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Dear All,
We've just had all the roads i drive to work on (when I'm not cycling) go to 20 mph limits. I got wondering what that meant for emissions and did some calculating. Eyeballing the rpm at different speeds and gears;
20 mph in 4th is 1,100 rpm. So over an hour thats 20 miles covered and 66,000 fuel burning and emissions creating engine revolutions (1100 60).
30 mph in 4th is 1500 rpm, but you cover the 20 miles in 40 minutes, so 60,000 revolutions (1500
40).
30 mph in 5th is 1200 rpm and again the 20 miles covered in 40 mins so 48,000 revolutions (1200 * 40).
So slowing the traffic is actually worse for the environment?
Or have I got it wrong somewhere

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You may (or might not) be right about 20mph being worse for emissions than 30mph.

Difficult to say whether or not for a given trip a higher number of total engine rotations is better or worse for total emissions for a given trip because emissions per rotation (firing cycle) will be different at different engine loads (and load isn't just about rpm, it's also about throttle position etc).

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Surely, in a manual, if 20MPH = 1,100 RPM, then 30MPH must be 1,650 RPM not 1,500 RPM, as there is a direct physical link, so the revs used to travel your 20 miles are the same regardless if you do it at 20MPH to 120MPH. If changing to 5th gear, different story.

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each engine revolution doesnt burn a fixed amount of fuel. What matters is power. Lower road speeds generally use less power to maintain, and thus should use less fuel. You also spend less time accellerating, and less waste going into the brakes stopping again. There are some losses ofcourse, and those often become a larger % of the total as the average engine power falls.

Its unfortunately all extremely complex with many factors all intermingling together. Different cars will also vary wildly making the same comparison.

Engines are typically maximally efficient at peak torque wide open throttle, but they dont spend much time there.

Futher, is the justification of those 20mph limits purely emissions based? There are many environmental factors which arent solely about emissions. Usually there are a host of proported benefits including safety and noise and whatnot for instance. Even slowing down cars such that people consider alternatives such as walking or cycling for instance is an environmental benefit. Theres more to "environment" than just emissions.

20 limits are also a bit odd. The residential streets round here are all 20, and tbh its fine. you cant realistically do much more than 20 anyway without it being a bit silly, as there are corners and parked cars and the like. But they are all proper residential estates. They seem to be extending them into places where they are less obvious though, like link roads and inner city roads that are long/streight/open where 20 feels like your literally stationary. It almost makes it worse in those places as you end up with a bigger stratifcation of road speeds. You'll get the vast majority doing 27-30ish because 20 feels too slow, a few idiots blasting at 40, and then someone doing 17mph (cos their speedo says 20), and i think if anything just casuses more anger and bad feeling on the roads.

But then they've spent the past 20 years doing the same thing turning NSL roads into 50 limits, so its probably not really a surprise.

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PC38: As others have remarked It's definitely much more complicated than mere RPM calculations .....
This has the main points: https://www.gov.uk/government/publications/20-mph-speed-limits-on-roads
Lots of Studies too performed over the years, eg. see ~ p192 in the second (technical) report there:

14.6. How do environmental outcomes compare in 20mph limits
and zones?
14.6.1. Air quality
Section 10.4 shows that air quality in 20mph limits can be affected by vehicle speeds, driver behaviour, and
volume of traffic.
Existing evidence suggests that vehicle emissions in 20mph zones may be adversely affected by changing
vehicle speed and acceleration rate (Boulter and Webster, 1997, cited in Grundy C et al., 2008). Although
vehicle emissions are usually less at lower speeds, emissions may increase in 20mph zones as vehicles use
more fuel to accelerate between calming measures.
Relatively few studies have attempted to quantify the energy and environmental impact of traffic calming
measures, and the results are mixed results with regards to the impact on emissions (greenhouse gases and
air quality).
• Pharoah (1991), cited in Ahn and Rakha (2009), found that traffic calming measures with smooth and
low speed driving in a high gear may result in relatively low emissions and that the effect of traffic
calming strategies on air quality depends on how the scheme influences both the average speed of
traffic and the amount of speed variation. While some studies found that traffic calming measures
benefit air quality, several concluded they increase vehicle fuel consumption and emissions.
• Litman (1999), cited in Ahn and Rakha (2009), studied the benefit and cost of traffic calming measures
and concluded that traffic calming strategies that reduce traffic speeds and smooth traffic flow can
generally reduce air pollution, while those that increase the number of stops may increase emissions. He
also found that when traffic calming reduces vehicle speeds from 50 km/h to 30 km/h for an ‘‘Easy
Driver,” savings in CO, HC, NOx, and fuel consumption in the range of 13%, 22%, 48%, and7%,
respectively, are achievable. In the case of the ‘‘Aggressive Driver” savings in CO, HC, and NOx in the
range of 17%,10%, and 32%, respectively are observable with increases in vehicle fuel consumption in
the range of 7%.
• TRL research undertaken by Boulter et al. (2001), also considered the effect of traffic calming measures
on air quality. The evidence reported that the mean emission rates of carbon monoxide, hydrocarbons,
nitrogen oxide, and carbon dioxide from petrol non-catalyst, petrol catalyst, and diesel cars increased by
up to 60% following the introduction of traffic calming measures. However, it was estimated by TRL that
the increased emission rates were not expected to lead to poorer local air quality.
• Daham et al. (2005), cited in Ahn and Rakha (2009), simulated braking and acceleration events to mimic
speed humps by driving a normal road using an on-road emission measurement device. He found that
speed humps increase HC, CO, NOx, and CO2 emissions by 148%, 117%, 195%, and 90%.
• According to Williams (2013), measures with the least detrimental impact on vehicle emissions are those
that induce the least variation in speed. Emissions were monitored on 10 routes with a range of different
traffic calming methods (vertical deflection, horizontal deflection and psychological). Vehicles were often
seen to exhibit a greater variability in speed on links with vertical deflection than those without; however,
the impact of such traffic calming features was not thought to be as large as that of other traffic
management features, such as pedestrian crossings and signalized junctions. Williams (2013) also
shows that a higher proportion of time spent accelerating and decelerating, is likely to be associated with
increased particulate matter associated with tyre and brake wear.

So, 'big shock' - Resultant Emissions Levels also depend on just how we drive in these 20MPH Zones... !

Observing folks 'interacting oddly' with speed humps, accelerate/brake/accelerate/brake etc illustrates that
issue particularly well, most notably when that acceleration/braking is unnecessarily harsh/hard ?!

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the problem with these types of tests is it is done under a controlled situation, when you are feathering the throttle or the brake, emissions go out the window, so no real difference in my opinion. It's just a case of i said so, so you do as i say.