The rise in Portsmouth reported KSI casualties from 2010 to 2011 was concentrated on roads with speed limits above 20 mphSummary
In July 2012, a press release was issued by the Association of British Drivers and articles appeared in several national and local newspapers expressing concern about the large increase in the 2011 reported KSI casualties in Portsmouth compared to 2010. The press release suggested that the rise was due to the introduction of 20 mph speed limits for many of Portsmouth's residential streets in 2007 to 2008.
A study summarised here of the reported road casualty databases released by the DfT shows these concerns to be unfounded: most of Portsmouth's KSI casualties occurred on the 30 mph roads, and the largest increases in 2011, both proportionately and in numbers, were on the 30 mph roads and the 40+ mph roads rather than the 20 mph roads.
Furthermore, a comparison of 2011 figures with the average for the previous 5 years (2006 to 2010) found that there were increases in reported KSI casualties throughout Hampshire: the increases in Portsmouth and Southampton were both 56%, and the increase in the rest of Hampshire was 19%.
It is concluded that there is no evidence from the released DfT databases and tables that the introduction of 20 mph speed limits in residential roads in Portsmouth in 2007 to 2008 has led to the large increase in Portsmouth reported KSI road casualties in 2011.
Background
In June 2012, the DfT released summary figures on reported road casualties for up to and including 2011. In July 2012, the Institute of Advanced Motorists (IAM) issued a press release [1] giving its calculation of the changes from 2010 in killed and seriously injured (KSI) casualties for each highway authority. This was followed by a press release from the Association of British Drivers, headed "20MPH Zones and Speed Humps Do Not Work" [2], and an article in the Sun headed "Official: Casualties soaring in 20mph zones" [3]. Articles appeared in other online publications linking the increase in Portsmouth reported KSI casualties to the introduction of 20 mph speed limits in residential roads in 2007 to 2008. The controversy was the subject of an article from the Full Fact group [4].
This report summarises the evidence contained in the DfT published database on whether there is a link between the 20 mph speed limits and the rise in Portsmouth reported KSI casualties. The methods are detailed in the Appendix.
Findings
1. The DfT table RAS30008 [5], which was the basis of the IAM press release, showed an increase in Portsmouth reported KSI road casualties from 91 in 2010 to 143 in 2011.
Examination of spreadsheets derived from the DfT databases showed that most of these reported KSI casualties occurred on the 30 mph roads. A breakdown of these 2010 and 2011 totals by class of road is as follows.
Portsmouth Reported KSI Road Casualties 2010 - 2011 Road category 2010 2011 Change % change 95% CI for change 20 mph roads 17 21 +4 +24% -48% to +95% 30 mph roads 58 95 +37 +64% +22% to +106% 40 - 70 mph roads 16 27 +11 +69% -12% to +149% Total 91 143 +52 +57% +24% to +90%
The relevant spreadsheet for Hampshire Constabulary reported KSI casualties for 2010 can be downloaded from here, and that for 2011 can be downloaded from here.
The chart above shows the figures graphically:
So almost all of the increase from 2010 to 2011 was on the 30 mph roads, or the 40 - 70 mph category of roads, rather than the 20 mph roads.
2. A comparison of the 2011 figures with the previous 5 years found increases across the whole of Hampshire, as shown in the following table.
Hampshire Reported KSI Road Casualties 2006 - 2011 Local authority 2006 2007 2008 2009 2010 2006-10 average 2011 Change: 2011 over 2006-10 average 95% CI for change Hampshire excluding Unitary Authorities 618 713 587 650 632 640 764 +19% +9% to +29% Portsmouth 93 79 99 97 91 91.8 143 +56% +27% to +85% Southampton 90 85 96 99 123 98.6 154 +56% +28% to +84%
There were increases of 56% in both Portsmouth and Southampton, and an increase of 19% in the rest of Hampshire.
Comment
Examination of the DfT databases showed that most reported KSI casualties in Portsmouth occur on 30 mph roads, and most of the increase from 2010 to 2011 occurred on 30 mph roads. Consequently, there seems to be no reason to attribute the increase in the total number of KSI casualties to the introduction of 20 mph speed limits on Portsmouth's residential roads in 2007 to 2008.
The findings of large increases in KSI casualties across Hampshire, not just in Portsmouth, again make a link to the introduction of 20 mph speed limits appear unlikely.
It is concluded that there is no evidence from the DfT STATS19 databases that the increase in reported KSI casualties in Portsmouth from 2010 to 2011 was due to the introduction of 20 mph speed limits on residential roads in 2007 to 2008.
This analysis is based only on the DfT databases and not on any local knowledge. It is therefore not a comprehensive study of Portsmouth road casualties, but merely a response to the concerns raised by the IAM comparisons.
Appendix: Methods used
The DfT STATS19 databases of collisions and casualties for 2005 to 2011 were downloaded from the DfT website [6]. Data from the collisions files was added to the casualties files (extending each record) by matching through the unique Acc_Index identifier. The Hampshire (Police_Force variable) and KSI (Casualty_Severity variable) cases were then selected, and the codes of the more informative variables were then augmented by the addition of the meaning in barckets using the Road Accidents and Safety Data Guide [7], in order to make the file more comprehensible. Files were then imported into a spreadsheet in Microsoft Excel format for analysis.
Because of year-to-year random variation, there are advantages to comparing time periods longer than one year. This study therefore included a comparison of the 2011 Portsmouth reported KSI figures with the average of the previous 5 years (2006 to 2010). The same was done for the two other Hampshire highway authorities. Figures were obtained from an analysis of the downloaded database and can also be obtained from the Tables 46a of the annual DfT Reported Road Casualty Reports [8].
The calculation of 95% confidence intervals (95% CIs) was carried out by standard techniques [9], treating road casualties as a Poisson variable.
References
[1] http://www.iam.org.uk/news/latest-news/1087-road-deaths-and-serious-injuries-up-road-safety-spending-down and see the footnote.
[2] http://www.abd.org.uk/pr/787.htm
[3] http://www.thesun.co.uk/sol/homepage/news/4481938/Official-Casualties-soaring-in-20mph-zones.html
[4] http://fullfact.org/factchecks/speed_humps_20_mph_limit_zones_plenty_Portsmouth_KSI-27588
[5] http://www.dft.gov.uk/statistics/releases/reported-road-casualties-gb-main-results-2011
[6] http://data.gov.uk/dataset/road-accidents-safety-data/
[7] http://data.dft.gov.uk/road-accidents-safety-data/Road-Accident-Safety-Data-Guide.xls
[8] http://webarchive.nationalarchives.gov.uk/20110503151558/http://dft.gov.uk/pgr/statistics/datatablespublications/accidents/casualtiesgbar/
[9] Statistics with Confidence 2nd Ed. (2000) Altman D, Machin D, Bryant T, and Gardner S. BMJ books. ISBN-10: 0727913751.
Footnote: The IAM press release is misleading in stating "The biggest increases in KSI numbers were in St Helens - 62 per cent, Portsmouth - 57 per cent, Stoke on Trent - 57 per cent, and Coventry - 51 per cent" since the largest increase was in St Helens (62%), the second largest was in Stoke-on-Trent (58%), the third largest was Portsmouth (57%), and fourth was Coventry (52%). There were also some errors in the numbers in the IAM footnote 3. Also, while the IAM made clear that its analysis was restricted to England, others have quoted the IAM press release as if it covered the whole of the UK.
Author
Ian Campbell (www.iancampbell.co.uk) October 2012
In July 2012, a press release was issued by the Association of British Drivers and articles appeared in several national and local newspapers expressing concern about the large increase in the 2011 reported KSI casualties in Portsmouth compared to 2010. The press release suggested that the rise was due to the introduction of 20 mph speed limits for many of Portsmouth's residential streets in 2007 to 2008.
A study summarised here of the reported road casualty databases released by the DfT shows these concerns to be unfounded: most of Portsmouth's KSI casualties occurred on the 30 mph roads, and the largest increases in 2011, both proportionately and in numbers, were on the 30 mph roads and the 40+ mph roads rather than the 20 mph roads.
Furthermore, a comparison of 2011 figures with the average for the previous 5 years (2006 to 2010) found that there were increases in reported KSI casualties throughout Hampshire: the increases in Portsmouth and Southampton were both 56%, and the increase in the rest of Hampshire was 19%.
It is concluded that there is no evidence from the released DfT databases and tables that the introduction of 20 mph speed limits in residential roads in Portsmouth in 2007 to 2008 has led to the large increase in Portsmouth reported KSI road casualties in 2011.
Background
In June 2012, the DfT released summary figures on reported road casualties for up to and including 2011. In July 2012, the Institute of Advanced Motorists (IAM) issued a press release [1] giving its calculation of the changes from 2010 in killed and seriously injured (KSI) casualties for each highway authority. This was followed by a press release from the Association of British Drivers, headed "20MPH Zones and Speed Humps Do Not Work" [2], and an article in the Sun headed "Official: Casualties soaring in 20mph zones" [3]. Articles appeared in other online publications linking the increase in Portsmouth reported KSI casualties to the introduction of 20 mph speed limits in residential roads in 2007 to 2008. The controversy was the subject of an article from the Full Fact group [4].
This report summarises the evidence contained in the DfT published database on whether there is a link between the 20 mph speed limits and the rise in Portsmouth reported KSI casualties. The methods are detailed in the Appendix.
Findings
1. The DfT table RAS30008 [5], which was the basis of the IAM press release, showed an increase in Portsmouth reported KSI road casualties from 91 in 2010 to 143 in 2011.
Examination of spreadsheets derived from the DfT databases showed that most of these reported KSI casualties occurred on the 30 mph roads. A breakdown of these 2010 and 2011 totals by class of road is as follows.
Portsmouth Reported KSI Road Casualties 2010 - 2011 Road category 2010 2011 Change % change 95% CI for change 20 mph roads 17 21 +4 +24% -48% to +95% 30 mph roads 58 95 +37 +64% +22% to +106% 40 - 70 mph roads 16 27 +11 +69% -12% to +149% Total 91 143 +52 +57% +24% to +90%
The relevant spreadsheet for Hampshire Constabulary reported KSI casualties for 2010 can be downloaded from here, and that for 2011 can be downloaded from here.
The chart above shows the figures graphically:
So almost all of the increase from 2010 to 2011 was on the 30 mph roads, or the 40 - 70 mph category of roads, rather than the 20 mph roads.
2. A comparison of the 2011 figures with the previous 5 years found increases across the whole of Hampshire, as shown in the following table.
Hampshire Reported KSI Road Casualties 2006 - 2011 Local authority 2006 2007 2008 2009 2010 2006-10 average 2011 Change: 2011 over 2006-10 average 95% CI for change Hampshire excluding Unitary Authorities 618 713 587 650 632 640 764 +19% +9% to +29% Portsmouth 93 79 99 97 91 91.8 143 +56% +27% to +85% Southampton 90 85 96 99 123 98.6 154 +56% +28% to +84%
There were increases of 56% in both Portsmouth and Southampton, and an increase of 19% in the rest of Hampshire.
Comment
Examination of the DfT databases showed that most reported KSI casualties in Portsmouth occur on 30 mph roads, and most of the increase from 2010 to 2011 occurred on 30 mph roads. Consequently, there seems to be no reason to attribute the increase in the total number of KSI casualties to the introduction of 20 mph speed limits on Portsmouth's residential roads in 2007 to 2008.
The findings of large increases in KSI casualties across Hampshire, not just in Portsmouth, again make a link to the introduction of 20 mph speed limits appear unlikely.
It is concluded that there is no evidence from the DfT STATS19 databases that the increase in reported KSI casualties in Portsmouth from 2010 to 2011 was due to the introduction of 20 mph speed limits on residential roads in 2007 to 2008.
This analysis is based only on the DfT databases and not on any local knowledge. It is therefore not a comprehensive study of Portsmouth road casualties, but merely a response to the concerns raised by the IAM comparisons.
Appendix: Methods used
The DfT STATS19 databases of collisions and casualties for 2005 to 2011 were downloaded from the DfT website [6]. Data from the collisions files was added to the casualties files (extending each record) by matching through the unique Acc_Index identifier. The Hampshire (Police_Force variable) and KSI (Casualty_Severity variable) cases were then selected, and the codes of the more informative variables were then augmented by the addition of the meaning in barckets using the Road Accidents and Safety Data Guide [7], in order to make the file more comprehensible. Files were then imported into a spreadsheet in Microsoft Excel format for analysis.
Because of year-to-year random variation, there are advantages to comparing time periods longer than one year. This study therefore included a comparison of the 2011 Portsmouth reported KSI figures with the average of the previous 5 years (2006 to 2010). The same was done for the two other Hampshire highway authorities. Figures were obtained from an analysis of the downloaded database and can also be obtained from the Tables 46a of the annual DfT Reported Road Casualty Reports [8].
The calculation of 95% confidence intervals (95% CIs) was carried out by standard techniques [9], treating road casualties as a Poisson variable.
References
[1] http://www.iam.org.uk/news/latest-news/1087-road-deaths-and-serious-injuries-up-road-safety-spending-down and see the footnote.
[2] http://www.abd.org.uk/pr/787.htm
[3] http://www.thesun.co.uk/sol/homepage/news/4481938/Official-Casualties-soaring-in-20mph-zones.html
[4] http://fullfact.org/factchecks/speed_humps_20_mph_limit_zones_plenty_Portsmouth_KSI-27588
[5] http://www.dft.gov.uk/statistics/releases/reported-road-casualties-gb-main-results-2011
[6] http://data.gov.uk/dataset/road-accidents-safety-data/
[7] http://data.dft.gov.uk/road-accidents-safety-data/Road-Accident-Safety-Data-Guide.xls
[8] http://webarchive.nationalarchives.gov.uk/20110503151558/http://dft.gov.uk/pgr/statistics/datatablespublications/accidents/casualtiesgbar/
[9] Statistics with Confidence 2nd Ed. (2000) Altman D, Machin D, Bryant T, and Gardner S. BMJ books. ISBN-10: 0727913751.
Footnote: The IAM press release is misleading in stating "The biggest increases in KSI numbers were in St Helens - 62 per cent, Portsmouth - 57 per cent, Stoke on Trent - 57 per cent, and Coventry - 51 per cent" since the largest increase was in St Helens (62%), the second largest was in Stoke-on-Trent (58%), the third largest was Portsmouth (57%), and fourth was Coventry (52%). There were also some errors in the numbers in the IAM footnote 3. Also, while the IAM made clear that its analysis was restricted to England, others have quoted the IAM press release as if it covered the whole of the UK.
Author
Ian Campbell (www.iancampbell.co.uk) October 2012
1. The DfT table RAS30008 [5], which was the basis of the IAM press release, showed an increase in Portsmouth reported KSI road casualties from 91 in 2010 to 143 in 2011.
Examination of spreadsheets derived from the DfT databases showed that most of these reported KSI casualties occurred on the 30 mph roads. A breakdown of these 2010 and 2011 totals by class of road is as follows.
| Portsmouth Reported KSI Road Casualties 2010 - 2011 | |||||
|---|---|---|---|---|---|
| Road category | 2010 | 2011 | Change | % change | 95% CI for change |
| 20 mph roads | 17 | 21 | +4 | +24% | -48% to +95% |
| 30 mph roads | 58 | 95 | +37 | +64% | +22% to +106% |
| 40 - 70 mph roads | 16 | 27 | +11 | +69% | -12% to +149% |
| Total | 91 | 143 | +52 | +57% | +24% to +90% |
The chart above shows the figures graphically:
So almost all of the increase from 2010 to 2011 was on the 30 mph roads, or the 40 - 70 mph category of roads, rather than the 20 mph roads.
2. A comparison of the 2011 figures with the previous 5 years found increases across the whole of Hampshire, as shown in the following table.
| Hampshire Reported KSI Road Casualties 2006 - 2011 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Local authority | 2006 | 2007 | 2008 | 2009 | 2010 | 2006-10 average | 2011 | Change: 2011 over 2006-10 average | 95% CI for change |
| Hampshire excluding Unitary Authorities | 618 | 713 | 587 | 650 | 632 | 640 | 764 | +19% | +9% to +29% |
| Portsmouth | 93 | 79 | 99 | 97 | 91 | 91.8 | 143 | +56% | +27% to +85% |
| Southampton | 90 | 85 | 96 | 99 | 123 | 98.6 | 154 | +56% | +28% to +84% |
Comment
Examination of the DfT databases showed that most reported KSI casualties in Portsmouth occur on 30 mph roads, and most of the increase from 2010 to 2011 occurred on 30 mph roads. Consequently, there seems to be no reason to attribute the increase in the total number of KSI casualties to the introduction of 20 mph speed limits on Portsmouth's residential roads in 2007 to 2008.
The findings of large increases in KSI casualties across Hampshire, not just in Portsmouth, again make a link to the introduction of 20 mph speed limits appear unlikely.
It is concluded that there is no evidence from the DfT STATS19 databases that the increase in reported KSI casualties in Portsmouth from 2010 to 2011 was due to the introduction of 20 mph speed limits on residential roads in 2007 to 2008.
This analysis is based only on the DfT databases and not on any local knowledge. It is therefore not a comprehensive study of Portsmouth road casualties, but merely a response to the concerns raised by the IAM comparisons.
Appendix: Methods used
The DfT STATS19 databases of collisions and casualties for 2005 to 2011 were downloaded from the DfT website [6]. Data from the collisions files was added to the casualties files (extending each record) by matching through the unique Acc_Index identifier. The Hampshire (Police_Force variable) and KSI (Casualty_Severity variable) cases were then selected, and the codes of the more informative variables were then augmented by the addition of the meaning in barckets using the Road Accidents and Safety Data Guide [7], in order to make the file more comprehensible. Files were then imported into a spreadsheet in Microsoft Excel format for analysis.
Because of year-to-year random variation, there are advantages to comparing time periods longer than one year. This study therefore included a comparison of the 2011 Portsmouth reported KSI figures with the average of the previous 5 years (2006 to 2010). The same was done for the two other Hampshire highway authorities. Figures were obtained from an analysis of the downloaded database and can also be obtained from the Tables 46a of the annual DfT Reported Road Casualty Reports [8].
The calculation of 95% confidence intervals (95% CIs) was carried out by standard techniques [9], treating road casualties as a Poisson variable.
References
[1] http://www.iam.org.uk/news/latest-news/1087-road-deaths-and-serious-injuries-up-road-safety-spending-down and see the footnote.
[2] http://www.abd.org.uk/pr/787.htm
[3] http://www.thesun.co.uk/sol/homepage/news/4481938/Official-Casualties-soaring-in-20mph-zones.html
[4] http://fullfact.org/factchecks/speed_humps_20_mph_limit_zones_plenty_Portsmouth_KSI-27588
[5] http://www.dft.gov.uk/statistics/releases/reported-road-casualties-gb-main-results-2011
[6] http://data.gov.uk/dataset/road-accidents-safety-data/
[7] http://data.dft.gov.uk/road-accidents-safety-data/Road-Accident-Safety-Data-Guide.xls
[8] http://webarchive.nationalarchives.gov.uk/20110503151558/http://dft.gov.uk/pgr/statistics/datatablespublications/accidents/casualtiesgbar/
[9] Statistics with Confidence 2nd Ed. (2000) Altman D, Machin D, Bryant T, and Gardner S. BMJ books. ISBN-10: 0727913751.
Footnote: The IAM press release is misleading in stating "The biggest increases in KSI numbers were in St Helens - 62 per cent, Portsmouth - 57 per cent, Stoke on Trent - 57 per cent, and Coventry - 51 per cent" since the largest increase was in St Helens (62%), the second largest was in Stoke-on-Trent (58%), the third largest was Portsmouth (57%), and fourth was Coventry (52%). There were also some errors in the numbers in the IAM footnote 3. Also, while the IAM made clear that its analysis was restricted to England, others have quoted the IAM press release as if it covered the whole of the UK.
Author
Ian Campbell (www.iancampbell.co.uk) October 2012
The DfT STATS19 databases of collisions and casualties for 2005 to 2011 were downloaded from the DfT website [6]. Data from the collisions files was added to the casualties files (extending each record) by matching through the unique Acc_Index identifier. The Hampshire (Police_Force variable) and KSI (Casualty_Severity variable) cases were then selected, and the codes of the more informative variables were then augmented by the addition of the meaning in barckets using the Road Accidents and Safety Data Guide [7], in order to make the file more comprehensible. Files were then imported into a spreadsheet in Microsoft Excel format for analysis.
Because of year-to-year random variation, there are advantages to comparing time periods longer than one year. This study therefore included a comparison of the 2011 Portsmouth reported KSI figures with the average of the previous 5 years (2006 to 2010). The same was done for the two other Hampshire highway authorities. Figures were obtained from an analysis of the downloaded database and can also be obtained from the Tables 46a of the annual DfT Reported Road Casualty Reports [8].
The calculation of 95% confidence intervals (95% CIs) was carried out by standard techniques [9], treating road casualties as a Poisson variable.
References
[1] http://www.iam.org.uk/news/latest-news/1087-road-deaths-and-serious-injuries-up-road-safety-spending-down and see the footnote.
[2] http://www.abd.org.uk/pr/787.htm
[3] http://www.thesun.co.uk/sol/homepage/news/4481938/Official-Casualties-soaring-in-20mph-zones.html
[4] http://fullfact.org/factchecks/speed_humps_20_mph_limit_zones_plenty_Portsmouth_KSI-27588
[5] http://www.dft.gov.uk/statistics/releases/reported-road-casualties-gb-main-results-2011
[6] http://data.gov.uk/dataset/road-accidents-safety-data/
[7] http://data.dft.gov.uk/road-accidents-safety-data/Road-Accident-Safety-Data-Guide.xls
[8] http://webarchive.nationalarchives.gov.uk/20110503151558/http://dft.gov.uk/pgr/statistics/datatablespublications/accidents/casualtiesgbar/
[9] Statistics with Confidence 2nd Ed. (2000) Altman D, Machin D, Bryant T, and Gardner S. BMJ books. ISBN-10: 0727913751.
Footnote: The IAM press release is misleading in stating "The biggest increases in KSI numbers were in St Helens - 62 per cent, Portsmouth - 57 per cent, Stoke on Trent - 57 per cent, and Coventry - 51 per cent" since the largest increase was in St Helens (62%), the second largest was in Stoke-on-Trent (58%), the third largest was Portsmouth (57%), and fourth was Coventry (52%). There were also some errors in the numbers in the IAM footnote 3. Also, while the IAM made clear that its analysis was restricted to England, others have quoted the IAM press release as if it covered the whole of the UK.
Author
Ian Campbell (www.iancampbell.co.uk) October 2012
Ian Campbell (www.iancampbell.co.uk) October 2012