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Re: [carfree_cities] More pollution from slower traffic

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  • Mark Jaroski
    ... Frankly, it s simply not true at all. Yes, an engine has an optimum running speed, but it s measured in rpm (rotations per minute) *NOT* km/h. The car is
    Message 1 of 9 , Feb 6, 2002
      Andras Toth wrote:
      > In Hungary where I come from there is an argument popping up time and again
      > about the harmful effect of slowing down the traffic. In many newspaper
      > articles we can read that cars going slower pollute more, and therefore
      > enlarging a road and building tunnels is justifiable and blahblah.
      > And it is true, as we know that the right speed for optimal fuel
      > consumption for a car is way over 50 km/h (not to mention 30 km/h). More
      > fuel = more pollution.

      Frankly, it's simply not true at all. Yes, an engine has an
      optimum running speed, but it's measured in rpm (rotations
      per minute) *NOT* km/h.

      The car is polluting the least when it is parked with the engine off.

      Other than that the pollution curve has more to do with
      things like the temperature of the engine (a cold engine has
      to run a richer fuel mixture which generates soot).

      Speed has nothing at all to do with it.


      --
      --
      =================================================================
      -- mark at geekhive dot net --
    • carfreecrawford
      ... and again ... newspaper ... therefore ... More ... convincing ... we have ... reducing ... not ... The only way to speed up cars is to add road capacity,
      Message 2 of 9 , Feb 6, 2002
        Andras Toth said:

        >In Hungary where I come from there is an argument popping up time
        and again
        >about the harmful effect of slowing down the traffic. In many
        newspaper
        >articles we can read that cars going slower pollute more, and
        therefore
        >enlarging a road and building tunnels is justifiable and blahblah.
        >And it is true, as we know that the right speed for optimal fuel
        >consumption for a car is way over 50 km/h (not to mention 30 km/h).
        More
        >fuel = more pollution.
        >
        >I can imagine a few counterarguments, but they are not entirely
        convincing
        >to me.
        >
        >1. "Slower cars mean fewer cars. This counterbalances the increase in
        >pollution per car." Has this been proved? Also, even if it is true
        we have
        >to face the much more far-reaching and difficult argument about
        reducing
        >the amount of car traffic, as some people would object that they do
        not
        >want to be restricted in their access to the town.

        The only way to speed up cars is to add road capacity, which
        seems always to draw more cars, slowing things down again
        within a few years. You end up worse off than you started:
        more cars driving slowly.

        >2. "Slower cars mean fewer accidents. Breathing in polluted air is
        not as
        >bad as being hit by a car." But with this argument, how can you be an
        >environmentalist and a protector of civil rights at the same time?

        You're proceeding from the assumption that one has to accept
        dangerous, polluting objects on the street. Don't accept that
        basic assumption.

        >3. "You can't live in a town turned into a network of highways." And
        their
        >answer could be: "If that's the price to pay for less pollution, why
        not?"

        The answer is simply that you can't build your way out of
        congestion (and therefore out of pollution), and that trying
        to do so ultimately makes the situation even worse.

        >And then you have been dragged again into the tricky grounds of town
        >aesthetics and social ethics etc. Isn't there a more straightforward
        and
        >simple answer?

        Don't be afraid to argue this point, but I grant you that you'll
        only be likely to win your argument on the technical issues--
        people just don't seem to care about aesthetics any more. sign.
      • turpin
        ... You re half-right. Yes, engines have an optimum RPM, FOR A FIXED LOAD. But the load depends on the speed and the car. That doesn t answer the question of
        Message 3 of 9 , Feb 6, 2002
          --- In carfree_cities@y..., Mark Jaroski <mark@g...> wrote:
          > Frankly, it's simply not true at all. Yes, an engine
          > has an optimum running speed, but it's measured in rpm
          > (rotations per minute) *NOT* km/h.

          You're half-right. Yes, engines have an optimum RPM, FOR
          A FIXED LOAD. But the load depends on the speed and the
          car. That doesn't answer the question of how to make a
          trip in an automobile with minimum pollution. And if you
          ask THAT QUESTION, then it depends on the car as well as
          the engine. And for most cars, the answer is that you
          achieve minimum pollution per mile by driving at a steady
          rate, near highway speeds. The "steady rate" is the more
          important part of the answer. A car driven at a constant
          30 mph will use less gas and pollute less than one driven
          stop-and-go, though sometimes up to higher speeds. In
          fact, stop-and-go driving has a lot of harmful effects,
          not only in pollution, but also in wear and tear on the
          car, increase in accidents, etc.

          Now: there's always the issue of how important that
          fact is in relation to the variety of concerns at hand.
          But please, don't deny a fact just because it's
          inconvenient. It only hurts your credibility when you
          raise those other concerns.
        • Andras Toth
          Sorry, I managed to put a simple question very complicatedly, because I started to look for answers myself. So, putting aside all theoretical argumentation
          Message 4 of 9 , Feb 6, 2002
            Sorry, I managed to put a simple question very complicatedly, because I
            started to look for answers myself.

            So, putting aside all theoretical argumentation about traffic and urban
            planning (in which I have always fully adhered to the opinions you defend
            on this list):

            Is it true or false that a car trapped in a traffic jam or slowed down by
            sleeping policemen pollutes more on a given distance than a car doing the
            same journey at an optimal speed/rpm whatever? If true, how significant is
            the difference?

            (I am secretly hoping that someone will come up with a reference to a
            scientific study proving what Mark Jaroski stated, that is speed has
            nothing to do with it.)

            Andras Toth
          • turpin
            ... Yes, it is true. And you can prove it yourself. For a given car, a good first-order measure of how much it pollutes on one trip compared to another trip is
            Message 5 of 9 , Feb 6, 2002
              --- In carfree_cities@y..., Andras Toth <toth_andras@y...> wrote:
              > Is it true or false that a car trapped in a traffic jam
              > or slowed down by sleeping policemen pollutes more on a
              > given distance than a car doing the same journey at an
              > optimal speed/rpm whatever? If true, how significant is
              > the difference?

              Yes, it is true. And you can prove it yourself. For a
              given car, a good first-order measure of how much it
              pollutes on one trip compared to another trip is the
              ratio of gasoline burned on the two trips. The more
              fuel burned, the more pollution emitted. How much does
              your mileage improve when driving on the highway, over
              driving on the city? For most cars, it's quite a bit.
              If you look, you will find articles on the optimum
              driving for gas mileage, and this is close to optimum
              driving to minimize pollution. A typical car will
              get the the best mileage by being driven at a steady
              speed somewhere around 45 mph. It varies from vehicle
              to vehicle, as does pollution per gallon burned.

              "First-order measure." In fact, the pollution increase
              from stop and go traffic is even worse than reduced
              gas mileage indicates, because a car starting from a
              dead stop generates more pollution per gallon burned
              than the same car cruising at steady speed. (Some
              decades ago, I programmed engine control computers for
              Ford.)

              Of course, the pollution generated by walking or
              bicycling is MUCH less than by driving! The question
              asked is far from the only, or even the primary, factor
              in urban planning, or even in figuring out how to
              minimize pollution. Asked that way, yep, it's true.
              Don't try to buck the facts. But don't let one narrow
              fact distract from other issues.
            • carfreecrawford
              ... urban ... defend ... down by ... doing the ... significant is ... Brake-specific fuel consumption of internal combustion engines generally reaches a
              Message 6 of 9 , Feb 7, 2002
                Andras Toth continued:

                >So, putting aside all theoretical argumentation about traffic and
                urban
                >planning (in which I have always fully adhered to the opinions you
                defend
                >on this list):
                >
                >Is it true or false that a car trapped in a traffic jam or slowed
                down by
                >sleeping policemen pollutes more on a given distance than a car
                doing the
                >same journey at an optimal speed/rpm whatever? If true, how
                significant is
                >the difference?

                Brake-specific fuel consumption of internal combustion engines
                generally reaches a minimum (optimum) at the engine RPM at which
                the greatest torque can be developed. Generally, this is about
                half the maximum RPM but varies quite widely. Further, the
                fuel efficiency of a car in any gear is generally best when the
                engine is running at the optimum RPM, but this is only for
                that gear. The most efficient gear is alwyas the highest gear
                except if steep hills are being climbed. That means that the
                top gear of a car is always the most efficient and that a
                speed of somewhere between 40 MPH and 65 MPH is almost always
                the most efficient speed in top gear (some sports cars may
                be most efficient at even higher speeds).

                Aerodynamic dag of a car increases as the square of the speed,
                so the energy required to overcome air drag over a given
                distances doubles as the speed doubles. The rolling resistance
                of pneumatic tires increases less slowly than the square law,
                but, unlike railroad wheels, increases as speed increases due
                to higher losses to hysteresis.

                One of the largest terms in fuel consumption is acceleration
                of the vehicle. This is because the energy used to accelerate
                the vehicle is always lost when the vehicle brakes (except
                that some hybrids recover a comparatively small proportion
                of that energy). In stop-and-go traffic, poor fuel efficiency
                is inescapable because the engine is idling some of the time
                (using fuel without doing useful work) and is accelerating
                the vehicle quite a lot of the time. When the vehicle must
                brake, that energy is lost.

                Thus, we see that a vehicle maintaining a steady 50 MPH is
                almost certainly more fuel-efficient than a vehicle caught
                in traffic. Fuel efficiency translates quite directly to
                pollution output, with these caveats:

                Cars equipped with catalytic converters emit far more
                pollutants when cold than once warmed up. There are two
                reasons: cold engines require much richer fuel mixtures
                to run, and the converter does not "light" until it reaches
                a certain, relatively high, temperature. Once this occurs,
                the vehicle runs fairly clean.

                When a car is accelerating, especially if the driver uses
                a lot of power for a fast start, more fuel, and a richer
                mixture, is required. This results in higher, and sometimes
                MUCH higher, emissions levels.

                So, there can be almost no arugment with the contention that
                modern cars pollute less for a given distance when moving
                in free-flowing traffic at highway speeds. (This equation
                would change somewhat if engine power were limited to, say,
                20 HP and top speed were limited to, say, 30 MPH; in that
                case, a car moving a steady 20 MPH would be operating most
                efficiently, and would, incidentally, be gettign FAR better
                fuel economy than modern, high-powered cars.)

                The argument against adding traffic capacity must thus be
                made on other grounds, of which there is no shortage.

                Hope this helps. For further info, see Mark's Handbook
                (published under varying titles for many, many years, and
                the standard reference for mechanical engineering).

                Regards,

                J.H. Crawford
              • Doug Salzmann
                In a message sent Today, carfreecrawford wrote: - - Andras Toth continued: - - So, putting aside all theoretical argumentation about traffic and - urban
                Message 7 of 9 , Feb 7, 2002
                  In a message sent Today, carfreecrawford wrote:

                  ->
                  -> Andras Toth continued:
                  ->
                  -> So, putting aside all theoretical argumentation about traffic and
                  -> urban planning (in which I have always fully adhered to the opinions
                  -> you defend on this list):
                  ->
                  -> Is it true or false that a car trapped in a traffic jam or slowed
                  -> down by sleeping policemen pollutes more on a given distance than a
                  -> car doing the same journey at an optimal speed/rpm whatever? If true,
                  -> how significant is the difference?

                  Joel and turpin have both provided accurate and fairly complete answers to
                  this question, the short forms of which are "yes" and "quite significant.
                  There is no question: they are correct.

                  However, the original inquiry was about the efficacy of increasing highway
                  capacity to alleviate congestion, based partly upon the assertion that
                  such action will reduce pollution by allowing autos to operate under
                  conditions that minimize tailpipe emissions. As has been pointed out
                  earlier in the thread, the phenomenon of induced traffic prevents such
                  reduction from being achieved in the real world.

                  A quick review should be sufficient to confirm that assertion. For
                  more than fifty years, autocentric societies have been madly adding to
                  highway capacity. Name one such society, or one metro area, where
                  increased capacity has resulted in overall reductions in tailpipe
                  emissions, longer than momentarily.

                  But cars don't belong in cities anyway. And they wouldn't belong even if
                  they were fueled by pleasant thoughts and emitted only music.

                  -Doug
                • Chris Bradshaw
                  ... Perhaps the problem is the question. A better one might be, If the top speed were held down and more stop-and-go introduced to the driving experience,
                  Message 8 of 9 , Feb 7, 2002
                    Andras Toth wrote:

                    > Is it true or false that a car trapped in a traffic jam or slowed
                    > down by sleeping policemen pollutes more on a given distance than
                    > a car doing the same journey at an optimal speed/rpm whatever?
                    > If true, how significant is the difference?

                    Perhaps the problem is the question. A better one might be, If the top
                    speed were held down and more stop-and-go introduced to the driving
                    experience, would fewer people choose to drive, and would the use their
                    car in a more efficient manner?

                    The argument your question implies is part of a
                    favour-the-filthiest-mode thinking: whichever road users pollute the
                    most, given them a favoured postion on the road. In the name of
                    "safety," road engineers also argue that vulnerable road users must
                    yield to motorists who, after all, could kill them. In both cases, the
                    pedestrian and cyclist are restrained and, surprise-surprise, over time
                    their numbers decrease, with many of those trips switching to driving,
                    making the congestion-caused delays greater. Please note that there is
                    an ecology at work here.

                    Speed and delays are a product of conflicts on the roads caused by a
                    demands for the road's use. If you could make sure only one person
                    could use the road system at a time, voila!, no conflicts. But would
                    that be politically acceptable?

                    I am glad the wind friction was mentioned, since it suggests that the
                    premise itself is wrong.

                    And as far as the stop-and-go factor is concerned, in general, there are
                    more occasions-per-mile to have to stop or slow in a dense environment
                    than a less dense one, partly due to the smaller number of intersections
                    in each mile. But lower densities translate into longer distances for
                    each trip, more than wiping out any savings in reduced need to use the
                    brakes and gas-pedal.

                    One final note: speed is a _personal_ benefit to the driver, not the
                    community. As speed increases, the distanes between vehicles has to
                    increase proportionally, meaning that fewer drivers can be accommodated
                    at higher average speeds than at lower ones (and lane widths must be
                    higher, too). Also, increased speeds, as you have noted, increase the
                    seriousness of collisions, and they increase the marginal utility of
                    driving over walking/cycling/transit, not to mention the longer-term
                    effect of the increases in modal split cutting at the critical mass the
                    other modes need to be viable.

                    Chris Bradshaw
                    Ottawa
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