Restricting motor vehicles increases air pollution

[Lance Dopestrong]

I never suggested that any of my awesome colleagues in Cycling UKville was saying otherwise, so I don't understand the comment?

[pwa]

I never suggested that any of my awesome colleagues in Cycling UKville was saying otherwise, so I don't understand the comment?

Don't worry. You were responding directly to the item in the OP. I understand your comment in that context. But I do think the idea that removing "obstructions" to motorised traffic can reduce pollution an interesting idea, if ultimately wrong. At least, wrong if you take it too far. In theory we could lose all the cycle/bus lanes, for a time make the roads easier for cars. But then the former cyclists/bus passengers would resort to car use, as would people drawn by the extra capacity of the roads, and pretty soon you would have more cars on the road and pollution levels back up.

[squeaker]

Must be careful not to confuse miles per gallon with 'efficiency', unless you define efficiency as miles per gallon. A lot of powertrain engineers will focus on efficiency as 'work done / energy in'

But surely fuel efficiency is a fairly accurate gauge of emissions.

Not as far as NOx goes. Its formation is related to peak combustion temperatures. High maximum combustion temperature tends to be related to high efficiency, but as has already been said, it's complicated...

[mjr]

Silly me. I always thought air pollution in this regard was caused by people driving cars. How daft was I, eh?

Nobody is saying otherwise. All the discussion is about the technicalities of how vehicle speed is related to pollution levels. The basic notion is that allowing vehicles to flow more freely can reduce emissions from vehicles, which is true in theory, but in practice it is far more complicated. One additional factor is that if traffic flows more freely on a particular road it encourages people to drive on that road and eventually congestion occurs and emissions increase.

Higher speed limits prevent traffic from flowing more freely because it increases the stopping distance required in front of each vehicle and so fewer vehicles fit on the same length of lane. It also means that vehicles accumulate at any obstruction or even just something surprising far more quickly and can be enough to cause "phantom jams" that last long after the obstruction is removed.

Today's demonstration of that: there was a lorry broken down on the A10, right near the north end. On my trip north, it had caused about 1.5 miles of jam. By the time I returned south, it had been removed from the lane by the police, but people just touching their brakes, apparently because they spotted a police car in the nearby petrol station (whose occupants seemed to be doing full checks of the lorry paperwork or something) was perpetuating a phantom jam which extended back past my destination, some 3 miles further, and out of sight - it was longer than when the lane was blocked and motorists were having to stop and wait for gaps in oncoming traffic!

If the speed limit through our three villages was 30mph instead of 40, I suspect it would slow the rate of new arrivals at such surprises (which are not unusual in villages) enough that the phantom jam wouldn't have been able to grow.

[[XAP]Bob]

Silly me. I always thought air pollution in this regard was caused by people driving cars. How daft was I, eh?

Nobody is saying otherwise. All the discussion is about the technicalities of how vehicle speed is related to pollution levels. The basic notion is that allowing vehicles to flow more freely can reduce emissions from vehicles, which is true in theory, but in practice it is far more complicated. One additional factor is that if traffic flows more freely on a particular road it encourages people to drive on that road and eventually congestion occurs and emissions increase.

Higher speed limits prevent traffic from flowing more freely because it increases the stopping distance required in front of each vehicle and so fewer vehicles fit on the same length of lane. It also means that vehicles accumulate at any obstruction or even just something surprising far more quickly and can be enough to cause "phantom jams" that last long after the obstruction is removed.

Today's demonstration of that: there was a lorry broken down on the A10, right near the north end. On my trip north, it had caused about 1.5 miles of jam. By the time I returned south, it had been removed from the lane by the police, but people just touching their brakes, apparently because they spotted a police car in the nearby petrol station (whose occupants seemed to be doing full checks of the lorry paperwork or something) was perpetuating a phantom jam which extended back past my destination, some 3 miles further, and out of sight - it was longer than when the lane was blocked and motorists were having to stop and wait for gaps in oncoming traffic!

If the speed limit through our three villages was 30mph instead of 40, I suspect it would slow the rate of new arrivals at such surprises (which are not unusual in villages) enough that the phantom jam wouldn't have been able to grow.

But it's not just distance that is important - if you take the 2 second rule to heart then a road's capacity is limited to 1 car every two seconds - irrespective of the speed of the traffic...

Obviously the 2 second rule is a naive approximation, but reality (assuming an infinite supply of perfect drivers*) is slightly more complex. You need a reaction time and a braking distance - so car separation goes as a sum of V and V^2. The distance between cars therefore has minimum at some speed (since this is a quadratic equation).

BUT the *time* between cars is this distance plus the vehicle length/V

(l+aV+bV^2)/V
Which is a more complex beast entirely...

I assert that is is obvious that at zero speed the road capacity is zero, and therefore not maximum.
I also assert that as V becomes large the V^2 element dominates and the capacity reduces.
Therefore I assert that there is some optimum speed for free flowing traffic.

Note of course that imperfect drivers often ignore b entirely - although minor discrepancies in the separation can be accepted and averaged out by good (not necessarily perfect) drivers. In smoothing out these changes the good driver will enhance the smooth flow behind them.

All of that goes to pot when anything unexpected happens and everyone is on 'minimum distance' driving. The whole lot grinds to a halt, and that traffic jam will never cease.


* Available from ACME stores, along with inextensible string, frictionless surfaces and spherical cows...

[squeaker]

Nice illustration - more here

[mjr]

Obviously the 2 second rule is a naive approximation, but reality (assuming an infinite supply of perfect drivers*) is slightly more complex. You need a reaction time and a braking distance - so car separation goes as a sum of V and V^2. The distance between cars therefore has minimum at some speed (since this is a quadratic equation).

BUT the *time* between cars is this distance plus the vehicle length/V

(l+aV+bV^2)/V
Which is a more complex beast entirely...

I assert that is is obvious that at zero speed the road capacity is zero, and therefore not maximum.
I also assert that as V becomes large the V^2 element dominates and the capacity reduces.
Therefore I assert that there is some optimum speed for free flowing traffic.

I think time between cars is basically the inverse of throughput rather than capacity, but let's look at it anyway:

a is basically reaction time, which is commonly assumed to be 2 seconds.

b is a constant of proportionality... let's assume (contrary to my belief) that the optimum is going to be a fairly high speed, so use 50mph aka about 22m/s. Textbook braking distance is 38m, so 38 = b 22^2, 38 = 484b, b = 38/484 = 19 / 242 = 0.078512 (5sf)

l is vehicle length. Let's say 4 metres.

Let's look at a graph (attached below), putting V on the X axis and so the time between cars is the Y access. So basically, the minimum time between cars is with V somewhere near 7m/s or roughly 16mph. That means my assumption about optimum speed was broken.

Let's revisit b, using the 6m braking distance for 20mph aka 8.9m/s, so 6 = b 8.9^2 = 79.2 b, so b = 6/79.2 = 0.075748 (5sf) rather than 0.078512 which I doesn't move the graph much.

[XAP]Bob's assertion 1 that at zero speed the road throughput is zero, and therefore not maximum, is shown by the time between cars tending to infinity as speed nears zero.
Assertion 2 that as V becomes large the V^2 element dominates and the throughput reduces, is shown by the slow increase of time between cars as speed increases beyond the optimum.

The optimum speed is expected to be around 16mph in dense traffic. Therefore, 20mph urban zones seems a good idea to optimise throughput, even for busy arterials, as it should result in peak throughput at most times and also reduce the risk of clogs forming at any obstructions that won't be cleared until flows fall below peak levels.

It also seems a pretty strong argument for using programmable road signs to close lanes or maybe even roads (except for access) and divert traffic as soon as an obstruction occurs at peak time, as a way to avoid heading up that left-hand slope up towards infinity, low traffic capacity and gridlock and reset things and get back onto the low part of the curve. When a road reopens after clearing it, you re-enter the graph from the right-hand edge.

Finally, it seems like even if cars are slightly less dirty at higher speeds, having a steady flow of 16mph cars would be better than having slightly fewer cars passing at higher speeds most of the time and risking a pollution surge from lots of cars sitting in a very slow queue, many of which will have their engines running.

[mjr]

Nice illustration - more here

As I've mentioned before, the solution advocated by that video falls into the same trap (wishing humans were better than they are) because some humans will reprogram their self-driving cars to be more aggressive and screw the system up anew.

[RickH]

It also seems a pretty strong argument for using programmable road signs to close lanes or maybe even roads (except for access) and divert traffic as soon as an obstruction occurs at peak time, as a way to avoid heading up that left-hand slope up towards infinity, low traffic capacity and gridlock and reset things and get back onto the low part of the curve. When a road reopens after clearing it, you re-enter the graph from the right-hand edge.

Using Google Maps on my phone as a satnav, I quite often find, particularly if I'm travelling around peak times (but have had it on a Sunday afternoon recently), that it will come up with a message asking If I want to re-route because of congestion on the way it was directing me.

Google seems more responsive to the situation on the road than the live traffic (via DAB) on the Garmin satnav that we also have.

[Vorpal]

Re-routing because of congestion is responsible for sending an excess of traffic down rural lanes where it has no business being.

[RickH]

Re-routing because of congestion is responsible for sending an excess of traffic down rural lanes where it has no business being.

It can do but it doesn't have to - the re-routes I've had recently have been mostly on motorways /major roads until near home, where there will almost inevitably be some driving on more minor roads, ie. when heading north on the M6 keeping me on the M6 for several more junctions rather than taking me towards Manchester on the M62/M60/M61.

[mjr]

Re-routing because of congestion is responsible for sending an excess of traffic down rural lanes where it has no business being.

That's why it should be done officially and not left to every sat nav, so we can seek to keep most traffic on the major routes. I'm thinking of situations like entering Norwich from the west on the A47, where the A1074, A11, B1108/A140/A1074 or even A140 or A146 might be viable routes, depending on where in Norwich you are going to.

[[XAP]Bob]

I think time between cars is basically the inverse of throughput rather than capacity, but let's look at it anyway:

Difference between lane throughput and road capacity?


I am surprised that the optimum came out so low - I was expecting it to be closer to 50.
Obviously the compromises we are willing to make change according to the style of road - and motorways are definitely one road where speed is possibly more important than throughput.

There was also a gif of a Japanese experiment where they set up a track and told everyone to drive at a steady 30kph. The ensuing jam travelled back around the track at 20kph...