The paper isn’t really research, it’s an exercise in modelling/simulation – something that I used to do professionally. Although the paper describes some model validation, this is limited to vehicle performance, rather than actual driving patterns. As a modelling exercise, it is intended to try to simulate ‘typical’ London traffic, although it is not clear whether that’s a typical period of time, or driving experience. I would expect any modelling that elaborates a ‘typical’ scenario to also provide a credible interval (to use a Bayesian term), as do the IPCC climate change scenarios. It’s very debateable as to how typical typical London traffic conditions are for most urban driving. As there is no relevant validation, it is not possible to know how representative the results would be for actual driving.

The results are not validated at all against actual driver behaviour. Some of the discussion is wrong, and, rather typically, shrouded in unnecessarily complex maths that is simultaneously over-simplistic (e.g. the discussion on how far a vehicle would have to accelerate to a higher speed to be more efficient than a car limited to 20mph.

There is value in educating drivers on how to drive efficiently. However, it’s debatable whether the best overall/ethical outcome arises from imposing regulations on peripheral characteristics (the speed limit, all the time), rather than, say appealing to the drivers’ pocket.

For the most part, the environmental and health impacts of driving result from specific locations where road layouts are not suitable, rather than large areas (e.g. look at the locations of STATS19 incidents to make this very evident from an accident point of view). And technology continues to improve both the impact in both dimensions.

The research on fuel consumption vs speed seems to show that the engineering loss differential is larger than you’re claiming. (https://bit.ly/3lzMR0Y) .e) is only relevant if a vehicle is oscillating between <20 and 30mph, which may be the case in heavy traffic with some over-competitive drivers, but that’s hardly a reason to constrain use at other times.

I’m not sure that the reductions quoted by the different studies make sense. To quantify the discussion below, from a year ago, the annual reduction in accidents in 30mph speed limit areas is 2.9% (based on STATS19 data). This is in line with the changes quoted above, where a whole impact is described. Some of the examples constrain the impact to the area where the new limit was introduced, thus the impact of behavioural changes, which could reduce traffic levels, would also have an impact.

It’s also worth noting that the original TRL reports (esp. 421) focus on the impact of speed change on accident rates, not speed limit. It also notes that a larger impact comes from changing the variation in driving speed on a given road. In fact, the reductions in accident rates observed between 2005 and 2019 exceed those projected as possible by reducing the speeds in 30mph zones across the country in TRL421, although hardly any changes were made between the report and now.

Much of the original data comes from non-UK roads from late last century. The WHO comments also note that road accidents are mostly an issue in the developing world.

(numbers and analysis available on request)