Evidence of impact of 20mph limits on vehicle speeds

The Evidence – I wonder if this is best as a seperate page somewhere else with just a link to it – it is interesting to me and probably no one else

The Atkins Report.The UK Government commissioned WS Atkins report 20mph Research Study[11]looked at a number of area-wide studies across the UK; these studies were identified at the time that the report was commissioned in 2014. As a result, this work does not take into account the more recent studies which appear below. We would stress too that the report only looked at what is called a “signs and lines” approach on very small sample sites. It did not involve the multiple dimensions that are recommended in the TfL Vision Zero Action Plan approach of a) lower speed limits, b) designing roads for 20mph limits when changes are made, c) a more robust approach to enforcement and d) the opportunity offered by new technology such as Intelligent Speed Adaptation (ISA). 

The report states that with the introduction of 20mph limits in terms of Vehicle Speed: 

The median speed has fallen by 0.7mph in residential areas and 0.9mph in city centre areas;

Faster drivers have reduced their speed more, with the 85th percentile speed falling by -1.1mph in residential areas and by -1.6mph in city centre areas; 

The overall change in speeds is greater where speeds were faster before. The median speed fell by -1.3mph on residential roads with a before speed of more than 24mph; and by -1.1mph on ‘important local roads’ which typically had higher before speeds. On ‘minor local roads’ the median speed was already below 20mph and dropped by just 0.1mph.

Recent Research – London Boroughs. To date we have not seen any reports about the impact on casualties of introducing 20mph speed limits in those London boroughs that have introduced wide 20mph limits. We have some research findings on the impact on vehicle speeds:

Islington (2014)[12]. On non-principal roads there was an average reduction of 1mph. On the principal road network average speed fell 1mph from 23mph to 22mph.

City of London (2015)[13]. Average motor vehicle speeds at the 46 monitoring sites are 1.5 mph lower than before the new speed limit was introduced.

Southwark (2017)[14]. Across 86 sites the mean speeds reduced from 21.6mph to 19.8mph – a reduction of 1.8mph.

Lewisham (2018). Lewisham Boroughwide 20mph Limit Review by The Project Centre. The top 20 roads identified with the highest 85th percentile speedin 2015, have all seen a reduction in speed, of on average 2.3mph. Of the top 20 locations identified in 2015 with the highest mean speeds, all except one location have recorded an overall reduction, with an average reduction of 2.0mph.

More Major Roads. There is evidence that these declines of an average of 2mph across all roads with the introduction of 20mph speed limits can mask greater declines in speeds across more major roads. when 20mph limits were introduced at the site of the IMAX roundabout (part of the TLRN) in 2013 there was a fall of 2.7mph in average speeds. This remains to be confirmed by TfL but is consistent with other findings (eg from Portsmouth) that speeds on more major roads declined to a greater degree than on the residential roads that were part of those schemes.

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