Big benefits from 20mph on faster roads

An analysis of seven 20mph area shows how Local Authorities that include faster roads in signed-only schemes reap the most benefit:

  1. Mean speed reductions of twice the previous estimate;
  2. Greatest reductions on previously faster roads;
  3. Close correlation between pre-existing speeds and speed reductions; and
  4. Including faster roads brings much greater road casualty reductions.

As Local Authorities become more receptive to including faster roads in 20mph schemes – Wales, Cornwall and Oxfordshire do not have pre-existing speeds as a criteria – the changing outcomes illustrate the shortcomings of the decade old DfT guidance.

Paragraph 96 in the 2013 DfT guidance suggested that 20mph schemes without traffic calming will reduce mean speeds by about 1mph, implying a likely reduction in casualties of about 6%. The latest data shows that speeds reduced by 2mph to 6mph, indicating casualty reductions of 12% to 40% (figure 1), if faster roads are included. This prediction is consistent with data from Edinburgh, which showed a 1/3 reduction in casualties, Cheshire West & Calderdale and, most recently, Transport for London’s arterial roads[1].

When Scottish Borders Council implemented 20mph in 100 communities across the authority, speeds reduced by 3mph on average and by 6mph on the faster roads. Pre-existing speeds averaged 25mph, ranging from 15mph to 36mph. Data from a further 250 sites in Edinburgh, Hampshire, Oxfordshire, Wales and Kent showed similar patterns:

  • Scottish Borders Council was a comprehensive trial of 20mph across 114 sites.
  • Edinburgh had two schemes: a pilot in South Edinburgh (28 sites) followed by a citywide 20mph rollout (59 sites)
  • Wales (14 sites) and Oxfordshire (130 sites) were trials to assess how best to roll out 20mph and included both rural and urban settings.
  • Kent trialled two town-wide schemes: Tonbridge (10 sites) and Faversham (11 sites).
  • Hampshire data (14 sites) was from an earlier trial, often on short stretches of road.

Over the entire 380 sites, average mean speeds fell from 23.7mph to 21.9mph - 1.9mph lower (8%). Data from Scottish Borders Council is probably the most robust because it has the highest correlation (R2 = 0.7) and has been analysed in most detail by academics from Napier University. Hampshire data is less useful since 20mph was implemented by a sceptical Local Authority on short stretches of road.

Bath and North-East Somerset schemes showed a similar pattern, but the data was not in a form which could be incorporated in the analysis – see figure 6.

The DfT’s freeflow speed report[2] (see figure 7) shows

  1. mean speeds are 5mph lower on 20mph than on 30mph roads; and
  2. in a 30mph speed limit, 50% of people drive at less than 30mph. In a 20mph speed limit, 80% drive less than 30mph.

With the caveat that there were few (8) 20mph sites and situated on atypical 20mph streets, this supports the view that 20mph signs alone bring worthwhile speed reductions.

Recent data from the Transport for London showed speeds reduced by 1.7mph to 5mph when 20mph was introduced on arterial roads, with casualties falling reducing by 25% to 64% for different severities and road user types – see figure 8.

Annex A from the DfT’s Speed Limit Assessment Tool, based on data from before 2010 and from many fewer sites, had a lower R2 of 0.39 and showed a lower expected reduction in speeds of 4.4%.

The latest results from multiple sites and a higher R2 show a speed reduction benefit of double that indicated by the DfT’s 2013 guidance in its Speed Limit Assessment Tool.


Figure 1: summary results


Figure 2: pre-speeds versus change in speed correlation


Figure 3: Speed distributions (before and after)


As well as a reduction in mean speeds, the chart shows a more tightly packed distribution in the 19mph to 25mph range in 20mph schemes.

Figure 4: cumulative benefit gap by pre-existing speed


Shows the speed and casualty reduction benefit foregone from excluding roads with mean speeds above specific mean speeds.


Figure 5: Speed reductions by road type (slow, medium, fast)




Figure 6: Bath & North East Somerset chart


Figure 7: DfT Freeflow speed comparison chart


Figure 8: TfL arterial roads (infographic only)


Figure 9 Projected speeds



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Showing 4 reactions

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  • James Egginton
    commented 2023-08-30 20:00:19 +0100
    These figures are fine, except, in our village using drivesafe figures, one third of vehicles are going over 36 mph (30mph zone). So I guess that these figures are collated from town streets where it is not so possible to speed. I wonder how much they apply to us if that is the case. We have not had any accidents, mainly because it is not safe to walk down the road.
  • Adrian Berendt
    commented 2023-05-03 12:51:31 +0100
    Apologies for typo,. In “4) Regarding Tonbridge, following the consultation, it was decided to revert parts of four roads back to 20mph”…should read “back to 30mph…”
  • Adrian Berendt
    commented 2023-05-02 08:15:39 +0100
    Thanks for your questions, Tim. I’ll deal with them in turn

    1) The relationship between mean speeds and collisions is widely available. E.g. In its guidance on setting local speed limits, the DfT says at #82 “There is clear evidence of the effect of reducing traffic speeds on the reduction of collisions and casualties, as collision frequency is lower at lower speeds; and where collisions do occur, there is a lower risk of fatal injury at lower speeds. Research shows that on urban roads with low average traffic speeds any 1 mph reduction in average speed can reduce the collision frequency by around 6% (Taylor, Lynam and Baruya, 2000). There is also clear evidence confirming the greater chance of survival of pedestrians in collisions at lower speeds.”

    2) The min and max speeds refer to the min/max mean speed for each group of locations

    3) Correlations between before and after speeds

    4) Regarding Tonbridge, following the consultation, it was decided to revert parts of four roads back to 20mph, where people had concerns. The remaining 470 have stayed at 20mph.
  • Tim Coote
    commented 2023-04-19 13:46:39 +0100
    What is the evidence that mean speed is relevant to accident rates? If accidents were due to random vehicles, then median speed would be a better measure. If the accident causing vehicles, are, say skewed towards the maximum, then mean would not be relevant. In the latter case in particular, the issue may be poor enforcement in existing 30mph zones, rather than punishing all drivers all the time.
    Is it really the case that the columns ‘min’ and ‘max’ in Figure 1 are min and max speeds, or, say, 95 centile speeds?
    It’s not clear what’s being correlated when you quote the correlation coefficients (R^2)

    Note that the Tonbridge schemes attracted an overwhelming rejection by the locals – I don’t know about the others.