 46 percent, in Canada by 50 percent, and in Australia by 51 percent.”

5. Waiting on a Train, by James McCommons. Shifting from road to rails, McCommon’s book is a cross-country trip into the modern-day heart of U.S. passenger rail (”service that the Bulgarians would be ashamed of,” notes James Howard Kunstler in his intro), laying bare the roots of its decline and offering a way forward for the country’s most embattled mode. And I’ve not read it yet, but Matthew Engel’s Eleven Minutes Late, a “train journey to the soul of Britain,” is definitely on my list.

6. Jeff Mapes, Pedaling Revolution. Another one I’ve banged on about before about, but the go-to work on cycling as a form of transportation in America today. And full disclosure: The guy did lend me a bike to ride in Portland.

7. City: Rediscovering the Center. By William H. Whyte.
One of those rare books — reissued in paperback in 2009 — that actually lives up to the promise of “changing the way you see the world.” Along with the writing of Joseph Mitchell, I can’t think of any other title that has so influenced my experience of living in New York City.

8.
Cars: Freedom, Style, Sex, Power, Motion, Colour, Everything (text by Stephen Bayley).
Because sometimes you just really want to look at a pretty picture of a 1955 Citroën DS.

9. Jeff in Venice, by Geoff Dyer. One of my favorite writers, and his description of driving in India does not disappoint.

Suggestions are welcome for others I may have left out.

The device at right — the “Plant Lock,” for locking bikes, not plants — was new to me.

It’s one of a number of features employed in a so-called “DIY Street” in the East End of London.

Writes the Guardian:

Getting cars cars to slow down instead of racing through backstreet rat runs benefits everyone from cyclists to residents. But a windswept street on a November night in the East End of London is not the first place you’d expect to find inspiration for how to do that – not only cheaply but also with the total approval of the people who live there.

“We were expecting a 20mph sign and we got all of this!” a local told me.

More on the project here.

Here’s an update, via the Los Angeles Times, on the case of the L.A. engineers — as alluded to in Traffic — charged in tampering with traffic signals as part of a labor action (in a reverse of The Italian Job, they wanted to slow traffic down):

Two L.A. traffic engineers who pleaded guilty to hacking into the city’s signal system and slowing traffic at key intersections as part of a labor protest have been sentenced to two years’ probation.

As part of their plea deal, the engineers agreed to pay $6,250 in restitution and completed 240 hours of community service.

Richard Stampfle writes:

This is a true picture of a bus in Thailand used to deliver school children to functions. It is representative of many vehicles in Thailand, it is not an exception. I have hundreds more photos I could have used. While we may recognize that the driver cannot see you will find it strange to learn that most Thais find no problem with this picture. I have asked several what is wrong with the picture; one commented that it should be Liverpool not Manchester United on the Glass. One felt the colors were somewhat gaudy but that is a matter of taste. No one commented on the safety issue. When I showed a similar picture at a meeting of the Thailand Global Road Safety Partnership the only suggestion was that I should do some research on the subject and gather sufficient statistics to get the attention of law makers — if indeed this was actually a problem. (There seemed to be some doubt.)

The always interesting and prolific Michael Sivak has a new paper, “Homicide Rate as a Predictor of Traffic Fatality Rate” — a pretty self-explanatory title — just out in the journal Traffic Injury Prevention.

Here’s the details:

Background: In the United States, traffic fatality rates per distance driven vary greatly from state to state, with the maximum rate being 2.9 times the minimum rate. This study was designed to examine factors associated with this variability.

Conclusion: This finding suggests that social aspects of human interaction may play an important role in traffic safety.

In other words, states with a high murder rate are filled with aggressive people, many of whom take it out on the roads.

One issue that pops up in my head is the validity of statewide comparisons for both driving and murder rates, for the primary reason that the places where traffic fatalities occur are not necessarily the same places where murders occur (I’m imagining that the places with the higher per capita murder rates are urban counties, where driving exposure would be much less, but I may be very wrong on this). As William Lucy has argued, for example, in the state of Virginia there was a much higher risk of leaving home in sparsely populated suburbs — not because of would-be murderers but because of driving risk. Conversely, “cities that often are considered dangerous, like Richmond and Norfolk, ranked 19th and 30th in the number of traffic and homicide-by-stranger deaths among the 49 metropolitan-area jurisdictions included in the ranking.”

Or maybe some states just have a lot of murders and a lot of traffic fatalities, with no causal link between the two. Or perhaps all the police are all working on murder investigations, and too busy for traffic enforcement (though I have suggested the two should not be viewed is isolation).

I’m also not sure how deaths by liver disease as a proxy for alcohol-related driving deaths — there would seem to be any number of other things to consider (e.g., health-care quality in a given place, or density — Manhattan is filled with heavy drinkers but certainly must have fewer drunk drivers per capita).