20mph speed limits

On 13/12/2011 16:25, GT wrote:
"The wrote in message
...
On 13/12/2011 15:21, The Weasel wrote:
On 13/12/2011 13:39, GT wrote:
"Simon wrote in message
...
On 06/12/2011 21:39, Just zis Guy, you know? wrote:
On Tue, 6 Dec 2011 13:08:33 -0000, wrote:

wrote in message
...
On 30/11/2011 20:15, GT wrote:

Serves communities less as the throughput of that 'serving' road
has
been
lowered by 33%.


Really? - how's that then?

Give exact figures to support your argument

At max road capacity, you will get 30/20 more cars through at 30mph
over
20mph. Not a hard problem to solve!!

That is, of course, wrong, first because it assumes that maximum road
capacity is independent of speed (which it isn't, due to turbulent
flow) and second because it assumes that the drivers at 20 and at
30mph maintain the same headway, which they definitely should not.

Guy

Without sitting down and working out the mathematics of it I would have
thought that traffic throughput increases as speed decreases.

Then perhaps you should sit down and work out the maths then! Get a few
pennies and a folded piece of paper, held vertically with the fold at
the
bottom. See how many you can push through the paper in 15 seconds when
you
push them slowly. Then see how many you can get across the piece of
paper
when you push them quickly...
As the speed increased an object's speed increases, you can get more
objects
through a measured space in a given time!


Pennies do not need space in front – traffic does.


I just worked out the math.

What is a 'math'?


http://dictionary.reference.com/browse/math

The only way that you will get a more efficient vehicle throughput is for
vehicles to drive with less distance between themselves and the vehicle
ahead. The way you can achieve that is to drive more slowly. There is a
cut off point but it would be at a very low speed. A lot lower than 20mph.

The gap is a factor, yes, but increasing the speed of a stream of vehicles
will never reduce the throughput? In reality, there will be a slightly
larger gap between those vehicles, but you would have to double the gap to
maintain the same throughput - this never happens, therefore faster traffic
gets more cars along a set stretch of road.


A two second gap is a two second gap. As speed increases, so does the
gap needed for safety. If anyone is going to risk driving too close to
the vehicle ahead it is safer they do that at 10mph than it is at
100mph. Therefore the only safe way to achieve a better throughput is to
reduce the speed of the traffic.

Multiple pile-ups tend not to happen at low speed.

Work out the math :-)

Go on, work it out.

The results might surprise you.



--
The Weasel

On Tue, 13 Dec 2011 13:39:26 -0000, "GT" wrote:

"Simon Weaseltemper" wrote in message
...
On 06/12/2011 21:39, Just zis Guy, you know? wrote:
On Tue, 6 Dec 2011 13:08:33 -0000, wrote:

wrote in message
...
On 30/11/2011 20:15, GT wrote:

Serves communities less as the throughput of that 'serving' road has
been
lowered by 33%.


Really? - how's that then?

Give exact figures to support your argument

At max road capacity, you will get 30/20 more cars through at 30mph over
20mph. Not a hard problem to solve!!

That is, of course, wrong, first because it assumes that maximum road
capacity is independent of speed (which it isn't, due to turbulent
flow) and second because it assumes that the drivers at 20 and at
30mph maintain the same headway, which they definitely should not.

Guy

Without sitting down and working out the mathematics of it I would have
thought that traffic throughput increases as speed decreases.

Then perhaps you should sit down and work out the maths then! Get a few
pennies and a folded piece of paper, held vertically with the fold at the
bottom. See how many you can push through the paper in 15 seconds when you
push them slowly. Then see how many you can get across the piece of paper
when you push them quickly...
As the speed increased an object's speed increases, you can get more objects
through a measured space in a given time!

Ah, so you didn't understand.

Traffic does not flow like pennies down a channel. First the headway
increases with increasing speed, so a car at 40mph occupies a larger
block of road than one at 30mph, and second, traffic exhibits
turbulent flow - that's why the M25 VSLs actually increase throughput
*and* average speed by reducing the speed limit. Motorways are a rare
example where traffic throughput is constrained by road capacity,
normally it's junction capacity (which is why traffic jams are almost
always at junctions).

There are some good texts on traffic management aimed at the general
reader if you're actually interested.

Guy
--
Guy Chapman, http://www.chapmancentral.co.uk
The usenet price promise: all opinions are guaranteed
to be worth at least what you paid for them.

On Tue, 13 Dec 2011 16:26:34 -0000, "GT" wrote:

"The Weasel" wrote in message
...
On 13/12/2011 13:40, GT wrote:
"Just zis Guy, you wrote in
message
...
On Tue, 6 Dec 2011 13:08:33 -0000, wrote:

wrote in message
...
On 30/11/2011 20:15, GT wrote:

Serves communities less as the throughput of that 'serving' road has
been
lowered by 33%.


Really? - how's that then?

Give exact figures to support your argument

At max road capacity, you will get 30/20 more cars through at 30mph
over
20mph. Not a hard problem to solve!!

That is, of course, wrong, first because it assumes that maximum road
capacity is independent of speed (which it isn't, due to turbulent
flow)

No it doesn't. It assumes that the throughput of the road is directly
linked
to flow speed, which it is!


You would of course get more through put at traffic lights if everyone set
off at the same time and drove bumper to bumper. They never do because
they need space ahead for safety. As they speed up, they need more space.

True, but not in the proportions that your calculations are based on.

And your reference for that is?...

Guy
--
Guy Chapman, http://www.chapmancentral.co.uk
The usenet price promise: all opinions are guaranteed
to be worth at least what you paid for them.

"The Weasel" wrote in message
...
On 13/12/2011 16:25, GT wrote:
"The wrote in message
...
On 13/12/2011 15:21, The Weasel wrote:
On 13/12/2011 13:39, GT wrote:
"Simon wrote in message
...
On 06/12/2011 21:39, Just zis Guy, you know? wrote:
On Tue, 6 Dec 2011 13:08:33 -0000, wrote:

wrote in message
...
On 30/11/2011 20:15, GT wrote:

Serves communities less as the throughput of that 'serving' road
has
been
lowered by 33%.


Really? - how's that then?

Give exact figures to support your argument

At max road capacity, you will get 30/20 more cars through at 30mph
over
20mph. Not a hard problem to solve!!

That is, of course, wrong, first because it assumes that maximum
road
capacity is independent of speed (which it isn't, due to turbulent
flow) and second because it assumes that the drivers at 20 and at
30mph maintain the same headway, which they definitely should not.

Guy

Without sitting down and working out the mathematics of it I would
have
thought that traffic throughput increases as speed decreases.

Then perhaps you should sit down and work out the maths then! Get a
few
pennies and a folded piece of paper, held vertically with the fold at
the
bottom. See how many you can push through the paper in 15 seconds when
you
push them slowly. Then see how many you can get across the piece of
paper
when you push them quickly...
As the speed increased an object's speed increases, you can get more
objects
through a measured space in a given time!


Pennies do not need space in front – traffic does.


I just worked out the math.

What is a 'math'?


http://dictionary.reference.com/browse/math

Nice link. According to your link, the word math means:
1. a mowing.
2. the crop mowed.

Origin: 1575–85; probably back formation from aftermath; compare Old
English m?th; cognate with German Mahd

The only way that you will get a more efficient vehicle throughput is
for
vehicles to drive with less distance between themselves and the vehicle
ahead. The way you can achieve that is to drive more slowly. There is a
cut off point but it would be at a very low speed. A lot lower than
20mph.

The gap is a factor, yes, but increasing the speed of a stream of
vehicles
will never reduce the throughput? In reality, there will be a slightly
larger gap between those vehicles, but you would have to double the gap
to
maintain the same throughput - this never happens, therefore faster
traffic
gets more cars along a set stretch of road.

A two second gap is a two second gap. As speed increases, so does the gap
needed for safety. If anyone is going to risk driving too close to the
vehicle ahead it is safer they do that at 10mph than it is at 100mph.
Therefore the only safe way to achieve a better throughput is to reduce
the speed of the traffic.

You are confusing theory with reality. If the gap did increase
proportionally with speed then the throughput would not change at all and
speed would be irrelevant, so you are still wrong - slower flow would not
increase throughput. My calculations are based on reality however. Higher
flow speed, with a small decreases in time gaps gives higher throughput.
Fact.

Multiple pile-ups tend not to happen at low speed.

Absolutely - because in reality, people leave less time gap at higher speed.
Your statement supports my point and emphasises that higher traffic speed,
with smaller time gaps gives higher throughput.

Work out the math :-)

I have no crops to mow thanks!

Go on, work it out.

I worked out the maths already - see above and previous posts.

The results might surprise you.

Nope. They confirmed what I already knew.

"Just zis Guy, you know?" wrote in message
...
On Tue, 13 Dec 2011 13:39:26 -0000, "GT" wrote:

"Simon Weaseltemper" wrote in message
...
On 06/12/2011 21:39, Just zis Guy, you know? wrote:
On Tue, 6 Dec 2011 13:08:33 -0000, wrote:

wrote in message
...
On 30/11/2011 20:15, GT wrote:

Serves communities less as the throughput of that 'serving' road has
been
lowered by 33%.


Really? - how's that then?

Give exact figures to support your argument

At max road capacity, you will get 30/20 more cars through at 30mph
over
20mph. Not a hard problem to solve!!

That is, of course, wrong, first because it assumes that maximum road
capacity is independent of speed (which it isn't, due to turbulent
flow) and second because it assumes that the drivers at 20 and at
30mph maintain the same headway, which they definitely should not.

Guy

Without sitting down and working out the mathematics of it I would have
thought that traffic throughput increases as speed decreases.

Then perhaps you should sit down and work out the maths then! Get a few
pennies and a folded piece of paper, held vertically with the fold at the
bottom. See how many you can push through the paper in 15 seconds when you
push them slowly. Then see how many you can get across the piece of paper
when you push them quickly...
As the speed increased an object's speed increases, you can get more
objects
through a measured space in a given time!

Ah, so you didn't understand.

One of us didn't!

Traffic does not flow like pennies down a channel

It does on most of the 40mph roads I'm talking about. Reducing that speed to
20mph can only reduce the flow and throughput of the road.

First the headway
increases with increasing speed, so a car at 40mph occupies a larger
block of road than one at 30mph, and second, traffic exhibits
turbulent flow - that's why the M25 VSLs actually increase throughput
*and* average speed by reducing the speed limit. Motorways are a rare
example where traffic throughput is constrained by road capacity,
normally it's junction capacity (which is why traffic jams are almost
always at junctions).

There are some good texts on traffic management aimed at the general
reader if you're actually interested.

Guy
--
Guy Chapman, http://www.chapmancentral.co.uk
The usenet price promise: all opinions are guaranteed
to be worth at least what you paid for them.

"Just zis Guy, you know?" wrote in message
...
On Tue, 13 Dec 2011 16:26:34 -0000, "GT" wrote:

"The Weasel" wrote in message
...
On 13/12/2011 13:40, GT wrote:
"Just zis Guy, you wrote in
message
...
On Tue, 6 Dec 2011 13:08:33 -0000, wrote:

wrote in message
...
On 30/11/2011 20:15, GT wrote:

Serves communities less as the throughput of that 'serving' road
has
been
lowered by 33%.


Really? - how's that then?

Give exact figures to support your argument

At max road capacity, you will get 30/20 more cars through at 30mph
over
20mph. Not a hard problem to solve!!

That is, of course, wrong, first because it assumes that maximum road
capacity is independent of speed (which it isn't, due to turbulent
flow)

No it doesn't. It assumes that the throughput of the road is directly
linked
to flow speed, which it is!


You would of course get more through put at traffic lights if everyone
set
off at the same time and drove bumper to bumper. They never do because
they need space ahead for safety. As they speed up, they need more
space.

True, but not in the proportions that your calculations are based on.

And your reference for that is?...

What? Do you live in the real world?

On Wed, 14 Dec 2011 10:00:10 -0000
"GT" wrote:
Traffic does not flow like pennies down a channel

It does on most of the 40mph roads I'm talking about. Reducing that speed to
20mph can only reduce the flow and throughput of the road.

Quite. Traffic is more like fluid flow than pennies in a slot but all things
being equal if you make the speed limit lower then the flow reduces. You
see a lot of BS about 50mph speed limits on some motorways actually
improving the traffic flow due to drivers not speeding up and slowing down
all the time and causing braking effects back down the road, but having
experienced its effects on the M25 I'm afraid I'm unconvinced. I don't think
drivers fit into the neat mathematical box that traffic modellers would like
to put them in.

B2003

wrote in message
...
On Wed, 14 Dec 2011 10:00:10 -0000
"GT" wrote:
Traffic does not flow like pennies down a channel

It does on most of the 40mph roads I'm talking about. Reducing that speed
to
20mph can only reduce the flow and throughput of the road.

Quite. Traffic is more like fluid flow than pennies in a slot but all
things
being equal if you make the speed limit lower then the flow reduces. You
see a lot of BS about 50mph speed limits on some motorways actually
improving the traffic flow due to drivers not speeding up and slowing down
all the time and causing braking effects back down the road, but having
experienced its effects on the M25 I'm afraid I'm unconvinced. I don't
think
drivers fit into the neat mathematical box that traffic modellers would
like
to put them in.

B2003

I made a similar point elsewhere. The posters arguing here that slower
speeds increase throughput seem to base their calculations on traffic
maintaining a 2 second gap at all times! Perhaps on paper, but in the real
world?! Besides, if the time gap is the same, then the throughput would be
the same, so they are still wrong!

On Wed, 14 Dec 2011 10:42:48 -0000
"GT" wrote:
I made a similar point elsewhere. The posters arguing here that slower
speeds increase throughput seem to base their calculations on traffic
maintaining a 2 second gap at all times! Perhaps on paper, but in the real

That 2 second gap is just farcical. On a busy motorway or A road like the M25
A406 in the rush hour you're probably looking at gaps of under a second.

B2003

On 14/12/2011 09:58, GT wrote:
"The wrote in message
...
On 13/12/2011 16:25, GT wrote:
"The wrote in message
...
On 13/12/2011 15:21, The Weasel wrote:
On 13/12/2011 13:39, GT wrote:
"Simon wrote in message
...
On 06/12/2011 21:39, Just zis Guy, you know? wrote:
On Tue, 6 Dec 2011 13:08:33 -0000, wrote:

wrote in message
...
On 30/11/2011 20:15, GT wrote:

Serves communities less as the throughput of that 'serving' road
has
been
lowered by 33%.


Really? - how's that then?

Give exact figures to support your argument

At max road capacity, you will get 30/20 more cars through at 30mph
over
20mph. Not a hard problem to solve!!

That is, of course, wrong, first because it assumes that maximum
road
capacity is independent of speed (which it isn't, due to turbulent
flow) and second because it assumes that the drivers at 20 and at
30mph maintain the same headway, which they definitely should not.

Guy

Without sitting down and working out the mathematics of it I would
have
thought that traffic throughput increases as speed decreases.

Then perhaps you should sit down and work out the maths then! Get a
few
pennies and a folded piece of paper, held vertically with the fold at
the
bottom. See how many you can push through the paper in 15 seconds when
you
push them slowly. Then see how many you can get across the piece of
paper
when you push them quickly...
As the speed increased an object's speed increases, you can get more
objects
through a measured space in a given time!


Pennies do not need space in front – traffic does.


I just worked out the math.

What is a 'math'?


http://dictionary.reference.com/browse/math

Nice link. According to your link, the word math means:
1. a mowing.
2. the crop mowed.

Origin: 1575–85; probably back formation from aftermath; compare Old
English m?th; cognate with German Mahd

You missed the bit that said:

math
noun
mathematics.

The only way that you will get a more efficient vehicle throughput is
for
vehicles to drive with less distance between themselves and the vehicle
ahead. The way you can achieve that is to drive more slowly. There is a
cut off point but it would be at a very low speed. A lot lower than
20mph.

The gap is a factor, yes, but increasing the speed of a stream of
vehicles
will never reduce the throughput? In reality, there will be a slightly
larger gap between those vehicles, but you would have to double the gap
to
maintain the same throughput - this never happens, therefore faster
traffic
gets more cars along a set stretch of road.

A two second gap is a two second gap. As speed increases, so does the gap
needed for safety. If anyone is going to risk driving too close to the
vehicle ahead it is safer they do that at 10mph than it is at 100mph.
Therefore the only safe way to achieve a better throughput is to reduce
the speed of the traffic.

You are confusing theory with reality. If the gap did increase
proportionally with speed then the throughput would not change at all and
speed would be irrelevant, so you are still wrong - slower flow would not
increase throughput. My calculations are based on reality however. Higher
flow speed, with a small decreases in time gaps gives higher throughput.
Fact.

Multiple pile-ups tend not to happen at low speed.

Absolutely - because in reality, people leave less time gap at higher speed.
Your statement supports my point and emphasises that higher traffic speed,
with smaller time gaps gives higher throughput.

Do they?

If that is true then people are pretty daft leaving less of a time gap
at higher speeds when it is more dangerous to do so than at lower speeds
when it is comparably safe.

Work out the math :-)

I have no crops to mow thanks!

Go on, work it out.

I worked out the maths already - see above and previous posts.

The results might surprise you.

Nope. They confirmed what I already knew.


The other point which is relevant to speed limits in urban areas is that
overall throughput is determined largely by movement through lights,
junctions, give ways etc. not by the maximum possible speed.

Just because it is possible to speed up to 30mph only to have to slow
down again and the next junction or queue, does not make the overall
throughput any better. It just makes the drive more stressful for the
driver, dangerous for the pedestrian, and uses more fuel.

--
The Weasel