15 Feb 2014.
For immediate release.

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Bristol's Claims of Walking and Cycling Increases are Bogus
Bristol City Council's claim that walking and cycling have increased by more than 20% in its pilot 20mph speed limit areas have been shown to be false, following a detailed investigation by the ABD's Malcolm Heymer 1.
The claim has been seized upon by proponents of blanket 20mph speed limits, arguing that the lower limit encourages more 'healthy' modes of transport.

The council's claim comes from a report in July 2012 2, which states that increases in walking ranged from 10% to 36%, and cycling from 4% to 37%. From these figures it declared that the average increases in walking and cycling were 23% and 20.5%, derived by simply averaging the highest and lowest figures.

As Mr Heymer points out:
"It is statistically invalid to calculate an average of percentages by simply adding the highest and lowest and dividing by two. The individual percentage changes have to be weighted according to the actual numbers involved to give a true average. That such an elementary error could be made in a local authority report is both extraordinary and concerning, especially when the figures so derived are used by others to justify speed limit changes that could affect millions of people."
Using a Freedom of Information request, Mr Heymer obtained detailed data for all the survey points used to count pedestrian and cyclist numbers in Bristol's two pilot 20mph areas. This showed that counts were made on just one day of the week and one weekend day at each point, before and after the speed limit was changed. All the counts were carried out in August, a school holiday period that is normally avoided for traffic surveys because it is not representative of the rest of the year. These limitations alone mean that the results cannot be relied upon to give a statistically valid picture of the changes that may have taken place.

After analysing the limited data correctly to obtain true average figures, Mr Heymer found that, over a 7-day week, walking and cycling increased in the first pilot area by about 3% and in the second by 9%, a long way below the claims made by the council. In the second area, the before and after surveys were undertaken two years apart, as opposed to one year apart for the first pilot area.

Even the modest increases calculated by Mr Heymer from the inadequate survey data cannot be attributed wholly to the effects of the 20mph speed limits. In addition to the limitations of one-day counts, the effects of the economic downturn continued to be felt throughout the three years of the Bristol surveys (2009-2011). As the recession bit, people might have walked or cycled more to cut the cost of car use. They may also have taken more holidays at home.

As Mr Heymer explains:
"It is impossible to quantify the impact these factors could have played in the apparent increases in walking and cycling. If the council had undertaken control counts at the same time, in areas of the city where 30mph speed limits were retained, the changes in the pilot areas could have been compared with those in the control areas. In the absence of such comparisons, and given the limitations of the survey data, the apparent increases in the pilot areas, even when properly calculated, must not be taken as evidence of increased walking and cycling in the 20mph speed limit areas."

 
 
NOTES FOR EDITORS
 
1. ABD — Bristol 20mph analysis [pdf]
2. Bristol City Council — Citywide Rollout of 20mph speed limits [pdf]
 
 
Notes for Editors about the ABD
 
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