Pollution from cars - flowing or crawling is worse?

As an engineer who currently is involved in engine controls development, I do have some experience with this. I suspect that the answer is "maybe, maybe not."  Moving a vehicle at higher speeds will generally require more energy, which means more fuel is used to move faster, which means that a faster moving vehicle produces more pollution than a slower moving vehicle.

But a vehicle which is stopping and starting will also use more fuel.  All cyclists should recognize that accelerating required more effort than steady state travel.

Reducing the number of lanes will probably decrease the overall speed of traffic.  But, if traffic speed limits and flow control devices, such as signal lights, are set to produce smooth flow of traffic at the posted speed, and if the operators of the vehicles will drive at the posted speed, then I would expect a reduction in pollution from engine powered vehicles.

Certainly, tire wear is less at lower speeds, and I am aware of two independent studies which indicated that a large fraction of the micro plastic particles found in littoral water come from tire wear. Note well that battery powered electric vehicles tend to be heavier than combustion engine vehicles and thus they will produce more micro plastic particles.

There is also the reduction in travel by motor vehicles which better cycling infrastructure will produce. This will clearly reduce the level of air pollution.

The problem that I see is that people will not follow the speed limits as they would rather drive faster to wait at traffic control points than drive slower and flow smoothly through the control points. They then point to the reduction in speed as the cause of pollution, when in reality it is their refusal to operate their vehicles in an optimal manner.

In areas of heavy congestion, automobile traffic is already moving slower than a bicycle can travel, yet many people choose to travel by automobile. Perhaps requiring automobile travel to pay the actual cost of the roads they travel on would go a long ways toward reducing the use of automobiles, or maybe it would result in a Yellow Vest movement in more countries

TFL - Speed, emissions & health:

http://content.tfl.gov.uk/speed-emissions-and-health.pdf

Quote:

The impact of cycle lanes on local air quality has not been evaluated. However, after protected cycle lanes are built, journey times tend to return to pre-construction levels, suggesting the lanes do not worsen congestion.

Furthermore, even where congestion marginally increases local pollution, the health impacts are likely to be negligible. Since air quality improves further away from traffic, protected cycle lanes can reduce pollutant exposure for people walking and cycling.

Reallocation of road space to prioritise the most space-efficient modes (walking, cycling, buses) and reduce private car use can decrease congestion and vehicle emissions city-wide.

As I understand it, this is something that is very difficult to analyse. In part because there are confounding variables - as you increase width of roads, you may increase vehicle speeds (in the short term at least, let's not include induced demand at this stage) but you also increase the number of vehicles passing through per unit of time. But if you keep the number of vehicles constant, and increase their average speed, then pollution goes up (more fuel is burned by faster-moving vehicles). But then, you need to factor in braking and acceleration, both of which create more pollution than a steady speed. 

So if you define things very very narrowly, you can argue that traffic jams create more pollution than moving traffic. But in the real world, pollution goes up with efforts to reduce congestion, because the number of vehicles passing through increases. Then you get induced demand, and ultimately, more congestion.

TL/DR - the argument that cycle lanes create pollution is false

Locally it will be worse, but overall it will be the same.

(Randomly) If you had a bag of peas with a hole in it, and each pea fell out at a constant 1/2 second interval.
If you walked along the street, you'd drop more peas in a shorter distance, than if you ran, drove and emptied the bag.

I'm not sure about mushy peas in a tin though