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
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
• 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 ?!