Selective Application of Smeed’s Law

A few months ago, in response to the Raquel Nelson case, author Tom Vanderbilt found an FHWA study from 2005 that finds that on wide, busy roads, pedestrian death rates are higher on marked crosswalks than on unmarked ones. The study itself is worth reading; its explanation of the finding is that,

These results may be somewhat expected. Wide, multilane streets are difficult for many pedestrians to cross, particularly if there is an insufficient number of adequate gaps in traffic due to heavy traffic volume and high vehicle speed. Furthermore, while marked crosswalks in themselves may not increase measurable unsafe pedestrian or motorist behavior (based on the Knoblauch et al. and Knoblauch and Raymond studies) one possible explanation is that installing a marked crosswalk may increase the number of at-risk pedestrians (particularly children and older adults) who choose to cross at the uncontrolled location instead of at the nearest traffic signal.

An even greater percentage of older adults (81.3 percent) and young children (76.0 percent) chose to cross in marked crosswalks on multilane roads compared to two-lane roads. Thus, installing a marked crosswalk at an already undesirable crossing location (e.g., wide, high-volume street) may increase the chance of a pedestrian crash occurring at such a site if a few at-risk pedestrians are encouraged to cross where other adequate crossing facilities are not provided. This explanation might be evidenced by the many calls to traffic engineers from citizens who state, “Please install a marked crosswalk so that we can cross the dangerous street near our house.” Unfortunately, simply installing a marked crosswalk without other more substantial crossing facilities often does not result in the majority of motorists stopping and yielding to pedestrians, contrary to the expectations of many pedestrians.

This is a rather standard application of Smeed’s law and similar rules governing traffic, whose one-line form is that traffic fatalities are determined primarily by psychology. This is not a problem; the problem is why such issues are only ever brought up in case of pedestrian fatalities.

In 1949, R. J. Smeed found a simple explanation for traffic fatalities: they depend less-than-linearly on the number of cars on the road. In the 1980s John Adams revised this to a more accurate rule based on VMT rather than the number of cars, and based on a constant decline in per-VMT accidents over time. Safety improvements do not bend or break the general trend. Quoting Adams again, the introduction of seat belts caused no reduction in traffic fatalities, and on the contrary caused pedestrian fatalities to temporarily inch up, as drivers felt safer and drove more recklessly. The only way to reduce the number of car accident victims is to reduce traffic.

And yet, government reaction is consistently on the side of accepting Smeed’s law when it implies there’s no need to improve pedestrian facilities, and rejecting it when its implication is bad for cars or good for pedestrians and cyclists. Local governments in the US routinely argue that safety is at stake when they want to upgrade a road with grade crossings into a full freeway. The FHWA helpfully adds that intersections are responsible to half of all car crashes and “FHWA will identify the most common and severe problems and compile information on the applications and design of innovative infrastructure configurations and treatments.”

In reality, all building freeways does is create more traffic, and cause more people to die in crashes. The average per-VMT death rate in the US has declined by 3.3% per year, but in the years following the Interstate Highway Act, it was practically flat – in other words, building freeways did nothing to accelerate the trend for reduction in per-VMT accident deaths. Although an individual freeway is undoubtedly safer than an individual road with intersections, the road network has to be viewed as a system: increase safety in one area and people will drive more recklessly elsewhere.

This systemwide view is clearly present in the case of pedestrians: the FHWA isn’t claiming that crosswalks are inherently unsafe, only that they cause more at-risk pedestrians to cross. In other words, the problem is that they cause too many of the wrong kind of pedestrians to cross. The implication is never used for roads. Traffic is never treated as variable, and if people shoot down freeway upgrades on the grounds that they’ll induce more traffic, it’s always on environmental or community grounds rather than on safety grounds.


  1. ValkRaider

    I would agree with most of these points, and I would actually take it a bit further and point out how the aspect of “false sense of security” plays in to these things (although that is not directly linked to the selective application of Smeed’s law – they should go together).

    Without actually reducing traffic or *controlling* traffic – often some perceived safety measures create a false sense of security which will lead to more fatalities.

    Examples include:

    The marked crosswalk example from above. People on foot will expect auto drivers to pay them more attention, because HEY – I’m in a crosswalk here! Pedestrians think they are safer so they take more risks, when the drivers still are not paying attention or slowing anyway. (There are laws of man and laws of physics – you can have the right of way and still be dead).

    Bike lanes can actually make people less safe. Studies have shown that auto drivers pass closer to bikes in bike lanes than they do bikes not in bike lanes. The simple white line lends a false sense of security. Bicyclists will take more risks because they feel like they have a protection with their own lane, and auto drivers will take more risk around a bike because they assume the bike will always be tucked away in it’s own lane.

    Technology in cars, like airbags and traction control systems. People as they feel safer and safer in cars will push cars further and further. Modern high horsepower cars have suspension and traction control systems that make them handle exponentially better than cars from 25 or 50 years ago. That sounds like a good thing, but in reality in an older car when it felt like it was out of control most drivers would slow down or take other precautions. Now cars feel like they are driving so good, drivers will go faster and take more risks – and when the systems fail (and they do fail) the results are a crash at much much higher speeds. Generally now with modern cars, people operate them much closer to the edges of the performance envelopes. In a 1970 car spinning out at 40mph in a corner might simply be embarrassing but in a 2010 car spinning out at 70mph in a corner is most often deadly. Everything like airbags and seatbelts make drivers feel safer and they take more risks (some people joke that drivers would drive much safer if there were big spikes on the steering wheel and dash board).

    Controlled access roads or super wide lanes and shoulders. When people get so used to never having to be aware of their surroundings – like on interstates or roads with few cross streets – they actually stop being aware of their surroundings. I am and I bet everyone else is – complete guilty of this. We get co comfortable buzzing along that we zone out – and we are completely not ready when something DOES surprise us. The safest streets tend to be narrow and full of people and oddities, because drivers are forced to pay attention – and they know it too as you can see by complaints when people drive around in old neighborhoods they complain about narrow streets with people walking in and out of cars, car doors opening, limited sight lines, etc etc etc. The fact that they complain shows that it makes them scared and as such they will reduce speeds and pay more attention.

    Road markings and signs. We have put up so many road markings and signs that people literally will not think for themselves. If a sign does not tell you to slow down for a curve, people will drive right off the road. People are so used to being told about everything on the road that they stop paying attention themselves. Why do we even have a sign that reads “Be prepared to stop” or “Hill Blocks View” or “Slippery When Wet”. All of those things should be expected, but we have grown so used to all these “safety features” that we assume that if something is not marked it is not a risk. We have such a sense of security that someone has gone before us and taken out all of the risks on the road – that we are no able to just read signs and not think.

    And finally a tangent: I have noticed both my own behavior and that of others around me – generally when participating in snow sports like skiing or snowboarding we will take more risks when wearing a helmet. Completely unscientific – just my own observation.

    I think the “false sense of security” aspect of the world is a very powerful force, and impacts people in ways they will never realize.

    • Alon Levy

      Yes, exactly.

      Another issue: if people are driving at 130 km/h on a road designed for 160 km/h, they’ll fall asleep. I’ve heard of an experiment done on French autoroutes, whose design and traffic level is such that 160 km/h is easy, in which test drivers drove at the statutory speed limit of 130 km/h. Using some sensor devices, the experimenters could tell that the drivers fell asleep multiple time while still driving, at one time for 30 seconds straight. When driving is too easy, it seems, drivers just pay less attention.

      The way Smeed phrased it is that technical changes in safety do have an effect, but it’ll be seen elsewhere. They’ll allow drivers to be less attentive, drive faster, or (with the analog of congestion, which obeys the same psychological rules) take more discretionary trips and drive longer distances.

      • John

        So that’s why rural interstate highways make me sleepy, but urban ones don’t. “Sorry officer, I had to drive 90 mph to stay awake.” 🙂

  2. ant6n

    So if we never install any crosswalks, then children and old people will never cross any roads — problem solved!

    • ValkRaider

      The point is more that we shouldn’t install any half-assed crosswalks that are designed to protect people with paint stripes alone – but rather if we are going to put in a crosswalk there needs to be something else other than just paint which will cause drivers to slow and stop…. Paint alone cannot protect a child or elder from an inattentive or impatient driver.

      I vote for massive spikes which impale cars that come too close. Those may be cost prohibitive though… 🙂

  3. Steve

    What about isolated cases–certain exceptionally dangerous roads (such as Philadelphia’s Roosevelt Boulevard)? Or how about when applied to shared space? I have the feeling that, like general relativity, Smeed’s Law may well start to break down at the margins.

    • Alon Levy

      Smeed’s Law is way, way too general to be tested on a single road.

      I don’t know what shared space is going to do. My guess is that it’s going to do nothing to car fatalities per VMT, but reduce VMT. On the other hand, shared space is the normal state of affairs in many third world cities, where road accident rates are the same as those of the US in the 1910s.

      • Eric

        “the normal state of affairs in many third world cities, where road accident rates are the same as those of the US in the 1910s.”

        And travel speeds and convenience, as well, are the same as in the US in the 1910s…

        • Alon Levy

          I’m not trying to idolize those cities. The comment about 1910 is that their accident death rates are vastly higher than in developed countries per VMT, but (generally) lower per capita, and are close to the situation in the US in 1910 on both metrics. The same is of course true of travel speed and its converse, the need to travel long distances on a daily basis. It’s not surprising if you think of what’s happening in most of the developing world these decades as a rapid catch-up industrialization, with the same social issues that arose in the West around 1900.

      • Steve S.

        I would suggest that Europe and Japan would be better models to look to for implementation of such shared space schemes, and that pilots of same attempted in the older urban centers (usually on the East Coast, esp. Boston, Philadelphia, and Baltimore).

        • Andre Lot

          Shared space only works on local narrow roads, where traffic is light. With the exception of very, very limited areas in older cities, ALL other major american urban centers have a street pattern that doesn’t warrant the concept of shared space.

          Moreover, this time I honestly think the mentality “war on traffic” took over an otherwise nice piece by Alon. Any expanded use of modern technologies carries certain level of increase in damages caused by such technology. The rate of commercial passenger air traffic death in 1900 was zero. So were zero the ratio of people killed by accidental electric shocks in 1850, the ratio of people killed by dam failures in 1300, the ratio of people whose cell phones exploded in their pockets in 1970, the ratio of people who lost their livers due to paracetamol intoxication in 1920, the ratio of people killed in train collision in 1800… we could expand the list ad infinitum.

          In the context of mobility, the amount of km an average citizen of France would travel in 1850 during his/her lifetime was certainly shorter than today, for instance. That is why we need to compare safety with km*passengers metrics, holding fixed the unit of mobility. Else, we enter the paradox of assuming the use of certain technology as inherently bad, thus eliminating the technology (no more remote water heating) eliminates all related problems (burns from shower with too hot water, water tank explosions etc)..

          Freeways (not the name, but the engineering solution) drastically reduce accidents and fatalities because they reduce interference of uses. Not only they avoid car-on-car accident, but they actually block pedestrians crossing the freeway at-grade, providing under/overpasses and other safety features that benefit everyone.

          Marked crossings need to be pre-signaled, well lighted and clearly defined. On-demand traffic lights provide uttermost safety to more remote pedestrian crossings, or those not used often to warrant a normal traffic light. The more sophisticated ones are even connected to a variable speed limit indicator that slows down cars when there are pedestrians crossing that odd middle-of-nowhere crossing, without slowing down traffic permanently.

          As for the biking citation, the problems persists: as I commented on the previous thread, there is good biking path engineering and bad biking path engineering. Merely painting a line does nothing to improve the safety of cyclists, grade-separating intersections for cyclists with small tunnels/overpasses, or – if they have traffic lights – creating a light phase scheme that eliminates turn conflicts cars x bikes or creating offset and elevated (in relate to pavement) bike crossings are good policies.

          When cycling in an area is high, there are a lot of other problems usually ignored as well, like pedestrians x cyclists conflicts. They are pervasive in Netherlands, with many serious injuries cause by collisions of bicycles and pedestrians. For instance, many cyclists do not respect pedestrian crossings over bike paths, as they don’t want to slow down and hold down, and regulations preventing cyclists in pedestrianized areas are often ignored.

          Bottom line is that shared space approaches only work in quiet streets or those where everybody use of the road is supposed to be degraded in performance and comfort – including pedestrians.

          • Alon Levy

            The whole point of Smeed’s law is that freeways do not reduce accidents drastically, even though on an individual basis they’re manifestly safer. Look again at what happened when the US started building freeways en masse: safety did not improve measured in accident deaths per vehicle-km, despite a multi-decade trend for a 3.3% annual decrease. Smeed himself noted that if people didn’t modify their behavior, the car-on-car accident death rate should be proportional to the square of the number of cars on the road. That the actual exponent of best fit is closer to 1/3 than to 2 is one of the most important insights of the law, the other being that no systematic attempt at doing things that are manifestly safer (mandating seat belts, building freeways, improving car technology) is going to have an effect on the per-vehicle-km accident trend.

          • ValkRaider

            “but they [freeways] actually block pedestrians crossing the freeway at-grade, providing under/overpasses and other safety features that benefit everyone.”

            That actually only benefits drivers on the freeways. The neighborhoods that the freeways cut through were destroyed and cut off from themselves, and people who need to get from one side of a freeway to another are either forced to walk/bike some times miles out of their way or to drive a car. Freeways cut off everyone but the auto driver, and freeways are almost always exclusively designed to move people farther and faster at the expense of the local population.

          • Andre Lot

            @ValkRider: by those standards, any major surface infrastructure to that, including high-speed rail, water diversion canals, even signal-based light-rail, let alone airports etc. Highways are far from being the only “intrusive” structure. Actually, I never understand this double standard on the blogosphere: most traditional railways and massive yards/stations cut cities in half much more than highways, and they have far less crossings than railways, while providing, if much, a station ever several miles wheres urban freeways have at least lot of exits for people to use. But the presence of highly disruptive rail yards or rail lines is usually praised as an “asset” rather than a liability.

            It’s quite easy to build overpasses for pedestrians over railways, highways, canals etc. Just take a bit of planning.

          • Alon Levy

            Modern cities don’t slice themselves with railyards. If you look at massive railyards, they all were built at the edge of the city, and the city simply grew farther out. It’s similar to how modern auto-oriented suburbs grew around freeways and intersections, and are not sliced by them the way older cities are. So things like Sunnyside Yards or Shinagawa are not really a problem (notably, despite the expansion of commuter rail, neither Tokyo or New York has built additional above-ground railyards in built-up territory), whereas the Cross-Bronx Expressway is.

            Cities do build elevated trains, but these are quite narrow, by highway standards. Even in the 1930s, there were a few observers who noted that American cities were tearing down 30-foot-wide els and building 100-foot-wide elevated highways.

  4. Charlie Gardner

    The implication isn’t used for roads, I’d guess, since the bedrock assumption for the highway engineer is the need to move cars as quickly as possible and in the largest volumes possible, and only secondarily to maximize safety within those parameters. Minimizing pedestrian interation with cars isn’t an irrational objective within that narrow framework. To question the need for additional highways on safety grounds would shake those assumptions to their foundations — I won’t expect to find a cost/benefit analysis incorporating the cost of 35,000 annual crash fatalities, 2 million permanent injuries and 6 million crashes against the economic benefits of personal car transportation on the FHWA website, for instance.

    • Andre Lot

      Personal cars serve a higher-order need of individual mobility that doesn’t depend on schedules. Even in countries well served with urban transit, it is often impractical to have a modern life without cars, be them yours, rented, taxis etc., unless everyone were to reduce – greatly – expectation like “if you live in a cold weather and fancy nightlife, you better move close to nightlife or assume you can’t get home on winter snowy nights after 10pm when last tram runs”.

      There are many items and behaviors that carry a lot of associated damage, but we’re not giving up on them. Banning alcohol outright (prohibition anyone?), if possible, would dramatically reduce incidents of violence, many other crimes, and affect positively the health of people. But it’s not feasible. On a very theoretical point, many health benefits could be extracted if we could make people have sex with one, and one only person throughout their life. And soon we’d be descending into eugenics, how people more prone to give birth to infants with genetic disorders like psychopathy should not have children at all to avoid the birth of serial killers.

      So to assume that personal, unimpeded, fast, motorized (not necessarily with a car, maybe in the future with PRT (personal rapid transit) mobility should be questioned is like to argue if we should roll back to a medieval lifestyle to reduce our energy use by 99% and slash carbon emissions, for instance.

      The answer is not less use of car, but a dramatic revamp of the “personal mobility motorized vehicle” to something that operates more or less automatically depending less on human inputs.

      • Simon

        Andre, your point about the neccesity of cars to moedern life is true, but only to the extent that our cities have been built on the assumption of car travel. In dense pre-automobile cities it’s easy enough to walk or cycle home after 10pm, whatever the weather.

        In other words, cars are an excellent mobility solution to the mobility problems caused by cars. We can continue living within this environment, or we can start the process of rebuilding it into something else.

        And if engineering for slower traffic speeds is social engineering, isn’t engineering for high traffic speeds also social engineering?

        • Andre Lot

          Simon, there are shortcomings to extremely dense medieval cities, like lack of proper air circulation, extremely small built-up area/inhabitant, lack of parks and so on. It just a take a look to well-preserved medieval cores of Italian cities (that, for a variety of reasons, haven’t seen much modernization) like Urbino, Camerino, Orvieto etc. It’s nice and cute as a tourist destination, impracticable if you think of an environment for people living in the millions.

          The grids of Manhattan, or Chicago, or Los Angeles, were all defined before, well before the pervasiveness and ubiquity of cars. So were Haussmanian Paris boulevards and revamped alignment. They had many reasons to depart from the un-sanitary living conditions of the medieval ages, and I don’t think one would advocate living in a place with 4m-wide streets in high-rise buildings (= no sunlight for 80% of the windows but for few minutes a day), lack of open areas, lack of stand-alone buildings etc. Throw issues like the need for fast access for firefighters and ambulances…

          As for: “In dense pre-automobile cities it’s easy enough to walk or cycle home after 10pm, whatever the weather.“, I beg to differ. It is not feasible to walk even 6km under snow or desert-like heat (try to do that in Tucson or Las Vegas in an August afternoon or a humid night in Memphis), and it is impossible to conceive any built-up sane environment that could accommodate even 1 million people living, say, within a – say – 1-mile radius of everyone else.

          Finally, in regard of: “And if engineering for slower traffic speeds is social engineering, isn’t engineering for high traffic speeds also social engineering?“, I think humans are bound to explore, go faster, further, higher, all the time. We’ve been like that since the dawn of human civilization. It’s part of our nature to progress, not go backwards.

          • Alon Levy

            Well, medieval cities aren’t extremely dense. They’re dense by the standards of most Western cities today, but the densities they obtained are in line with the denser first-world cities. My recollection reading conworlding guides is that a medieval European city had about 20,000 people per km^2 within city walls, which is unremarkable by the standards of Paris, Barcelona, and Manhattan today, let alone at their historic population peaks. The buildings were lower, but not by all that much. For example, Ancient Rome had 5- and 6-story apartment buildings, and preserved European medieval cities range from about 3 to 5 stories, vs. about 6 in today’s Paris. There is a tradeoff between street width and building height, which limits density somewhat: Tokyo is full of desirable neighborhoods with 2-3-story buildings and even single-family detached houses, fronting 5-meter streets at high density; Manhattan is full of desirable neighborhoods with 20-story buildings fronting 30-meter avenues. The unsanitary conditions are more a matter of wealth than urban form; a person whose experience was limited to rural India and the Upper West Side would conclude rural areas are inherently unsanitary, while a person whose experience was limited to the old Lower East Side and patrician country homes would conclude the opposite.

  5. Thomas Riehle

    Why did the chicken cross the road? Because Smeed’s crosswalk was there.

  6. EngineerScotty

    There is one poster at Portland Transport–a longtime critic of public transportation (and a user frequently referred to as a “troll” by other readers), who tries to extrapolate this sort of argument into a generic argument against any sort of traffic calming or other means of improving the pedestrian environment. He is fond of pointing to a study where addition of painted crosswalks increased pedestrian fatalities on a particular road–a road which happens to be Queens Boulevard in NYC… and thereby alleging that all traffic calming devices have the potential for side-effects, and should not be deployed pending further study. Of course, his concern is not for pedestrian safety–his real objection to traffic calming is that it inconveniences motorists…

    • Andre Lot

      I usually oppose traffic calming in non-local streets on grounds that it reduces overall speed of traffic in the area and thus increases the time spend on traffic (by whatever mode) for everyone. I even presented a case on a public meeting about that couple years ago. The counter-argument is that less speed is better because it forces people to relocate, but that is easy to attack on ground of social engineering.

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  8. dejv

    Re quote – in my country, length of new unsignalized crosswalks is limited to 6 m and two lanes in opposite direction – there can be exception for the lanes in the same direction, but no excuse for curb-curb length or lane count, refuge must must be installed when it is exceeded.

    The rules used to be similarly strict for existing crosswalks but that spurred opposition from large cities because that would effectively mandate installing traffic lights on every crosswalk on streets with tram tracks or cancelling it altogether.

  9. Adirondacker12800

    Has anyone controlled for the number of pedestrians? Years ago, looking at raw numbers, the most dangerous place to be a pedestrian was New York City. But pedestrians would get run down in New York City because there were pedestrians to run down. It’s very difficult to run down pedestrians if there are few or none.
    Nobody measures pedestrian miles traveled which, from the point of view of pedestrians is much more interesting than vehicles miles traveled. Or compares the fatality rate for “walk to mass transit to go to work” versus “drive to work/drive for work”.

    • Alon Levy

      In the study I link to, they compare the number of accidents with the number of pedestrians who cross. If they hadn’t, I’d have opened with that (as I’ve continually excoriated the “PPW is safer than 9th Avenue” line), and this post would’ve gone under shoddy studies rather than under good studies.

  10. EngineerScotty

    There’s a proposed project in Portland, courtesy of ODOT, to improve safety on a busy urban arterial by removing the curbside bus lane, on the grounds that this poses a danger to motorists who fail to notice that busses are there and sideswipe them while turning right. Of course, the net result will likely be an increase in speed or throughput for motorists (the road is frequently congested, so the bus lane gives transit an advantage through the area), not an increase in safety; with busses finding themselves more often stuck in the traffic.

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