On Passenger Miles and Greenness

Route 522 in Downtown Bothell

Matt Yglesias had a great short post on why passenger-miles-travelled is a poor metric for measuring the greenness of a mass transit system:

The whole idea of trying to talk about which city’s mass transit system is greenest in terms of emissions per passenger-mile is terribly flawed.

Just think of it in terms of cars. Driving 5 miles in a 20 mpg car takes a quarter of a gallon of gasoline. Driving 25 miles in a 45 mpg car takes over half a gallon. Being the guy with the 5 mile commute and the 20 mpg car is considerably greener than being the guy driving much further in his Prius. The point of intra-urban transportation networks—whether you’re talking about the mass transit element or the private cars or bicycles or whatever—isn’t to transport people arbitrary distances, it’s to get people where they’re going. Having trips that aren’t very long isn’t cheating, it’s a great way to achieve efficiency.

Of course, greenness isn’t the only useful measure of a transportation system. I’ll just add two points on top of what Matt has said.

1) I’d go a step further and say it’s actually a greener transportation system in general that allows people to make shorter trips. Shorter car trips mean less time on the road, which itself means less congestion, less fuel consumed and less wear on the roads. Shorter bus trips mean less time on the bus, less fuel consumed (though the difference is very small), less wear on the buses and more room for others. I’m not saying it’s bad to take long trips, I’m just saying a transportation system that would allow everyone to walk everywhere they could possibly need to go would be the greenest of all.

2) While we’re on the topic of terrible greenness measures, miles-per-gallon is actually a rather poor way of comparing fuel efficiency of different vehicles. Fuel consumed is actually inversely proportional to mpg, so the slope of the fuel consumed graph is very steep at low values, and very flat at high values. This means that there’s a lot of savings from going from, say, 5 MPG to 10 MPG, but much less savings going from 50 to 100. Imagine if you had two cars, one getting 10 MPG and another getting 20. Supposed you drove 100 miles per week on each. With the two, you consume 15 gallons of gas. If you wanted to get an average of 20 MPG, you could replace the 10 MPG car with one getting twenty and consume only 10 gallons per week, or you could replace the 20 MPG car with one getting 30 and consume 13 1/3 gallons per week. This would be obvious if we used gallons per 100 miles (or litres per 100km as they do in Australia). So if we want to measure the “greenness of cars” it makes sense to put the thing we care about (gallons) as the numerator, not the denominator.

Comments

  1. David L says

    Liters per 100 km is used throughout the rest of the world (except the UK), not just in Australia. It’s a much more sensible measure. In our silly units, we could use gallons per hundred miles exactly the same way.

    MPG lets truly atrocious cars off the hook, while over-penalizing ordinary cars relative to extremely fuel-efficient ones. Which difference in fuel usage is bigger: 9 mpg vs. 12 mpg or 31 mpg vs. 40 mpg?

    • Andrew Smith says

      I actually don’t have a problem with miles or gallons, the problem is that we’re taking the wrong ratio.

  2. John Bailo says

    Wow…another thinly veiled argument for density. Why don’t we all just move into Walla Walla State Prison and live in 120 sq foot cells — I mean apodments.

    • Jeffrey J. Early says

      Or, it might just be that increased density is the solution to a lot of different problems.

      • John Bailo says

        Let’s make all the rail and bus lines only a mile long and anyone who doesn’t want to live in that half mile radius can suffer in quiet sparsity.

      • Andrew Smith says

        John, I think you may have misunderstood. What I am trying to say is that the greenest trips are the shortest ones, not the longest ones.

      • Jim Cusick says

        @Andrew Smith

        “What I am trying to say is that the greenest trips are the shortest ones, not the longest ones.”

        Not if an internal combustion engine is involved. The worst part of the emissions (and mileage) is in the warm-up period, about 4-5 miles of driving.

        Now if one used an electric vehicle, charged with Solar, Hydro, or Wind for those trips, then you’d be talking.

    • Cascadian says

      Density comes in different forms. It’s possible to get high levels of density without cramming people into studio apartments in high rises. If development was oriented to transit, then you could put the really dense developments (and most of the retail and commercial) within a block or two of the station, have everything within a quarter of a mile in moderate density, and then have single family (on more reasonably sized lots than is typical now) outside of that and still have almost everyone about a half mile or less walking distance from everything local and the same distance from a transit system that will get them half a mile or so from anything else.

      Even within each housing type, more density is usually better. An older single-family neighborhood on smaller lots on smaller blocks has more of a sense of community than a sprawling development of large lots on a meandering street. An apartment building on the street above retail is better than the same number of same-sized apartments in the typical suburban parking lot style.

      • John Bailo says

        The problem with density is that everything becomes entrenched and inflexible. Take me, I just hopped into my car, I went to QFC for some canned tomatoes, then I went Carpinito Bros. for fresh vegetables, then to Wild Wheat for fresh bread. These businesses can exist because of low rents and free access to them from many households as well as efficient delivery of goods.

        The high risk-high reward of pricey density eventually decays into slums…as we have often seen.

      • Stephen F says

        That’s nonsense, John, and you know it. Have you looked around Kent? The supermajority of the city is in decay–massive decay. It’s pathetic. The only bit of gentification taking place–well, mostly static, not really improving–is the city centre. That’s it.

      • David L says

        John, you don’t think that similar businesses — just more of them — are located in Seattle’s urban neighborhoods? You need to open your eyes a bit when you come up here…

        I’m sure the residents of lower Queen Anne, which has been dense for 70 years, would be interested to hear of its decay into a slum.

      • Mark Y says

        John,
        I walk to Pike Place Market every Saturday morning, and get all those things at once. I’m not sure what your point is.
        All I see when I walk is building after building going up.

      • Mike Orr says

        How much gas did you use? Doesn’t it seem silly to take a ton+ vehicle to three places for tomatoes and bread?

      • Mike Orr says

        Peripheral cities with lower rents and perhaps a variety of low-budget business startups is not the issue. It may be smart to live in those cities for those reasons. The issue is that in the current era, this means cities unchanged since the 1970s, which was an especially bad period for walkability and transit-readiness. Buildings that could easily have been 3 story are 1 story, with an excessive amount of space around them that people have to walk past every time, and large parking lots in front rather than hidden in the back.

        If those cities had been more like 1920s cities, I’d say great, let’s keep them as-is. But 1970s-era buildings and land-use give you an automobile-oriented landscape and excessively long walks to anywhere. That’s the problem.

      • Mike Orr says

        Actually, John has said he lives next to a shopping center around the south end of 104th, so he does have a lot of everyday-need businesses within walking distance. And Fred Meyer is at the opposite end of East Hill, a short bus ride or mile-walk away. The library (which I would care about) is a short bus ride away near Kent Station. So he does live in the closest suburban equivalent to an “urban neighborhood”. Similar neighborhoods exist in Crossroads and Overlake, and they’re the most urban places to live if you’ve crossed off Seattle and the city centers. Metro has started to get the word, with RapidRide B and, we hope, a future RapidRide 169.

        The main thing that struck me is, since I don’t know where these shops are John mentioned, couldn’t he have walked to them or to another acceptable supermarket or produce store?

      • Stephen F says

        Mike, no. Wild Wheat is in the city centre off Gowe, Carpenitos is off of Central Ave/SR-167, and QFC is near 240th/132nd. A round-trip of at least 8 miles and that would take forever by bus. Although, all are possible without a car…sort of.

    • Stephen F says

      Density is an abitrary term. But, the point is that modest density could even reduce need for bus. I bike or walk 5 to 9 blocks to the grocery store, 2 to the library, 2 to church, 10 to Greenlake, one to the bus stop, 1 to 10 for a whole assortment of services and restaurants and I don’t even live in a “dense” neighbourhood. I live in a walkable neighbourhood. And yeah, some people are lucky enough to work in the neighbourhood. What’s so terrible about that, John? That is almost impossible anywhere in Kent–anywhere.

    • says

      Whether one likes density or not, if a modern/specialized lifestyle is a given (one where you have a job, earn money, and use that money to buy most of your necessities instead of making or growing your necessities themselves), all else equal, traveling short distances on foot is most energy-efficient.

      Density of certain sorts can concentrate certain types of pollution, but by the time you’ve reached intense suburbia you’re already there (see: LA’s smog), and dispersing an equal-sized urban population wouldn’t.

      • John Bailo says

        Good luck finding tenants…

        U.S. birth rate plummets to its lowest level since 1920

        The overall birth rate declined by 8 percent between 2007 and 2010, with a decrease of 6 percent among U.S.-born women and 14 percent among foreign-born women. The decline for Mexican immigrant women was more extreme, at 23 percent. The overall birth rate is now at its lowest since 1920, the earliest year with reliable records.

        http://www.washingtonpost.com/local/us-birth-rate-plummets-to-its-lowest-since-1920/2012/11/29/ee7e8d16-3a3f-11e2-b01f-5f55b193f58f_story.html

      • aw says

        This is good news because the planet already has plenty of people. However, it does not imply that the population of the US or the Seattle region will decrease. With migration, it’s likely that this area will continue to grow.

      • Mike Orr says

        And so a drop in the birth rate leads straight to boarded-up apartments.

        Hint: it also depresses demand for single-family exurban houses.

        The US has been keeping a stable population due to immigration but it’s about to dip like Europe.

      • Nathanael says

        If we made this country more attractive again, people would start immigrating again.

        But instead MEXICO has cheap single-payer health care and the US doesn’t. Remind me why anyone would move north of the border if they could possibly avoid it?

      • Mike Orr says

        Because the US average income is far higher than Mexico’s? Because we don’t have drug cartels beheading women and asassinating police chiefs?

      • says

        John, small populations are perfectly conducive to short trips.

        And, of course, the size of the national population doesn’t dictate how that population is distributed into urban areas and within them. A general lack of population growth militates against changes in population distribution, of course.

  3. Daniel says

    The metric one uses to evaluate potential solutions to a problem should match the problem as closely as possible.

    MPG is a useful metric for comparing the marginal fuel consumption of otherwise similar vehicles. Passenger-miles per gallon allows one to compare vehicles of a variety of sizes.

    If you’re interested in reducing carbon emissions, your metric should be one that takes into account both technology and lifestyle choices. Tons of carbon per person-year is the most specific, and allows for the most honest comparisons.

    • Andrew Smith says

      MPG is a useful metric for comparing the marginal fuel consumption of otherwise similar vehicles.

      My point is that gallons per mile would do that, and do that better. Plus you get the added bonus of being able to absolutely compare otherwise dissimilar vehicles.

      • Andrew Smith says

        No, they are not. Did you read the post? I give an example of how they are absolutely not the same thing whatsoever.

      • Norman says

        I read the post. 20 miles per gallon is exactly the same thing as 1 gallon per 20 miles, is exactly the same thing as 1/20 gallon per mile.

        They say exactly the same thing in different ways. But, they all describe exactly the same fuel efficiency.

      • David L says

        It’s in human perception of the result. We care about the gallons used more than the miles traveled, so presenting the result as a number of gallons is less misleading to us humans.

        MPG makes small gains at the high end of the fuel economy spectrum look better and large losses at the low end look less important.

        Think about this: look at these numbers and you’ll think the best car is just head and shoulders above an ordinary pack.

        10 mpg
        16.7 mpg
        25 mpg
        40 mpg

        But when we say it this way…

        10 gallons/100 miles
        6 gallons/100 miles
        4 gallons/100 miles
        2.5 gallons/100 miles

        …it’s much clearer just how bad the 10 mpg car is.

      • Andrew Smith says

        You may have read it, but you clearly didn’t understand it.

        1) For a single car MPG is simply the inverse of GPM. But that stops being true when you talk about combining them. So it makes this government program incredibly stupid:

        Just as they do today, the rules will allow automakers to average their fuel economy across a number of models. A guzzler that doesn’t meet the standard can be “canceled out” by one or more vehicles that better the mpg standard.

        As my example showed, averaging MPG gives you a metric that is actually pretty divorced from the actual fuel economies.

        I’ll make it very clear. You have two cars. One gets 10 mpg, the other gets 20. The government says you need to get to an average of 20 mpg. So you can switch either the 10 mpg car for a 20 mpg car, or switch the 20 mpg car for a 30 mpg car. However, they give you very different actual fuel economies:

        10 mpg is 10 gallons per 100 miles (g/100m)
        20 mpg is 5 g/100m
        30 mpg is 3.33 g/100m.

        So the 10 mpg car plus the 30 is a combined fuel efficiency of 13.33 g100m. While two 20 mpg cars have a combined fuel efficient of 10 g/100m, which is significantly better.

        2) Because the inverse curve slopes the way it does, it substantially misstates the actual fuel efficiency. Comparing a 10 mpg (10g/100m) car to a 20 mpg car is not like comparing a 20 mpg car (5g/100m) to a 30 mpg car (3.33g/100m). Which is to say, in terms a fuel economy, not every marginal mpg is worth the same amount. The value of any given marginal mpg is dependent on which marginal mpg it is.

        Mathematically speaking, we say that the first derivative of the mpg by gallons is not a constant. So dg/d(mpg) is – mpg^-2, which is to say that the change in gallons per mpg is quadratically decreasing in magnitude as we increase mpg.

        This shows that MPG is a crappy way of discussing fuel economy because 1) mpgs are related to fuel economy, but the relationship means that not all MPGs are the same, 2) you can’t aggregate the measurement without losing the relationship to fuel economy.

        People generally say a good metric has consistency. This one doesn’t (because not all marginal mpgs are the same as I’ve shown).
        People also say a good metric has comparability. This one doesn’t (because you cannot aggregate them).

      • Bernie says

        CAFE standards work on a fleet average and light trucks are treated as a separate category than passenger cars which is an issue as it tends to favor selling more F150s than Fusions. Overall the goal is to reduce the amount of crude oil consumed and while I’m not typically a big proponent of government mandates CAFE has worked; as have mandatory breaklights, seatbelts and collapsible steering columns. There will be gaming the system no matter what units are used.

      • Andrew Smith says

        That’s good to know. Harmonic means may be fine for government work, but try explaining it to joe every man.

      • Norman says

        “Think about this: look at these numbers and you’ll think the best car is just head and shoulders above an ordinary pack.

        10 mpg
        16.7 mpg
        25 mpg
        40 mpg

        But when we say it this way…

        10 gallons/100 miles
        6 gallons/100 miles
        4 gallons/100 miles
        2.5 gallons/100 miles

        …it’s much clearer just how bad the 10 mpg car is.”

        I see 40 mpg as being 4 times as efficient as 10 mpg.

        I see 2.5 gallons/100 miles as being 4 times as efficient as 10 gallons/100 miles.

        They both have a factor of four: 10 x 4 = 40; 2.5 X 4 = 10.

        What exactly do you see as the difference there?

      • Norman says

        So, Andrew you are saying there is a difference between 10 mpg and 1 gallon per 10 miles? lol Not in a car, there isn’t.

      • says

        Here’s the problem with using MPG to compare vehicles if you wanted to reduce your gas use, equal increases in mpg do not result in equal fuel savings. That’s what Andrew is talking about in point 2.

        Using David L’s numbers above:

        Going from 10 mpg to 25 mpg, a difference of 15 mpg, saves 6 gallons/100 miles (or $1,800/year less in gas costs at $3/gallon and 10,000 miles driven/year).

        Going from 10 mpg to 40 mpg, a difference of 30 mpg, saves 7.5 gallons/100 miles ($2,250/year). That’s 4 times the mpg but only 25% more savings compared to going from 10 mpg to 25 mpg (2.5x).

        Going from 25 mpg to 40 mpg, also a difference of 15 mpg, saves only 1.5 gallons/100 miles ($450/year).

        Add another 15 mpg, 40 mpg to 65 mpg, saves only 0.96 gallons/100 miles ($288/year).

      • Norman says

        Your little mathematic equations are amusing but meaningless. All I can say is “so what”?

        “Comparing a 10 mpg (10g/100m) car to a 20 mpg car is not like comparing a 20 mpg car (5g/100m) to a 30 mpg car (3.33g/100m)”

        Right. Because 20 mpg is TWICE 10 mpg, but 30 mpg is NOT TWICE 20 mpg. If you want to double your mpg from a 20 mpg car, you need to switch to a 40 mpg car, not a 30 mpg car. I think this is obvious to anyone. Not sure why you feel it’s necessary to point that out.

        Yes, you are right: 2 X 10 = 20, but 2 X 20 does NOT = 30.

        So, what?

      • Andrew Smith says

        What don’t I understand? I wrote the post, it’s clear that you don’t understand it as you have demonstrated (‘so what?’).

      • Jim Cusick says

        @Andrew Smith

        “I am not surprised you do not understand, Norman. [Ad hom]”

        @Norman

        “I’m not surprised you don’t understand, Andrew.”

        The math is valid, but the big problem is
        the public doesn’t care to understand.

        Most people make these transportation judgements at a more aggregate level. When they can see the gas gauge visibily dropping for a given trip, that’s when they start thinking about it.

        When they find they’re filling up twice a week as opposed to once a week, then they use a variation of a ‘gallons per mile’ thought process.

        But if they have no choice of transportation options, they solve it using a ‘miles per gallon’ evaluation.(which vehicle’s MPG compares favorably with another’s MPG.

        These various calculations are interesting for us transportation wonks, it’s just how the rest of the world thinks.

        People make transportation choices on more of a gut level.

      • says

        Put y=x and y=1/x into a graphing calculator and you’ll see why “it’s just the inverse” is a far more profound statement than you think.

        The point is that it’s easier to look at it additively, not multiplicatively. If you’re just given a bunch of numbers, you’re likely to think that moving from 10 to 20 is the same as moving from 20 to 30. But if you flip the numbers around, suddenly you’re comparing moving from 10 to 5 to moving from 5 to 3 1/3, and the actual relation between the numbers becomes more apparent.

  4. Bernie says

    So if we want to measure the “greenness of cars” it makes sense to put the thing we care about (gallons) as the numerator, not the denominator.

    People like to use whole numbers, not fractions. That’s why we express interest rates in percent and changes in interest rates as basis points. Or cost per mile in cents ;-) It would be cumbersome to compare .038 gallons per mile with .025 gpm. And people don’t really care about the gallons per mile as much as how many miles they can go without running out of gas. It’s much easier to calculate, “I’ve got forty miles to go, I get 26mpg. Gas gage is on 1/4 which is ~4 gallons so I’m good.” Compare that calculating in your head, “I get .038 gpm, how much gas do I need to go 40 miles or how many miles can I go on 4 gallons?”

    • Andrew Smith says

      That’s why in Australia they do it in litres per 100 km. We could do gallons per 100 miles.

      Your calculation is “I’ve got a quarter tank, my tank is about 16 gallon, and I get about 25 miles per gallon, so I’ve got 1/4*16 * 25 = 100 m, which is more than 40″.

      My calculation is “I’ve got a quarter tank, my tank is about 16 gallons, and it takes 4 gallons to get me 100 miles. I’ve got 4 gallons which is 100 m which is more than 40.”

      I think I win for simplicity, to be totally honest.

      • Bernie says

        It’s [1/4*16 * 25] vs [1/4*16 * (100/25)] = 100 m. Either one is fairly simple because we’re dealing with whole numbers. Your gas gauge you get used to interpreting in gallons (i.e. your brain reads the analog gauge in gallons). 4*12mpg is easier than 4*(100/8.3). Of course IRL everyone would just memorize the inverse for their car! Boats tend to measure consumption in gallons per hour. Again because that’s a whole number and people care about how many hours they can be out water skiing or how many hours the run to Port Townsend is. Gallons per mile doesn’t work because you have to use 3 place decimals to get the same resolution as mpg. Gallons(liters) per 100 miles(km) is the same trick as using percent to express interest rates.

      • Andrew Smith says

        Yeah you are correct it is more difficult when the number is a fraction compared to a whole number (which is why I rounded to 25 in my example).

    • David L says

      In Europe and Asia, numbers are widely expressed as xx.x l/100 km (with one digit after the decimal). People use that measure just fine.

    • Nathanael says

      Bernie, people also prefer addition to multiplication and prefer subtraction to division. You can only compare mpg using multiplication and division. You can compare gpm using addition and subtraction.

      That’s why gpm is better.

    • Nathanael says

      Anyway, hopefully this will soon be obsolete — everyone will refer to kilowatt-hours per 100 kilometers. :-)

  5. Bernie says

    there’s a lot of savings from going from, say, 5 MPG to 10 MPG, but much less savings going from 50 to 100.

    True there’s diminishing returns if your miles driven is fixed. But if you’re looking to increase your range (i.e. be able to commute from Camano Island instead of Lynnwood) then your more interested in maximizing the miles per gallon improvement. One thing it points out is why trucking companies are obsessed with small gains in mpg and why the biggest “green” incentive the government could create is investment in the conversion of the nations long haul trucking fleet to naturally gas.

    • Andrew Smith says

      But if you’re looking to increase your range (i.e. be able to commute from Camano Island instead of Lynnwood) then your more interested in maximizing the miles per gallon improvement.

      I think this is a more rare case, because most people move house less often than they change cars.

      In reality very few people think “I have 40 bucks to spend on gas each week, which means I can get 10 gallons, and which car is going to let me drive the most?” Most people are “I have to drive 30 mile a day, which car is going to cost me the least?”

    • Mike Orr says

      It’s also a rare case becase as density decreases in the exurbs, fewer people live there. Only a few people live as far as Camano Island, and only a fraction of them commute to Lynnwood or Seattle. I know a woman in Stanwood who comes to Seattle about once a month, and does all her “city errands” together on the same day.

    • Briana says

      In any case, as clearly stated in the title of the post, the goal under discussion is not “to increase your range”, but to reduce total fuel use, by either making all trips shorter, or using less fuel for the same trips.

    • Nathanael says

      Actually, long haul trucking mostly shouldn’t exist. (Corner cases include Alaska, high-and-wide loads, etc.)

      In most actual long-haul cases there’s sufficient volume that it should be put on rail. Trucks are more efficient than rail for short hauls, but practically never for long hauls, which is why UPS and FedEx are two of the biggest railway customers in the US.

  6. Norman says

    “The whole idea of trying to talk about which city’s mass transit system is greenest in terms of emissions per passenger-mile is terribly flawed.

    “Just think of it in terms of cars. Driving 5 miles in a 20 mpg car takes a quarter of a gallon of gasoline. Driving 25 miles in a 45 mpg car takes over half a gallon. Being the guy with the 5 mile commute and the 20 mpg car is considerably greener than being the guy driving much further in his Prius. The point of intra-urban transportation networks—whether you’re talking about the mass transit element or the private cars or bicycles or whatever—isn’t to transport people arbitrary distances, it’s to get people where they’re going. Having trips that aren’t very long isn’t cheating, it’s a great way to achieve efficiency.”

    This is absurd. Once again, some transit apologist uses the wrong comparison. comparing someone taking a 5-mile trip in a 20 mpg car to someone taking a 25 mile trip in a 45 mpg car is the wrong comparison.

    The correct comparison is someone taking a 5 mile trip in a 20 mpg car to someone taking the same 5 mile trip in a 45 mpg car. Which is greener? Clearly the person driving the 45 mpg car.

    It does not matter how long your daily trip is, a 45 mpg car is always using less fuel than a 20 mpg car for the same trip.

    In other words, the higher mpg car is always better than the lower mpg car, no matter how long the trip is. Just like having 2 people in the car is always better than having 1 person in the car no matter how long the trip is.

    Energy per passenger-mile is always the correct way to compare different transportation modes. Whether your trip is 30 miles or 5 miles, it always uses less energy to use a higher-energy-efficiency mode of transportation than a lower-energy-efficiency mode.

    • Andrew Smith says

      This is not at all accurate. Imagine I’ve got a hummer and you’ve got a prius. I drive 1 mile to work and consume 1/6 of a gallon. You drive 40 miles to work and consume 1 gallon. Who’s greener? Per passenger mile I’m a massive arsehole, but over all, I’m a saint. Per passenger mile you’re a saint, but overall, you’re an arsehole (in this hypothetical, I’m sure you’re actually wonderful in real-life).

      • Norman says

        Completely wrong. If you are talking about being green, anyone who drives a HUmmer anywhere is not green. He is wasting a lot of fuel driving a Hummer, no matter how short his trip is.

        If you are just trying to say that shorter trips are greener than longer trips, then you have an amazing grasp of the obvious.

      • Andrew Smith says

        I cannot tell if you are a fool or just being obtuse on purpose. You miss the obvious point and accuse us of some confusion by saying it’s “obvious”.

    • Matt the Engineer says

      “Energy per passenger-mile is always the correct way to compare different transportation modes.” I think this is where you’re missing the point. We’re not just comparing transportation modes, we’re comparing transportation systems. Rather than fixing your route (everyone lives in the suburbs, so how can I get them to the city most efficiently), allow that to vary (do I send more buses way out to the far suburbs, or do I run more buses between dense neighborhoods?).

      • Norman says

        Again, if you are just trying to say that the fewer miles people travel, the greener they are, then congratulations for grasping the obvious.

        This is a good argument against any transit systems and for everyone working at home, by telecommuting. If you don’t want to use any energy, don’t take any trips at all.

      • Matt the Engineer says

        “Again, if you are just trying to say that the fewer miles people travel, the greener they are, then congratulations for grasping the obvious.”

        No. The question is how you use this data. In my example, it would be in considering where to add more bus service. If you only care about $ per passenger-mile, you’d add more exurban routes. But that would be using the wrong metric. We want to get more people to their jobs/schools/grocery stores, so it’s the number of trips that’s valuable, not the number of miles.

      • Bernie says

        Can’t look at trips or passenger miles in isolation. If your transit choice is a solution that moves 200 people to their jobs 2.5 miles away it doesn’t reduce emissions as much as replacing 50 peoples 10 mile commute. Of course it would be better if all 250 lived within walking distance of work but that’s not reality. Couples often have to choose being near one of the others employer or splitting the difference. Boeing workers are routinely bounced from plant to plant which can add 30 miles or more to their commute.

      • Nathanael says

        If you’re measuring the “greenness” of a transportation / land use system, you want to look at total carbon dioxide emissions. If you assume you can’t control population and that excess population will just go “somewhere else”, then carbon dioxide emissions per capita.

        However, you also have to measure the utility of the transportation / land use system. To do that is hard: you have to actually measure the level of *unfulfilled desires*. Basically, ask people what sort of amenities they want to be able to get to (starting with work) and firstly whether that is possible, and secondarily whether that is too damn expensive or takes too damn long.

        The problem of figuring this out is not helped by people’s lack of self-awareness about alternative possibilities. I know one woman who was *sure* she needed to live in a single-family house in the suburbs to raise her kid, and a few years later she is happily raising her kid downtown and an ardent advocate of living downtown. Um, so.

        The result is that I think only political analysis suffices for determining the *utility* of a transportation / land use system. And only one type of political analysis: Are lots of people complaining or unhappy? Then you need to change something. Quite likely what you need to change is not what they THINK you need to change, but you need to change something. Is everyone happy? Then how can you make them happier?….

    • David L says

      Evaluating transportation in terms of passenger miles, rather than completed trips, inherently biases your evaluation in favor of sprawl.

      • Norman says

        If you don’t take passenger-miles into consideration, then comparisons of different modes’ energy efficiency is meaningless.

        Why do you think the Transportation Energy Data Book uses passenger-miles to compare different modes?

      • David L says

        Hardly. Not every trip to accomplish the same function is the same length, and available modes are related to trip length. It’s absolutely an apples-to-apples comparison to compare the energy use of someone riding a trolley half a mile up the counterbalance on the 13 to get to Safeway with someone driving their car to their suburban Safeway 3 miles away. Without that trolley there, you couldn’t have the development patterns that enable the shorter trips.

      • Norman says

        So, what this is all about is the theory that transit creates density, and you all love density?

        Actually, it is patently obvious that it is ZONING that creates density.

        If you want multi-use buildings seven stories high, just zone for that, and you will get it, no matter if there is transit serving that area, or not.

        Likewise, if an area is zoned for single-family homes, that is what it will be, even if you put a transit station right next to it, as long as it is zoned for single family homes.

        It is all about zoning. I actually thought you people were beginning to figure that out. Maybe not.

      • David L says

        If you zone for high density, but don’t put traffic in, you’ll get a slum.

        Transit doesn’t create density, but it enables density with comfort. Without it, your density is either impossibly choked with cars or (like third-world slums) impossible to access without walking an hour or risking your life in a likely unlicensed jitney.

      • Nathanael says

        Zoning creates lack of density.

        I have never seen a zoning rule which prohibits underbuilding. I’m not sure it’s even possible — if there were such a rule, the lot might well be left vacant, which constitutes, you guessed it, underbuilding.

      • Nathanael says

        Norman, consider the “unzoned” rural areas in many very rural states. Do they have 20-story buildings? No, they don’t. Zoning only restricts density; it can’t force it.

      • Stephen F says

        Norman, that’s not correct. Zoning places regulations on development standards. There can be mimimums on density and maximums. But nothing forces that density from being built except for high property taxation that essentially makes absentee or low-utilisation ownership high unviable.

    • says

      “This is absurd. Once again, some transit apologist uses the wrong comparison. comparing someone taking a 5-mile trip in a 20 mpg car to someone taking a 25 mile trip in a 45 mpg car is the wrong comparison.

      The correct comparison is someone taking a 5 mile trip in a 20 mpg car to someone taking the same 5 mile trip in a 45 mpg car. Which is greener? Clearly the person driving the 45 mpg car.

      It does not matter how long your daily trip is, a 45 mpg car is always using less fuel than a 20 mpg car for the same trip.”

      Except Andrew’s entire point is that length of the trips is as important if not more important a variable than fuel efficiency. Andrew isn’t arguing that fuel-efficient cars are evil, but that shorter trips are good. If you’re going to accuse your opponents of conveniently overlooking something, don’t accuse them of overlooking the very point they’re making.

  7. RossB says

    I agree, passenger miles is a poor metric for greenness. Furthermore, miles per hour is a poor metric for the speed of a transportation system. It makes less densely populated areas sound far more successful. Compare Moses Lake with Manhattan. I can jump in a car and travel a few miles much faster in Moses Lake than Manhattan. But then where will I be? From a business standpoint, not very far. In other words, in modern business, the key is to get people together. There are still plenty of people who have to get to the factory (or port, etc.) but by and large, it makes sense to measure a transportation system by how quickly it is to get from a person to another person (whether that person is traveling for business or pleasure). To use the example cited earlier, Moses Lake has about 20,000 people. I can probably visit any particular person in about 15 minutes. In Manhattan, I can visit that many people by just taking an elevator ride. A more appropriate comparison is something like Manhattan versus Seattle (proper). Manhattan has way more people, but it takes way less time to get from one person to another. Part of this is do to the often underrated elevator. One of the best transportation systems around.

    • Nathanael says

      In the earlier days of Manhattan, even getting to the port or the factory was an easy walk, but the loss of seaside industry (some is still in New Jersey) reduced that.

  8. Jei says

    OK, everyone. Here is the number that really matters:

    “Planners have long noted a phenomenon they call “transit leverage.” Simply put, transit leverage means that people who ride transit some of the time tend to drive less the rest of the time. Every mile a transit rider goes on public transit results in between four and nine fewer driving miles (depending on the community).”
    http://grist.org/cities/move-a-little-closer-please-carbon-zero-chapter-3/

    That means that the energy use per passenger mile can be 4 times as much in transit compared to a car, and we still break even (on energy use, we benefit a lot from many other aspects of transit).

    That means that this doesn’t matter::
    “…in 2010 transporting each passenger one mile by car required 3447 BTUs of energy. Transporting each passenger a mile by bus required 4118 BTUs, surprisingly making bus transit less green by this metric. Rail transit admittedly fares better, at 2520 BTUs per passenger mile, but even this is not the kind of slam-dunk advantage over the auto that transit advocates might hope for.”
    http://www.freakonomics.com/2012/11/07/can-mass-transit-save-the-environment-right-wing-or-left-wing-heres-a-post-everybody-can-hate/

    I’ll show you slam dunk, Freakonomics.

    • asdf says

      The extent of this transit leverage varies greatly depending on the type of transit. If it’s buses from Capitol Hill to downtown, the idea makes sense. But if it’s commuter expresses from North Bend to downtown, they’re still going to drive just as much for every other trip, regardless of whether there’s a bus to downtown or not.



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