Re: [carfree_cities] More pollution from slower traffic
- Andras Toth wrote:
> In Hungary where I come from there is an argument popping up time and againFrankly, it's simply not true at all. Yes, an engine has an
> 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.
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 --
- Andras Toth said:
>In Hungary where I come from there is an argument popping up timeand again
>about the harmful effect of slowing down the traffic. In manynewspaper
>articles we can read that cars going slower pollute more, andtherefore
>enlarging a road and building tunnels is justifiable and blahblah.More
>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).
>fuel = more pollution.convincing
>I can imagine a few counterarguments, but they are not entirely
>to me.we have
>1. "Slower cars mean fewer cars. This counterbalances the increase in
>pollution per car." Has this been proved? Also, even if it is true
>to face the much more far-reaching and difficult argument aboutreducing
>the amount of car traffic, as some people would object that they donot
>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 isnot as
>bad as being hit by a car." But with this argument, how can you be anYou're proceeding from the assumption that one has to accept
>environmentalist and a protector of civil rights at the same time?
dangerous, polluting objects on the street. Don't accept that
>3. "You can't live in a town turned into a network of highways." Andtheir
>answer could be: "If that's the price to pay for less pollution, whynot?"
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 townand
>aesthetics and social ethics etc. Isn't there a more straightforward
>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.
- --- In carfree_cities@y..., Mark Jaroski <mark@g...> wrote:
> Frankly, it's simply not true at all. Yes, an engineYou're half-right. Yes, engines have an optimum RPM, FOR
> has an optimum running speed, but it's measured in rpm
> (rotations per minute) *NOT* km/h.
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.
- 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
(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.)
- --- In carfree_cities@y..., Andras Toth <toth_andras@y...> wrote:
> Is it true or false that a car trapped in a traffic jamYes, it is true. And you can prove it yourself. For a
> 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?
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
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.
- Andras Toth continued:
>So, putting aside all theoretical argumentation about traffic andurban
>planning (in which I have always fully adhered to the opinions youdefend
>on this list):down by
>Is it true or false that a car trapped in a traffic jam or slowed
>sleeping policemen pollutes more on a given distance than a cardoing the
>same journey at an optimal speed/rpm whatever? If true, howsignificant 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).
- 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.
- Andras Toth wrote:
> Is it true or false that a car trapped in a traffic jam or slowedPerhaps the problem is the question. A better one might be, If the top
> 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?
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.