The rise in Portsmouth reported KSI casualties from 2010 to 2011 was concentrated on roads with speed limits above 20 mph
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.
1. The DfT table RAS30008 , 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.
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.
The DfT STATS19 databases of collisions and casualties for 2005 to 2011 were downloaded from the DfT website . 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 , 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 .
The calculation of 95% confidence intervals (95% CIs) was carried out by standard techniques , treating road casualties as a Poisson variable.
Ian Campbell (www.iancampbell.co.uk) October 2012
|Last updated: 20 Jul 2015|