OT: 20 & 30mph pollution differences

On 21 Aug, 18:09, Tom Crispin <[email hidden]>

Quoted message said:

On Tue, 21 Aug 2007 16:38:13 +0100, Matt B

matt.bourke'@nospam.london.com said:

Tom Crispin wrote:

Quoted message said:

Quoted message said:

Parks are there for recreational use, not as a motorists thoroughfare.

Quoted message said:

Explain why you think (if indeed you do think) that pedestrians and
cyclists can use the park recreationally as a thoroughfare, but
motorists can't.

How nice, a Strawman.

The Greenwich Cyclists' Campaign is to have the speed limit through
Greenwich Park lowered to 20 mph and overtaking prohibited. This
limit would apply equally to cyclists, as do all speed limits in the
Royal Parks, including park vehicle roads where the limit is 5mph.

The son of the cyclist killed in Greenwich Park posted briefly on
Cycling Plus.

The car coming downhill was overtaking another car and on the wrong
side of the road so the cyclist would have been slowly meandering up
the hill when the driver, who's been arrested, smashed into him in the
cyclist's lane and killed him.

Marc Brett said:

On Tue, 21 Aug 2007 16:38:13 +0100, Matt B

matt.bourke'@nospam.london.com said:

Tom Crispin said:

Parks are there for recreational use, not as a motorists thoroughfare.

Explain why you think (if indeed you do think) that pedestrians and
cyclists can use the park recreationally as a thoroughfare, but
motorists can't.

It's the same reason that I do not allow horses to walk through my
living room to get to the back garden, but people, cats, and dogs are
welcome.

And that reason is?

--
Matt B

Tom Crispin said:

On Tue, 21 Aug 2007 16:38:13 +0100, Matt B

matt.bourke'@nospam.london.com said:

Tom Crispin said:

Parks are there for recreational use, not as a motorists thoroughfare.

Explain why you think (if indeed you do think) that pedestrians and
cyclists can use the park recreationally as a thoroughfare, but
motorists can't.

How nice, a Strawman.

How so? You actually meant that neither pedestrians, cyclists nor
motorists should be allowed to use the parks as a thoroughfare? I'm
delighted by your consistency, but amazed.

Quoted message said:

The Greenwich Cyclists' Campaign is to have the speed limit through
Greenwich Park lowered to 20 mph and overtaking prohibited.

And rely on what type of enforcement?

Quoted message said:

This
limit would apply equally to cyclists, as do all speed limits in the
Royal Parks, including park vehicle roads where the limit is 5mph.

Yes. Is the 5 mph strictly enforced?

--
Matt B

Martin Dann said:

When I was at school I was told that the most efficient speed for cars
was about 56mph. (Obviously cruising, not continually speeding up and
slowing down). I don't know where they got this figure from, or why they
seemed to think that is was the same for all cars.
This was also the speed that some car makers used for their mpg figures.
(makes sense).

Suggests a causal relationship: if everyone quotes their 56mph figures,
car makers have an incentive to improve efficiency at that speed ;-)

-dan

Daniel Barlow said:

Martin Dann said:

When I was at school I was told that the most efficient speed for cars
was about 56mph. (Obviously cruising, not continually speeding up and
slowing down). I don't know where they got this figure from, or why they
seemed to think that is was the same for all cars.
This was also the speed that some car makers used for their mpg figures.
(makes sense).

Suggests a causal relationship: if everyone quotes their 56mph figures,
car makers have an incentive to improve efficiency at that speed ;-)

-dan

Some mornings I have seen policemen with speed cameras checking
vehicle speeds down the hill, but despite regularly hitting 30mph
(always riding in the primary position), the only police action I've
incurred has been a cheery wave! It does seem perverse to have a 20mph
speed limit in residential streets near the park, and then allow the
cagers to race down the hill at 30 - particularly bearing in mind the
number of pedestrians, kids playing football, etc who are running
around the park, some of whom won't appreciate that it is used as a
rat-run.

Start quote:

It was my father who was killed in the park. He was a wonderful man
who had been cycling for all his his working life.

The car went into the wrong damn lane. THE POLICE TOLD ME. So no, I am
not speculating.

If you can't drive don't get behind the wheel.

He will be greatly missed. Thank you for your thoughts and sympathy,
as fellow cyclists you all can appreciate the kind of drivers he
faced.

My poor daddy, but he died doing what he loved.

http://www.cyclingplus.co.uk/forum/topic.asp?TOPIC_ID=134382&whichpage=3

You kind of predicted this death Tom, looking at that thread.

When motorists can't refrain from speeding through a PARK!

On Tue, 21 Aug 2007 18:29:27 +0100, Matt B

matt.bourke'@nospam.london.com said:

Tom Crispin said:

On Tue, 21 Aug 2007 16:38:13 +0100, Matt B

matt.bourke'@nospam.london.com said:

Tom Crispin wrote:
> Parks are there for recreational use, not as a motorists thoroughfare.
Explain why you think (if indeed you do think) that pedestrians and
cyclists can use the park recreationally as a thoroughfare, but
motorists can't.

How nice, a Strawman.

How so? You actually meant that neither pedestrians, cyclists nor
motorists should be allowed to use the parks as a thoroughfare? I'm
delighted by your consistency, but amazed.

Lovery, another Strawman.

You do know what a Strawman is, don't you?

Quoted message said:

Quoted message said:

The Greenwich Cyclists' Campaign is to have the speed limit through
Greenwich Park lowered to 20 mph and overtaking prohibited.

And rely on what type of enforcement?

In Richmond Park the police use hand held radar guns (or whatever
they're called). According to the Park Manager, Derrick Spurr, they
are very effective.

Quoted message said:

Quoted message said:

This
limit would apply equally to cyclists, as do all speed limits in the
Royal Parks, including park vehicle roads where the limit is 5mph.

Yes. Is the 5 mph strictly enforced?

Define strictly.

It's probably more strictly enforced than the average residential
street. I know that a lot of verbal warnings are issued to cyclists.
I have no idea what would happen to a park employee caught at 8mph.

elyob said:

Hi,

Just about to respond to a letter in my local rag r.e. Richmond Park.
The writer claims that driving at 20mph creates more pollution, he
doesn't back it up and wondered if there is any evidence to the
contrary?

yeah but the distance would probably end up making close to zero effect.

also the 20zone makes it clear this is a shared road, much like bushy
park the car isn't king.

Quoted message said:

He believes that the RPA is trying to close the park to all cars and
that the park is the main route for vehicles travelling between
Kingston and west London.[1]

He wants the RPA team retired and new 21st Century thinking people
brought in. I'm going to suggest congestion charge cameras which make
sure people obey the 20mph limits by averaging their time within the
park. Also, for anyone who uses it solely to 'cut-through' a £5
toll ... is that 21st Century enough?

Nick

[1] Funny, I thought the A3 was there for that reason.

me too..

though it's rare for me to use a car for that sort of journey it's
normally by train or bike etc.

roger
--
www.rogermerriman.com

On Tue, 21 Aug 2007 11:04:39 -0700, spindrift <[email hidden]>

Quoted message said:

You kind of predicted this death Tom, looking at that thread.

When motorists can't refrain from speeding through a PARK!

I had a meeting with the park manager this time last year. The
meeting was to discuss my use of a park road, closed to cyclists and
other road traffic other than park vehicles, to teach children road
positioning before going onto public roads. As we walked the route we
discussed the provision of cycle parking facilities in the park and
the two-way shared use cycle lane, on the footway next to the downhill
motor traffic lane. I said that I would not use it downhill as it is
dangerous as pedestrians stray into it without looking, and that I
wouldn't use it uphill as it means crossing the road twice.

When I go uphill with children I use the footway on the other side of
the road from the cycle path, i.e. on the left. I do not use the
carriageway with children because when inexperienced cyclists when
going up steep hills have a tendancy to unexpectedly swerve, and that
presents an unacceptable risk which is easily avoided. Besides, park
regulations allow for children under 11 to cycle on the footway.

I have never, in my wildest nighmares, envisaged an death such as
occurred on 26th June. This appears to be utter recklessness and
disregard for human life on the part of the driver.

I intend to attend the inquest on 22nd January.

The Royal Parks' cycling policy is currently under review.
www.royalparks.org.uk/docs/Cycling%20Policy%20Consultation%20Document.pdf

I like to think that this was partly as a result of a letter I sent to
Tessa Jowell, the then Culture Secretary who had responsibility for
the Royal Parks.

On Tue, 21 Aug 2007 14:09:01 +0100, elyob wrote
(in article <[email hidden]>😉:

Quoted message said:

Hi,

Just about to respond to a letter in my local rag r.e. Richmond Park.
The writer claims that driving at 20mph creates more pollution, he
doesn't back it up and wondered if there is any evidence to the
contrary?

I think closing some of the gates during weekday rush hour would do a fairly
effective job of cutting the traffic.

From a cycling POV, as a lap-munching roadie, having cars limited to 20mph is
a right pain in the [censored]. I'd much rather they overtook "properly", and
didn't hold me up.

--
simonk

Tom Crispin said:

On Tue, 21 Aug 2007 18:29:27 +0100, Matt B

matt.bourke'@nospam.london.com said:

Tom Crispin said:

On Tue, 21 Aug 2007 16:38:13 +0100, Matt B
<"matt.bourke"@nospam.london.com> wrote:

> Tom Crispin wrote:
>> Parks are there for recreational use, not as a motorists thoroughfare.
> Explain why you think (if indeed you do think) that pedestrians and
> cyclists can use the park recreationally as a thoroughfare, but
> motorists can't.
How nice, a Strawman.

How so? You actually meant that neither pedestrians, cyclists nor
motorists should be allowed to use the parks as a thoroughfare? I'm
delighted by your consistency, but amazed.

Lovery, another Strawman.

You do know what a Strawman is, don't you?

I thought I did. Based on previous experience I rather assumed you
wanted to exclude motorists from passing through, but allow others
(pedestrians and cyclists). So I posed my first question. Your answer,
and my understanding of a strawman, led me to ask the second question
instead.

So to clear it up, and to leave no question unanswered... Are you in
favour anyone using parks as thoroughfares? If yes, who, as we know you
don't think motorists should? Other questions may follow - feel free to
anticipate them and save me asking them in a follow-up ;-)

Quoted message said:

Quoted message said:

Quoted message said:

The Greenwich Cyclists' Campaign is to have the speed limit through
Greenwich Park lowered to 20 mph and overtaking prohibited.

And rely on what type of enforcement?

In Richmond Park the police use hand held radar guns (or whatever
they're called). According to the Park Manager, Derrick Spurr, they
are very effective.

Define "effective".

Quoted message said:

Quoted message said:

Quoted message said:

This
limit would apply equally to cyclists, as do all speed limits in the
Royal Parks, including park vehicle roads where the limit is 5mph.

Yes. Is the 5 mph strictly enforced?

Define strictly.

There are unlikely ever to be any contraventions.

Quoted message said:

It's probably more strictly enforced than the average residential
street. I know that a lot of verbal warnings are issued to cyclists.
I have no idea what would happen to a park employee caught at 8mph.

So, is that limit necessary, if yes, why, and how could it be enforced
100% then?

--
Matt B

On Tue, 21 Aug 2007 20:51:34 +0100, Matt B

matt.bourke'@nospam.london.com said:

Tom Crispin said:

On Tue, 21 Aug 2007 18:29:27 +0100, Matt B

matt.bourke'@nospam.london.com said:

Tom Crispin wrote:
> On Tue, 21 Aug 2007 16:38:13 +0100, Matt B
> <"matt.bourke"@nospam.london.com> wrote:
>
>> Tom Crispin wrote:
>>> Parks are there for recreational use, not as a motorists thoroughfare.
>> Explain why you think (if indeed you do think) that pedestrians and
>> cyclists can use the park recreationally as a thoroughfare, but
>> motorists can't.
> How nice, a Strawman.
How so? You actually meant that neither pedestrians, cyclists nor
motorists should be allowed to use the parks as a thoroughfare? I'm
delighted by your consistency, but amazed.

Lovery, another Strawman.

You do know what a Strawman is, don't you?

I thought I did.

What is it then?

Quoted message said:

Based on previous experience I rather assumed you
wanted to exclude motorists from passing through, but allow others
(pedestrians and cyclists). So I posed my first question. Your answer,
and my understanding of a strawman, led me to ask the second question
instead.

So to clear it up, and to leave no question unanswered... Are you in
favour anyone using parks as thoroughfares?

Your question does nothing to clear anything up.

Quoted message said:

If yes, who, as we know you
don't think motorists should?

Strawman.

Quoted message said:

Other questions may follow - feel free to
anticipate them and save me asking them in a follow-up ;-)

Quoted message said:

Quoted message said:

> The Greenwich Cyclists' Campaign is to have the speed limit through
> Greenwich Park lowered to 20 mph and overtaking prohibited.
And rely on what type of enforcement?

In Richmond Park the police use hand held radar guns (or whatever
they're called). According to the Park Manager, Derrick Spurr, they
are very effective.

Define "effective".

I suggest you ask the person who said it.

Quoted message said:

Quoted message said:

Quoted message said:

> This
> limit would apply equally to cyclists, as do all speed limits in the
> Royal Parks, including park vehicle roads where the limit is 5mph.
Yes. Is the 5 mph strictly enforced?

Define strictly.

There are unlikely ever to be any contraventions.

Then no.

Quoted message said:

Quoted message said:

It's probably more strictly enforced than the average residential
street. I know that a lot of verbal warnings are issued to cyclists.
I have no idea what would happen to a park employee caught at 8mph.

So, is that limit necessary, if yes, why, and how could it be enforced
100% then?

Silly question. Nothing, not even murder, can be enforced 100%.

CoyoteBoy said:

Quoted message said:

Just anecdotal, but some years ago I drove across Mali in an old
Landrover. We were on dirt roads/no roads and took a week to cover a
couple of hundred miles mostly in low ratio. Very worried about
running out of fuel but turned out that consumption was almost
identical to if we'd cruised the same distance in top gear.

Quoted message said:

Thats because landies would struggle to be any less economical 🙂 But
in all seriousness in a low-range you're running the engine so
unloaded its barely trying.

I used to 'drive for economy' on my daily commute by trying to get into as
high a gear as I could, as often as I could. The car (2 litre) would
regularly manage 30-32mpg averaged over a full tank (includes some heavy
acceleration when overtaking and the like).

Then I did the IAM advanced driving course, and got into 'appropriate gear'
driving, which is, roughly, 2nd gear for 20mph, 3rd for 30, 4th for 40, and
5th for 50 and up. I still boot it for overtaking, but my fuel consumption
hasn't changed at all - still 30-32 on average.

Interestingly, on a recent 450 mile blast down the motorway at exactly 70mph
(or thereabouts, sort of, if you get my meaning), it returned just over
32mpg. A more sedate 100 miles of motorway at around 60mph, followed by 100
miles of Welsh A road, also at 60mph and occasionally 'sprited', returned
over 35mpg - the best it's ever done.

--
Wally
www.wally.myby.co.uk
If it ain't broke, fix it until it is.

Tom Crispin said:

On Tue, 21 Aug 2007 20:51:34 +0100, Matt B

matt.bourke'@nospam.london.com said:

Tom Crispin said:

On Tue, 21 Aug 2007 18:29:27 +0100, Matt B
<"matt.bourke"@nospam.london.com> wrote:

> Tom Crispin wrote:
>> On Tue, 21 Aug 2007 16:38:13 +0100, Matt B
>> <"matt.bourke"@nospam.london.com> wrote:
>>
>>> Tom Crispin wrote:
>>>> Parks are there for recreational use, not as a motorists thoroughfare.
>>> Explain why you think (if indeed you do think) that pedestrians and
>>> cyclists can use the park recreationally as a thoroughfare, but
>>> motorists can't.
>> How nice, a Strawman.
> How so? You actually meant that neither pedestrians, cyclists nor
> motorists should be allowed to use the parks as a thoroughfare? I'm
> delighted by your consistency, but amazed.
Lovery, another Strawman.

You do know what a Strawman is, don't you?

I thought I did.

What is it then?

Is it a misspelling of straw man?

Quoted message said:

Quoted message said:

Based on previous experience I rather assumed you
wanted to exclude motorists from passing through, but allow others
(pedestrians and cyclists). So I posed my first question. Your answer,
and my understanding of a strawman, led me to ask the second question
instead.

So to clear it up, and to leave no question unanswered... Are you in
favour anyone using parks as thoroughfares?

Your question does nothing to clear anything up.

No, but the idea of a question is that an answer is provided. If you
answer it, all will become apparent.

Quoted message said:

Quoted message said:

If yes, who, as we know you
don't think motorists should?

Strawman.

No, it's not. It is a perfectly legitimate question, if you answer
"yes" to the one before it.

Let's put it another way: do you think that cyclists should be allowed
to use the park as a thoroughfare?

Quoted message said:

Quoted message said:

Other questions may follow - feel free to
anticipate them and save me asking them in a follow-up ;-)

Quoted message said:

>> The Greenwich Cyclists' Campaign is to have the speed limit through
>> Greenwich Park lowered to 20 mph and overtaking prohibited.
> And rely on what type of enforcement?
In Richmond Park the police use hand held radar guns (or whatever
they're called). According to the Park Manager, Derrick Spurr, they
are very effective.

Define "effective".

I suggest you ask the person who said it.

Ah.

Quoted message said:

Quoted message said:

Quoted message said:

>> This
>> limit would apply equally to cyclists, as do all speed limits in the
>> Royal Parks, including park vehicle roads where the limit is 5mph.
> Yes. Is the 5 mph strictly enforced?
Define strictly.

There are unlikely ever to be any contraventions.

Then no.

So a guaranteed appropriate speed, rather than a partially effective
speed limit would be better.

Quoted message said:

Quoted message said:

Quoted message said:

It's probably more strictly enforced than the average residential
street. I know that a lot of verbal warnings are issued to cyclists.
I have no idea what would happen to a park employee caught at 8mph.

So, is that limit necessary, if yes, why, and how could it be enforced
100% then?

Silly question.

No, is the limit necessary? If it is then what is the consequence of
not enforcing it?

Quoted message said:

Nothing, not even murder, can be enforced 100%.

Ah, you're getting close. A "speed limit" is notoriously difficult, to
enforce, and therefore unsustainable. Presumably the limit was set to
solve a problem. A solution which cannot work (because it cannot be
enforced 100%) isn't really a solution - is it. A real solution is
something which actually solves the original problem - sustainably.

--
Matt B

"Clive George" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

"Tony Raven" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Sounds like that old one of being low on fuel and having to drive fast to
get to the petrol station before it ran out ;-)

I know people who have done that - one who was a reasonably respected
physics professor who knew it wouldn't help, but he did it anyway :-)

What he did know however was the faster he was going the closer to the
garage he could get by declutching and dropping into neutral as soon as the
engine coughed.

and on the original topic
If you are driving at 20mph in 1st gear its probably going to be more
poluting and less fuel economical than choosing the appropriate gear for the
road conditions. Most cars I have driven are happy between 20 and 30 in 3rd,
not struggling at 20 in 3rd unless a nasty little hill gets in the way in
which case a breif blast in 2nd before switching back to 3rd does the trick,
though at 30 its little more than a similar blast of acceleration in 3rd
probably resulting not much difference in waste output.

Niall

On Tue, 21 Aug 2007 22:23:23 +0100, Matt B

matt.bourke'@nospam.london.com said:

Quoted message said:

Quoted message said:

> You do know what a Strawman is, don't you?
I thought I did.

What is it then?

Is it a misspelling of straw man?

Please explain what a straw man is.

Quoted message said:

Quoted message said:

Quoted message said:

Based on previous experience I rather assumed you
wanted to exclude motorists from passing through, but allow others
(pedestrians and cyclists). So I posed my first question. Your answer,
and my understanding of a strawman, led me to ask the second question
instead.

So to clear it up, and to leave no question unanswered... Are you in
favour anyone using parks as thoroughfares?

Your question does nothing to clear anything up.

No, but the idea of a question is that an answer is provided. If you
answer it, all will become apparent.

How?

Quoted message said:

Quoted message said:

Quoted message said:

If yes, who, as we know you
don't think motorists should?

Strawman.

No, it's not. It is a perfectly legitimate question, if you answer
"yes" to the one before it.

But you've answered for me.

Quoted message said:

Let's put it another way: do you think that cyclists should be allowed
to use the park as a thoroughfare?

What do you think? Do you think that a legitimate use of a Royal Park
is for a cyclist to travel from one point to another as part of a
longer trip?

Do you think that a legitimate use is as a rush hour relief route,
open to non-comercial motor traffic dawn - 10am and 4pm - dusk?

Quoted message said:

Quoted message said:

Quoted message said:

Other questions may follow - feel free to
anticipate them and save me asking them in a follow-up ;-)

>>> The Greenwich Cyclists' Campaign is to have the speed limit through
>>> Greenwich Park lowered to 20 mph and overtaking prohibited.
>> And rely on what type of enforcement?
> In Richmond Park the police use hand held radar guns (or whatever
> they're called). According to the Park Manager, Derrick Spurr, they
> are very effective.
Define "effective".

I suggest you ask the person who said it.

Ah.

Quoted message said:

Quoted message said:

>>> This
>>> limit would apply equally to cyclists, as do all speed limits in the
>>> Royal Parks, including park vehicle roads where the limit is 5mph.
>> Yes. Is the 5 mph strictly enforced?
> Define strictly.
There are unlikely ever to be any contraventions.

Then no.

So a guaranteed appropriate speed, rather than a partially effective
speed limit would be better.

Straw man.

Quoted message said:

Quoted message said:

Quoted message said:

> It's probably more strictly enforced than the average residential
> street. I know that a lot of verbal warnings are issued to cyclists.
> I have no idea what would happen to a park employee caught at 8mph.
So, is that limit necessary, if yes, why, and how could it be enforced
100% then?

Silly question.

No, is the limit necessary? If it is then what is the consequence of
not enforcing it?

Straw man.

Quoted message said:

Quoted message said:

Nothing, not even murder, can be enforced 100%.

Ah, you're getting close. A "speed limit" is notoriously difficult, to
enforce, and therefore unsustainable. Presumably the limit was set to
solve a problem. A solution which cannot work (because it cannot be
enforced 100%) isn't really a solution - is it. A real solution is
something which actually solves the original problem - sustainably.

And how do you propose that is achieved?

"Daniel Barlow" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Martin Dann said:

When I was at school I was told that the most efficient speed for cars
was about 56mph. (Obviously cruising, not continually speeding up and
slowing down). I don't know where they got this figure from, or why they
seemed to think that is was the same for all cars.
This was also the speed that some car makers used for their mpg figures.
(makes sense).

Suggests a causal relationship: if everyone quotes their 56mph figures,
car makers have an incentive to improve efficiency at that speed ;-)

DIN standard for extra-urban fuel economy. Simplest way to get good fuel
economy at that speed? Set 5th gear to drive at around Idling speed plus a
wee bit more (so the engine isn't screaming to be dropped down a gear) then
put a very good economy driver in the driving seat.

I am tempted to obtain a Polo bluemotion 1 and see if it will manage 70mpg
and then strip out all the extra weight I see in the sepc and see if it can
manage 100mpg. All with me driving.

Niall

"Marc Brett" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

On Tue, 21 Aug 2007 16:38:13 +0100, Matt B

matt.bourke'@nospam.london.com said:

Tom Crispin said:

Parks are there for recreational use, not as a motorists thoroughfare.

Explain why you think (if indeed you do think) that pedestrians and
cyclists can use the park recreationally as a thoroughfare, but
motorists can't.

It's the same reason that I do not allow horses to walk through my
living room to get to the back garden, but people, cats, and dogs are
welcome.

Shhh, Don't let the Ramblers association find out, you will have 200 oif
them demanding a gree signpost pointing through your house.

Tom Crispin said:

Matt B said:

Tom Crispin said:

Matt B wrote:
> Tom Crispin wrote:

You do know what a Strawman is, don't you?

You do know what a Troll is?

--
Tony

" I would never die for my beliefs because I might be wrong."
Bertrand Russell

elyob said:

Hi,

Just about to respond to a letter in my local rag r.e. Richmond Park.
The writer claims that driving at 20mph creates more pollution, he
doesn't back it up and wondered if there is any evidence to the
contrary?

IMHO (takes deep breath), assuming (always dangerous) that the gearing
is chosen such that the engine is at roughly the same rev/min at
either 20 or 30mile/hr vehicle speed, then:
1) engine load and energy needed/mile will be less at 20 (unless gear
chosen is incredibly inefficient) due to aero drag (and a few other
terms - final drive losses etc.)
2) engine will be consuming less fuel/sec (as power output is
reduced), but operating less efficiently at 20 - as a generalisation,
efficiency = (work out) / (energy in) increases with increasing load
3) without knowing the engine and vehicle's exact characteristics it's
impossible to say definitely which speed would be more efficient
(mpg), hence would produce less CO2/mile, but my guess would be that
it would have a higher mpg at 20mile/hr
4) lower load at 20 would result in lower oxides of nitrogen emissions
(good) and probably lower hydrocarbon emissions (good)
5) overall noise level probably less (although some diesel engines
have a negative noise vs load characteristic at some engine speeds,
but tyre noise would probably be dominant)

If the same gear were used, then (within reason = engine not operating
below a 'smooth running' speed) the engine would probably be more
efficient at 20mph = less CO2/mile (maximum efficiency does fall off
as speed drops below peak torque speed) as at a higher load, but
oxides of nitrogen emissions/mile would be greater (bad).

In other words, to answer the OP's original question, 'depends'..... :~