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Review of 20 mph zones in London Boroughs. by D C Webster and R E Layfield. Published Project Report PPR243 - PDF
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1 Review of 20 mph zones in London Boroughs by D C Webster and R E Layfield Published Project Report PPR243
2 Review of 20 mph zones in London Boroughs by D C Webster and R E Layfield PUBLISHED PROJECT REPORT
4 TRL Limited PUBLISHED PROJECT REPORT PPR243 REVIEW OF 20 MPH ZONES IN LONDON BOROUGHS by D C Webster and R E Layfield (TRL Limited) Prepared for: Project Record: Client: Review of 20 mph zones in London, under Term Consultancy Contract TFL Specialist Road Safety Data Analysis Services London Accident Analysis Unit (LAAU), Transport for London, (Jonathan Fray) Copyright TRL Limited June 2003 This report has been prepared for Transport for London, London Accident Analysis Unit. The views expressed are those of the author(s) and not necessarily those of TfL. Published Project Reports are written primarily for the Customer rather than for a general audience and are published with the Customer s approval. Project Manager Approvals Quality Reviewed
5 This report has been produced by TRL Limited, under/as part of a Contract placed by Transport for London. Any views expressed are not necessarily those of Transport for London. TRL is committed to optimising energy efficiency, reducing waste and promoting recycling and re-use. In support of these environmental goals, this report has been printed on recycled paper, comprising 100% postconsumer waste, manufactured using a TCF (totally chlorine free) process.
6 CONTENTS Executive summary 1 1 Introduction mph zones in London Data collection Implementation of schemes Types of schemes Types of traffic calming measures used Spacing of measures Length of roads Size of schemes 10 3 Impact on injury accidents Data collection Changes in annual accident frequency Accident severity Accident migration Accidents at/near junctions Comparison with trends for accidents on roads in London mph zones Boundary roads Regression to the mean 14 4 Impact on casualties Changes in annual casualty frequency Severity of injury Age and class of road user casualty Pedestrians Child pedestrians Pedal cyclists Child pedal cyclists Powered two wheelers Car occupants Child car occupants Goods vehicle occupants Bus and coach occupants All child casualties Summary of casualty changes after installation of 20 mph zones, by road user Comparison with trends for casualties on roads in London All casualties Fatal and serious (KSI) casualties 24 5 Traffic speeds and flows in 20mph zones Speeds after 20 mph zones were installed Before to after changes in traffic flows Before to after changes in speeds and accident frequencies 26 TRL Limited PPR243
7 6 Comparison with previous work 29 7 Summary, conclusions and future work 29 8 Acknowledgements 31 9 References 31 APPENDIX A APPENDIX B APPENDIX C APPENDIX D APPENDIX E APPENDIX F 20 mph zones in London Traffic calming measures used Numbers of accidents in 20 mph zones in London Chi-squared test Numbers of casualties (all ages) in 20 mph zones in London Numbers of child casualties in 20 mph zones in London TRL Limited PPR243
8 Executive summary Previous research at TRL has shown that a reduction in vehicle speeds generally leads to a reduction in the number and severity of accidents. On average, each 1mph reduction in speed is expected to reduce the injury accident frequency by about 5%. There are a number of ways in which substantial speed reductions can be achieved and one of these is the 20 mph zone in which the 20 mph limit is indicated by boundary signing and physical traffic calming measures are used to provide a self enforcing speed reducing element. The use of 20 mph speed limit signs alone, without supporting traffic calming features is likely to lead to only small speed reductions of about 1 mph. A national study of two hundred 20 mph zones in Great Britain was carried out by TRL in 1996 for the DfT, which showed that 20 mph zones were beneficial in reducing speeds and accidents. Speeds within the zones were reduced by 9 mph, traffic flows were reduced by 27%, injury accidents (all categories combined) were reduced by 61% and fatal and serious accidents (KSI accidents) were reduced by 70%. In October 2002, TRL were commissioned by the London Accident Analysis Unit (LAAU) to review the performance of 20 mph zones in London. This present study applies a similar methodology used for the DfT study to the many 20mph zones in London, allowing (among other things) a comparison of the changes in accident frequency for the London schemes against 20 mph zones in other areas. Only five of the 20mph zones in London were included in the previous study as, at that time, many had not been installed long enough to obtain the required minimum of 12 months after accident data. Initial contact with the London Boroughs indicated that the number of 20 mph zones being installed in London had increased from about 5 per year (up to 1999) to over 30 per year by 2002 with a current total of about 137 installed zones. Further information was requested from the Boroughs and detailed information was received for 115 of the 137 zones. Most of the zones were in residential areas with over half containing schools or colleges. The main traffic calming measures used within the zones were road humps, raised junctions and speed cushions. On average, the measures were spaced between 50 and 95 metres apart. Seventy eight of the 20 mph zones in London had been in long enough for at least a year of after accident data to be available for analysis. Before periods of 5 years were used and the average length of the after periods was about 3 years. The results indicate that: the overall average after mean traffic speed within the 20 mph zones in London was about 17 mph (22 zones). mean traffic speeds have reduced by an average of about 9 mph and traffic flows by an average of about 15% since the installation of the 20 mph zones (note: only a limited amount of speed (14 zones) and flow (11 zones) data was available). allowing for background changes in accident frequency on unclassified roads in London, the installation of 20 mph zones in London has reduced the frequency of injury accidents within the zones by about 42% and reduced the frequency of accidents involving fatal or serious injury (KSI) by about 53% (78 zones). the average ratios of KSI accidents to all injury accidents fell from 0.17 to 0.13 following the zone installations (78 zones). little, if any, accident migration has taken place from the 20 mph zones (38 zones). over the before period, the numbers of accidents per km per year on 20mph zone roads were, on average, more than twice those on other unclassified roads in London. allowing for background changes in road user casualty frequency on unclassified roads in London, the installation of 20 mph zones in London has reduced the frequency of road user TRL Limited 1 PPR243
9 casualties within the zones by about 45% and reduced the frequency of fatal or serious (KSI) casualties by about 57% (78 zones). The average ratios of KSI casualties to all casualties fell from 0.16 to 0.12 following the zone installations (78 zones). the average annual reduction in fatal and serious (KSI) casualties per 20 mph zone is This is equivalent to an annual saving of about 66 KSI casualties across all one hundred and thirty seven 20 mph zones in London. NB Each serious casualty is currently valued at 134,000 by DfT (DfT, 2002) which is therefore equivalent to 8.84 million. This is a conservative estimate because fatal casualties are valued at 1.19 million each which would increase the overall value but predicting reductions in fatal casualties is imprecise because of the small numbers involved. there were statistically significant reductions in the KSI casualty frequency within the 20 mph zones for the following classes of road user: pedestrians (50%), child pedestrians (61%), pedal cyclists (50%), powered two wheelers (68%), car occupants (77%), and all child casualties (60%) [78 zones]. allowing for background changes in KSI casualty frequency on unclassified roads in London, the installation of 20 mph zones has reduced the frequency of child KSI casualties within the zones between 45% and 60%, pedestrian KSI casualties between 39% and 50%, pedal cyclist KSI casualties between 30% and 50% and powered two wheeler casualties between 79% and 68% (78 zones). The results of this study are very encouraging, suggesting large accident and casualty savings. However, the results also leave many questions unanswered. To answer the questions was either outside the scope of the present study or due to a lack of available data and so it is recommended that:! further work is undertaken to assess the potential benefit of more extensive use of 20mph zones! future schemes are more fully monitored and analysed, preferably in a comprehensive way, in order to better understand good practice! an in depth study into the effect of 20mph zones on powered two-wheelers be undertaken. TRL Limited 2 PPR243
10 1 Introduction Research at TRL has shown that a reduction in vehicle speeds generally leads to a reduction in the number and severity of accidents, (Finch et al, 1994) and (Taylor et al, 2000). Broadly, each 1 mph reduction in speed is expected to cut injury accident frequency by 5%. There are a number of ways in which substantial speed reductions can be achieved and one of these is the 20 mph zone in which the 20 mph limit is indicated by boundary signing and physical traffic calming measures are used to provide a self enforcing speed reducing element (DfT, 1999). The use of 20 mph speed limit signs alone, without supporting traffic calming features is likely to lead to only small speed reductions of about 1 mph (Mackie, 1998). The review of urban speed management methods by Mackie also indicated that there was little reduction in injury accidents in 20 mph zones and (30kph zones) which used signs alone apart from the city of Graz in Austria (13% reduction) where the signs-only installations were accompanied by a comprehensive publicity and enforcement campaign. A national study of two hundred 20 mph zones in Great Britain was carried out by TRL for the DfT, which showed that 20 mph zones were beneficial in reducing speeds and accidents (Webster and Mackie, 1996). The main results of the report were that: Speeds were reduced by 9 mph Flows were reduced by 27% Injury accidents (all severities) were reduced by 61% Fatal and serious accidents (KSI accidents) were reduced by 70% Pedestrian injury accidents were reduced by 63% Pedal cyclist injury accidents were reduced by 29% Motor cyclist injury accidents were reduced by 73% Child (pedestrian and cyclist) injury accidents were reduced by 67% The ratio of fatal and serious accidents (KSI accidents) as a proportion of the total was reduced from 0.21 to Injury accidents were reduced by 6.2% for each 1mph speed reduction There was no apparent overall accident migration on the surrounding roads This level of reduction in accidents and casualties is supported by more recent data from the City of Hull where 26% (191 km) of the city s roads are subject to 20 mph speed restrictions. One hundred and twenty 20 mph zones have been installed since 1994 covering 500 streets. Injury accidents in the zones have decreased by 56%; KSI accidents by 90%; pedestrian casualties by 54%; child casualties by 64%: and child pedestrian casualties by 74% (Brightwell, 2003). In October 2002, TRL were commissioned by the London Accident Analysis Unit (LAAU) to review the performance of 20 mph zones in London. Only five of the 20mph zones in London were included in the earlier DfT national study as, at that time (1996), many had not been installed long enough to obtain the required minimum of 12 months after accident data. This present study applies a similar methodology used for the DfT project to the many 20mph zones in London, allowing (among other things) a comparison of the changes in accident frequency for the London schemes against 20 mph zones in other areas. Section 2 of this report provides information about the 20 mph zones in London, Section 3 considers their impact on injury accidents, Section 4 considers their impact on casualties, Section 5 considers the changes in changes in traffic speeds and flows within the 20 mph zones and Section 6 contains the summary of results and conclusions. TRL Limited 3 PPR243
11 2 20mph zones in London 2.1 Data collection In November 2002, TRL contacted all London Boroughs to determine how many 20 mph zones they had installed and to obtain information concerning the location of the zones, the installation period, plans of the zones and the types of measures used in the zones. It was estimated from the initial contact that there were about 137 zones (see Figure 2.1). Further information was requested from the Boroughs and detailed information was received for 115 of the 137 zones (see Appendix A). Seventy eight of the 115 zones had been in long enough for at least a year of after accident data to be available for analysis. The Boroughs were also asked for any speed or flow measurements made before or after the zones were installed. This data was added to the information that TRL had already acquired on 20 mph zones in London from previous studies Figure 2.1 Numbers of 20 mph zones in London as at November 2002 (137 total) 2.2 Implementation of schemes The implementation dates of the schemes are summarised in Figure 2.2 which shows how the number of 20 mph zones installed in London has increased from about 5 per year (up to 1999) to over 30 per year by The number for 2003 (which is a part year) is on course for a similar number to No Boroughs reported that any 20 mph zones had been removed after installation and it was believed that the roads within the zone were originally subject to a 30 mph speed limit. TRL Limited 4 PPR243
12 35 30 Number of 20 mph zones installed Year Figure mph zones in London number installed in each year (03 part year) 2.3 Types of schemes It was found that 102 (89%) of the hundred and fifteen 20 mph zones were area zones and 13 (11%) were linear zones. The linear zones were those that consisted of only one road. There were 40 (35%) purely residential zones, 70 zones (61%) containing schools and 5 zones (4%) which were town /city centres or mainly commercial zones. 2.4 Types of traffic calming measures used Information on the types of traffic calming measures used within the 20 mph zones was provided by the London Boroughs for 95 of the zones (see Appendix B). Examples of the measures used in Ealing (zone 29), Kingston (zone 55) and Sutton (zone 101) are given in Figures 2.3, 2.4 and 2.5. Road humps (flat-top and round-top) Roads humps are an effective and versatile traffic calming measure. Round-top humps were used in 49 zones (52%) and flat-top humps in 70 zones (74%). The most common height for the humps was 75 mm (see DfT, 1999). This height was used in about 80%, of the zones with road humps; the remainder used humps in the range 75 to 100 mm high. Raised junctions Raised junctions are a form of flat-top road hump covering the whole junction. The extent to which a raised junction extends into the side road depends on the local factors at the site. They were used in 26 zones (27%) mainly combined with road humps. Speed cushions Speed cushions were developed to reduce the discomfort / delay to large vehicles such as buses and fire appliances. For cars, the speed reduction achieved is mainly determined by cushion width with narrower cushions allowing higher speeds. Speed cushions were used in 42 zones (44%), mainly combined with road humps and raised junctions. Four zones in LB Southwark used only speed cushions (Zones 96, 97, 98 and 100). The most common height of the cushions used in the 20 mph zones was 75 mm (94%); the others were 80 mm high (6%). The most common width of the cushions was 1.6 m (64%). Other widths of cushion used were 1.7 m (17%), 1.8 m (11%) and 1.9 m (8%). TRL Limited 5 PPR243
13 Chicanes Chicanes generally cause less discomfort than road humps but they are more expensive and not so effective at reducing speeds. They can be unpopular with residents for a number of reasons: little speed reduction for powered two wheelers, fear of collisions with opposing traffic, loss of parking space and difficulty in using accesses. Chicanes were not very common and were used at only 2 (2%) of the 20 mph zones in London. Raised footway Raised footways are a form of flat-top hump which is used to raise the footway at the entrance to a zone. TRL Limited 6 PPR243
14 Figs 2.3a and 2.3b Signing and raised footway across entrance to 20 mph zone a b Fig 2.3c Raised junction Fig 2.3d Flat-top hump and controlled parking bays Figure 2.3 Five Roads 20 mph Zone, LB Ealing (zone 29 - first part of phased installation of Five Roads Home Zone) TRL Limited 7 PPR243
15 Figs 2.4a, 2.4b and 2.4c showing 20 mph zone signing and road narrowing at entrance gateway a b c Fig 2.4d Round-top hump Figure 2.4 The Groves 20 mph Zone, LB Kingston (zone 55) TRL Limited 8 PPR243
16 Fig 2.5a Road narrowing and signing at entry to 20 mph zone Fig 2.5b Signing on exit from the 20 mph zone Fig 2.5c Parking along road with round-top humps Figure 2.5 Worcester Park 20 mph zone, LB Sutton (zone 101) TRL Limited 9 PPR243
17 2.5 Spacing of measures Ideally the physical traffic calming measures used in the 20 mph zones should not only keep speeds low but also encourage a smooth driving style through the zone. In order to achieve this, the measures should be spaced about 60m to 70m apart (DfT, 1999). Information on the spacing of the traffic calming measures was available for 74 of the 20 mph zones from the plans supplied by the Boroughs. The maximum and minimum spacings of the measures were recorded for each of the zones and overall averages obtained. The average minimum spacing was about 49m and the average maximum spacing was about 94m. 2.6 Length of roads The maximum length of road comprising one zone was 14.5 km in Plashet Residential Haven, LB Newham (zone 76) and the minimum was 0.15 km in Watling Street, City of London (zone 26). The average length of road in each zone was 3.4 km. The total length of all roads in the zones was 391 km including short lengths that did not have speed-reducing measures. The total length of the roads in the 20 mph zones used for the accident analysis was 253.4km (see Section 3.2). 2.7 Size of schemes The largest 20 mph zone was in Waverley, LB Southwark (zone 97) which has an area of 2.08 km 2. The smallest (excluding the linear zones) was Cabul Road, LB Wandsworth (zone 114) with an area of 0.02 km 2. The average size of all of the zones was 0.35 km 2. 3 Impact on injury accidents 3.1 Data collection As was mentioned in Section 2.1, detailed information was received from the London Boroughs for 115 of the 137 zones. Seventy eight of these 115 zones had been in long enough for at least a year of after accident data to be available for analysis. The following information was determined for each of the 78 zones: The date that construction started The date that the 20 mph zone was finished The roads (wholly or partly) contained within the 20 mph zone. Whether there were well defined boundary roads onto which any displaced traffic might have transferred. This information enabled a before injury accident period of 5 years to be specified for each zone. This was generally taken as 5 calendar years. Calendar years were used because some Boroughs could not be precise as to when the scheme started e.g. in the summer of Schemes which had traffic calming measures installed before the introduction of the 20 mph zone and schemes which were installed in phases, had their 5 year before period finishing before the traffic calming or first phases were installed. The 3 phases in North Tottenham (zones 43, 44 and 45) were self contained and separated by main roads that were not part of the 20 mph zones and hence they were all treated individually. TRL Limited 10 PPR243
18 The dates when the 20 mph zones were completed were generally known so there was less of a problem with the specification of the after periods. The start of each after injury accident period was taken as the first complete month after the 20 mph zone was completed. The lengths of the after injury accident periods were constrained by the data available. The maximum length was five years and the minimum was one year. The information for each zone specifying the before and after injury accident periods and the roads contained within the zone was sent to the London Accident Analysis Unit (LAAU) who extracted the relevant accident and casualty data. LAAU also supplied accident data for the boundary roads around some thirty eight 20 mph zones (see Section 3.4), and control data for unclassified roads and other A and B class roads in London (see Section 3.6). Most zones were on minor roads and most boundary roads were main roads. Therefore, it was decided to use all unclassified roads in London as control data for zone roads and all A and B roads in London as control data for boundary roads. This data was supplied by LAAU. 3.2 Changes in annual accident frequency The numbers of before and after reported injury accidents (slight, serious and fatal) in each of the seventy eight 20mph zones are given in Appendix C. In the before period, the annual frequency of injury accidents ranged from 0 accidents per year (six schemes) to about 20 accidents per year (St Peter s Street, LB Islington). Four schemes had no accidents in the before or after periods. Details of the changes in the annual frequency of injury accidents are given in Table 3.1. During the before period there were 1660 reported injury accidents in 4680 site months giving an overall before accident frequency of about 4.3 accidents per year per site. During the after period there were 590 reported injury accidents in 2930 site months giving an overall after accident frequency of about 2.4 accidents per year per site. Table 3.1 Accidents in 20 mph zones in London (78 zones) No of accidents No of months Accs/year/site % diff. Stat. Before After Before After Before After Before to after All injury accidents Yes Signif. KSI accidents Yes The overall annual accident frequency has reduced by 43% (1.84 accidents per year per 20 mph zone) between the before and after periods. This reduction in accidents is highly statistically significant at the 99% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance. (See Appendix D for details of the significance test.) The total length of roads in the seventy eight 20 mph zones used for the accident analysis was km. Giving a before overall accident frequency of 1.31 accidents per year per km and an after accident frequency of 0.74 accidents per year per km. 3.3 Accident severity Accidents in which there are fatalities or serious injuries are grouped together as killed and seriously injured (KSI) accidents. Details of the changes in the annual frequency of KSI accidents are also given in Table 3.1. TRL Limited 11 PPR243
19 During the before period there were 282 KSI accidents (7 fatal and 275 serious) in 4680 site months giving an overall before KSI accident frequency of about 0.7 accidents per year per zone. During the after period there were 77 KSI accidents (4 fatal and 73 serious) in 2930 site months giving an overall after KSI accident frequency of about 0.3 accidents per year per zone. The annual frequency of KSI accidents has reduced by 56% (0.41 accidents per year per 20 mph zone) between the before and after periods. This reduction in accidents is statistically significant at the 95% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance. The average before severity ratio (proportion of total injury accidents that are fatal and serious) was The after severity ratio (KSI accidents/all reported accidents) reduced to This change in the proportion is also statistically significant at above the 95% level according to the Z test statistic. 3.4 Accident migration A possible concern of traffic calming schemes is that traffic and accidents may be redistributed to the neighbouring surrounding area, as drivers choose different routes, so as to avoid the speed reducing measures. This possibility was investigated for 38 of the 20 mph zones where there were clear boundary roads onto which driver route changes may have occurred and increased the potential for transfer of accidents. Details of the changes in the annual frequency of injury accidents on the boundary roads are given in Table 3.2. On the boundary roads, there were 4605 reported injury accidents in 2280 before site months and 3066 reported accidents in 1554 after site months. The overall annual accident frequency reduced by 2% (0.56 accidents per year per site) between the before and after periods. This reduction in accidents is small and not statistically significant. The annual KSI accident frequency also reduced by 2% (0.07 accidents per year per site) between the before and after periods. This reduction in accidents is also small and not statistically significant. These results suggest that little, if any, accident migration is taking place onto the boundary roads but see also Section 3.6. Table 3.2 Accidents in 20 mph zone boundary roads in London (38 zones) No of accidents No of months Accs/year/site % diff. Stat. Before After Before After Before After Before to after All injury accidents No KSI accidents No Signif. 3.5 Accidents at/near junctions A high proportion of accidents take place at or near junctions because of the inevitable conflicts with other road users when vehicles turn into or out of a junction. The proportions of all accidents that occurred at junctions were 70% before the implementation of the 20 mph zones and 68% after. This before to after change was small and not statistically significant. An analysis of the following accident types were carried out:- Car/Car (Accidents involving only cars) Car/Pedestrian (Accidents involving a car and a pedestrian) TRL Limited 12 PPR243
20 Car/Pedal cyclist Car/Powered 2 wheeler Goods and buses/pedestrian (Accidents involving a car and a pedal cyclist) (Accidents involving a car and a powered 2 wheeler) (Accidents involving a goods vehicle or bus and a pedestrian) The proportion of Car/Car accidents that occurred at junctions was 81% in the before period and 79% in the after period. The corresponding proportions for Car/Pedestrian accidents were 54% before and 57% after. These were the largest two categories of accidents, and both the changes were small and not statistically significant. There was no evidence that the proportions of the other accident types given above, that took place at a junction, were affected by the installation of the 20 mph zone. 3.6 Comparison with trends for accidents on roads in London mph zones The results in Sections 3.2 and 3.3 indicate that the 20mph zones in London have had a large impact on accidents within their boundaries. However some adjustments to the magnitude of the changes are required to allow for general underlying changes in accident frequency on roads within London during the time covered by the before and after periods. The reductions in injury accident frequency in the 20mph zones (43% all accidents, 56% KSI accidents) are substantially greater than the trends on unclassified roads within London, which if the average before and after periods are taken as 5 years (1 st September 1992 to 31 st August 1997) and 3 years (1 st November 1998 to 31 st October 2001) respectively, show a 4% reduction in all accidents and a 12% reduction in KSI accidents (see Table 3.3). Table 3.3 Accidents on all unclassified roads in London No of accidents No of years Accs/year % diff. Before After Before After Before After Before to after All injury accidents KSI accidents Making full allowance for these trends on unclassified roads (i.e. assuming that they have been brought about by other factors than 20 mph zones improved education and training for example) the present results for 20mph zones in London represent an overall reduction in injury accidents of 41% and a 50% reduction of accidents involving serious or fatal injury (both changes remain statistically significant). In reality, however, the 20 mph zones will have contributed to some of the accident reductions on unclassified roads within London; and the present results for the 20 mph zones are best interpreted as bringing about: a reduction of between 41% and 43% for all injury accidents. a reduction of between 50% and 56% for accidents involving fatal or serious injury. TRL Limited 13 PPR243
21 3.6.2 Boundary roads The reductions in injury accident frequency on the boundary roads outside the 20 mph zones (2.3% all accidents, 1.9% KSI accidents) are not substantially different from the trends on A and B classified roads within London, which if the average before and after periods are taken as 5 years (December 1991 to November 1996) and 3 years (June 1998 to May 2001) respectively show a 0.5% reduction in all accidents and a 3.6% reduction in KSI accidents. (see Table 3.4) Table 3.4 Accidents on all A and B class roads in London No of accidents No of years Accs/year % diff. Before After Before After Before After Before to after All injury accidents KSI accidents Making full allowance for these trends on A and B roads, the present results for the boundary roads represent an overall reduction in injury accidents of 1.8% and an increase of 1.7% in accidents involving serious or fatal injury. Both changes are small and not statistically significant and still indicate that little, if any, accident migration is taking place from the 20 mph zones. 3.7 Regression to the mean Regression to the mean (or bias by selection ) is a statistical phenomenon that also needs to be considered in before and after studies, if, what appear to be, above average accident sites are treated. It recognises that high accident frequencies in one year are likely to be followed by lower ones the next year, even if there is no intervention, or treatment, due to random fluctuation. Regression to the mean was not thought likely to be a major problem with this study as many of the areas will not have been selected for 20mph zones because of high accident rates (six zones had no accidents in the 5 year before period). An additional safeguard was that long (5 year) before periods were used. These give more reliable estimates of the true underlying accident frequencies before the schemes were implemented and reduce the likelihood of overstating the effect of the treatment (in this case 20mph zones) on accidents by more than a few per cent. (DfT, 2001). The use of a control (of similar, untreated roads) can also help to assess the true effect of a measure. However, identifying similar roads is not always easy and some further investigations were carried out to ascertain whether the roads chosen for 20mph zones were representative of the control roads (other unclassified roads in London) before they were treated. First, the proportions of accidents involving the various different types of road users were compared for 20mph zone roads and the non-20mph zone roads. Second, the proportions of accidents that resulted in fatal or serious injury were compared for 20pmh zone roads and non-20mph zone roads, and for different road user types. These two investigations revealed broadly similar accident patterns (with the rather surprising exception that the roads that were chosen for 20mph zones had a much higher KSI casualty ratio for powered two-wheeler casualties). Third, for each 20mph zone, the number of before accidents was plotted against the percentage change in accident frequency observed after the 20mph zones were installed. There was no clear evidence that the sites with the highest accident frequencies before achieved the greatest percentage reductions. Finally, data were obtained about unclassified road lengths in London and numbers of accidents/km/year were calculated, as shown in Table 3.5 below. TRL Limited 14 PPR243
22 Table 3.5: Accidents on unclassified roads in London Accidents per year per km Accidents per year per km All Saved % saved KSI Saved % saved All unclassified Before roads After % % 20 mph zones Before After % % Non 20 mph zones Before After % % The table shows that in terms of accident frequency per km, the sites selected for 20mph zones did have the largest values. Further, these values did not reduce to those observed on non-20mph zone roads after treatment. These findings may have consequences for the future installations of 20mph zones in London and these are discussed further in section 7. 4 Impact on casualties The numbers of before and after road user casualties (all ages) that resulted from reported injury accidents within each of the seventy eight 20 mph zones are given in Appendix E. The numbers of child road user casualties for each of the seventy eight 20 mph zones are given in Appendix F. 4.1 Changes in annual casualty frequency Table 4.1 contains details of the before to after changes in road user casualties within 20 mph zones in London. During the before period, there were 1936 reported road user casualties (all categories) in 4680 site months, giving an overall before casualty frequency of about 5.0 casualties per year per site. During the after period, there were 650 reported road user casualties (all types) in 2930 site months, giving an overall after casualty frequency of about 2.7 casualties per year per site. The overall annual casualty frequency has reduced by 46% (2.30 casualties per year per 20 mph zone) between the before and after periods. This reduction in casualties is highly statistically significant at the 99% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance. Table 4.1 Casualties in 20 mph zones in London (78 zones) No of casualties No of months Cas/year/site % diff Stat Before After Before After Before After Before to after Signif All casualties Yes KSI casualties Yes TRL Limited 15 PPR243
23 4.2 Severity of injury Fatalities or serious injuries are grouped together as killed and seriously injured (KSI) casualties. Table 4.1 contains details of the changes in KSI casualties following the introduction of the 20 mph zones in London. During the before period there were 308 KSI casualties (7 fatal and 301 serious) in 4680 site months giving an overall before KSI casualty frequency of about 0.80 casualties per year per zone. During the after period there were 77 KSI accidents (4 fatal and 73 serious) in 2930 site months giving an overall after KSI accident frequency of about 0.32 casualties per year per zone. The annual frequency of KSI accidents has reduced by 60% (0.48 accidents per year per 20 mph zone) between the before and after periods. This reduction in accidents is statistically significant at the 95% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance. The before severity ratio (proportion of total injury accidents that are fatal and serious) was The after severity ratio (KSI casualties/all reported casualties) reduced to This reduction is also highly statistically significant. 4.3 Age and class of road user casualty Pedestrians Table 4.2 contains details of the before to after changes in pedestrian casualties within 20 mph zones in London. Table 4.2 Pedestrian casualties in 20 mph zones in London (78 zones) No of casualties No of months Cas/year/site % diff. Stat. Before After Before After Before After Before to after Signif. All casualties Yes KSI casualties Yes During the before period there were 536 pedestrian casualties in 4680 site months giving an overall before casualty frequency of about 1.37 casualties per year per zone. During the after period there were 202 pedestrian casualties in 2930 site months giving an overall after casualty frequency of about 0.83 casualties per year per zone. The annual frequency of pedestrian casualties has reduced by 40% (0.55 casualties per year per 20 mph zone) between the before and after periods. This reduction in casualties is statistically significant at the 99% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance. The corresponding number of KSI pedestrian casualties were 125 before (0.32 casualties per year per zone) and 39 pedestrian casualties after (0.16 casualties per year per zone) giving a reduction of 50% (0.16 casualties per year per zone) which was statistically significant at the 95% level. The average before severity ratio (proportion of total injury casualties that are fatal and serious) was The after severity ratio (KSI casualties/all reported casualties) reduced to This change was not statically significant. TRL Limited 16 PPR243
24 4.3.2 Child pedestrians Table 4.3 contains details of the before to after changes in child pedestrian casualties within 20 mph zones in London. Table 4.3 Child pedestrian casualties in 20 mph zones in London (78 zones) No of casualties No of months Cas/year/site % diff. Stat. Before After Before After Before After Before to after Signif. All casualties Yes KSI casualties Yes During the before period there were 291 child pedestrian casualties in 4680 site months giving an overall before casualty frequency of 0.75 casualties per year per zone. During the after period there were 94 child pedestrian casualties in 2930 site months giving an overall after casualty frequency of 0.39 casualties per year per zone. The annual frequency of child pedestrian casualties has reduced by 48% (0.36 casualties per year per 20 mph zone) between the before and after periods. This reduction in casualties is statistically significant at the 99% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance. The corresponding number of KSI child pedestrian casualties were 73 before (0.19 casualties per year per zone) and 18 child pedestrian casualties after (0.07 casualties per year per zone) giving a reduction of 60.6% (0.11 casualties per year per zone) which was statistically significant at the 95% level. The average before severity ratio (proportion of total injury casualties that are fatal and serious) was The after severity ratio (KSI casualties/all reported casualties) reduced to This change was not statistically significant Pedal cyclists Table 4.4 contains details of the before to after changes in pedal cyclist casualties within 20 mph zones in London. Table 4.4 Pedal cyclist casualties in 20 mph zones in London (78 zones) No of casualties No of months Cas/year/site % diff. Stat. Before After Before After Before After Before to after Signif. All casualties Yes KSI casualties Yes During the before period there were 250 pedal cyclist casualties in 4680 site months giving an overall before casualty frequency of 0.64 casualties per year per zone. TRL Limited 17 PPR243
25 During the after period there were 105 pedal cyclist casualties in 2930 site months giving an overall after casualty frequency of 0.43 casualties per year per zone. The annual frequency of pedal cyclist casualties has reduced by 33% (0.21 casualties per year per 20 mph zone) between the before and after periods. This reduction in casualties is statistically significant at the 99% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance. The corresponding number of KSI pedal cyclist casualties were 38 before (0.10 casualties per year per zone) and 12 pedal cyclist casualties after (0.05 casualties per year per zone) giving a reduction of 49.6% (0.05 casualties per year per zone) which was statistically significant at the 95% level. The average before severity ratio (proportion of total injury casualties that are fatal and serious) was The after severity ratio (KSI casualties/all reported casualties) reduced to Child pedal cyclists Table 4.5 contains details of the before to after changes in child pedal cyclist casualties within 20 mph zones in London. Table 4.5 Child pedal cyclist casualties in 20 mph zones in London (78 zones) No of casualties No of months Cas/year/site % diff. Stat. Before After Before After Before After Before to after Signif. All casualties Yes KSI casualties No During the before period there were 97 child pedal cyclist casualties in 4680 site months giving an overall before casualty frequency of 0.25 casualties per year per zone. During the after period there were 25 child pedal cyclist casualties in 2930 site months giving an overall after casualty frequency of 0.10 casualties per year per zone. The annual frequency of child pedal cyclist casualties has reduced by 59% (0.15 casualties per year per 20 mph zone) between the before and after periods. This reduction in casualties is statistically significant at the 99% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance. The corresponding number of KSI child pedal cyclist casualties were 16 before (0.041 casualties per year per zone) and 4 child pedal cyclist casualties after (0.016 casualties per year per zone) giving a reduction of 49.6% (0.025) which was not statistically significant at the 95% level. The average before severity ratio (proportion of total injury casualties that are fatal and serious) was The after severity ratio (KSI casualties/all reported casualties) reduced to Powered two wheelers Table 4.6 contains details of the before to after changes in powered two wheeler casualties within 20 mph zones in London. TRL Limited 18 PPR243
26 Table 4.6 Powered two wheeler casualties in 20 mph zones in London (78 zones) No of casualties No of months Cas/year/site % diff. Stat. Before After Before After Before After Before to after Signif. All casualties Yes KSI casualties Yes During the before period there were 207 powered two wheeler casualties in 4680 site months giving an overall before casualty frequency of 0.53 casualties per year per zone. During the after period there were 77 powered two wheeler casualties in 2930 site months giving an overall after casualty frequency of 0.32 casualties per year per zone. The annual frequency of powered two wheeler casualties has reduced by 41% (0.22 casualties per year per 20 mph zone) between the before and after periods. This reduction in casualties is statistically significant at the 95% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance (see Table 8). The corresponding number of KSI powered two wheeler casualties were 54 before (0.138 casualties per year per zone) and 11 powered two wheeler casualties after (0.045 casualties per year per zone) giving a reduction of 67.5% (0.093 casualties per year per zone) which was statistically significant at the 95% level. The average before severity ratio (proportion of total injury casualties that are fatal and serious) was The after severity ratio (KSI casualties/all reported casualties) reduced to Car occupants Table 4.7 contains details of the before to after changes in car occupant casualties within 20 mph zones in London. Table 4.7 Car occupant casualties in 20 mph zones in London (78 zones) No of casualties No of months Cas/year/site % diff. Stat. Before After Before After Before After Before to after Signif. All casualties Yes KSI casualties Yes During the before period there were 870 car occupant casualties in 4680 site months giving an overall before casualty frequency of 2.23 casualties per year per zone. During the after period there were 233 car occupant casualties in 2930 site months giving an overall after casualty frequency of 0.95 casualties per year per zone. The annual frequency of car occupant casualties has reduced by 57% (1.28 casualties per year per 20 mph zone) between the before and after periods. This reduction in casualties is statistically significant at the 99% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance. TRL Limited 19 PPR243
27 The corresponding number of KSI car occupant casualties were 82 before (0.21 casualties per year per zone) and 12 car occupant casualties after (0.05 casualties per year per zone) giving a reduction of 76.6% (0.16 casualties per year per zone )which was statistically significant at the 95% level. The average before severity ratio (proportion of total injury casualties that are fatal and serious) was The after severity ratio (KSI casualties/all reported casualties) reduced to Child car occupants Table 4.8 contains details of the before to after changes in child car occupant casualties within 20 mph zones in London. Table 4.8 Child car occupant casualties in 20 mph zones in London (78 zones) No of casualties No of months Cas/year/site % diff. Stat. Before After Before After Before After Before to after Signif. All casualties Yes KSI casualties No During the before period there were 75 child car occupant casualties in 4680 site months giving an overall before casualty frequency of 0.19 casualties per year per zone. During the after period there were 23 child car occupant casualties in 2930 site months giving an overall after casualty frequency of 0.09 casualties per year per zone. The annual frequency of child car occupant casualties has reduced by 51% (0.10 casualties per year per 20 mph zone) between the before and after periods. This reduction in casualties is statistically significant at the 99% level using the chi-square test with 1 degree of freedom showing that the reduction is unlikely to be due to chance. The corresponding number of KSI child car occupant casualties were 3 before (0.008 casualties per year per zone) and 1 child car occupant casualty after (0.004 casualties per year per zone) giving a reduction of 46.8% (0.004 casualties per year per zone) which was not statistically significant at the 95% level. The average before severity ratio (proportion of total injury casualties that are fatal and serious) was The after severity ratio (KSI casualties/all reported casualties) increased to Goods vehicle occupants Table 4.9 contains details of the before to after changes in goods vehicle occupant casualties within 20 mph zones in London. Table 4.9 Goods vehicle occupant casualties in 20 mph zones in London (78 zones) No of casualties No of months Cas/year/site % diff. Stat. Before After Before After Before After Before to after Signif. All casualties No KSI casualties No TRL Limited 20 PPR243
28 During the before period there were 36 goods vehicle occupant casualties in 4680 site months giving an overall before casualty frequency of casualties per year per zone. During the after period there were 15 goods vehicle occupant casualties in 2930 site months giving an overall after casualty frequency of casualties per year per zone. The annual frequency of goods vehicle occupant casualties has reduced by 33% (0.031 casualties per year per 20 mph zone) between the before and after periods. This reduction in casualties is not statistically significant using the chi-square test with 1 degree of freedom showing that the reduction is likely to be due to chance. The corresponding number of KSI goods vehicle occupant casualties were 4 before (0.008 casualties per year per zone) and 3 goods vehicle occupant casualties after (0.012 casualties per year per zone) giving an increase of 19.8% which was not statistically significant at the 95% level. The average before severity ratio (proportion of total injury casualties that are fatal and serious) was The after severity ratio (KSI casualties/all reported casualties) increased to Bus and coach occupants Table 4.10 contains details of the before to after changes in bus and coach occupant casualties within 20 mph zones in London. Table 4.10 Bus and coach occupant casualties in 20 mph zones in London (78 zones) No of casualties No of months Cas/year/site % diff. Stat. Before After Before After Before After Before to after Signif. All casualties No KSI casualties No During the before period there were 18 bus and coach occupant casualties in 4680 site months giving an overall before casualty frequency of casualties per year per zone. During the after period there were 14 bus and coach occupant casualties in 2930 site months giving an overall after casualty frequency of casualties per year per zone. The annual frequency of bus and coach occupant casualties has increased by 24% (0.011 casualties per year per 20 mph zone) between the before and after periods. This increase in casualties is not statistically significant using the chi-square test with 1 degree of freedom showing that the increase is likely to be due to chance. The corresponding number of KSI bus and coach occupant casualties were 2 before (0.005 casualties per year per zone) and 0 bus and coach occupant casualties after (0.000 casualties per year per zone) giving a decrease of 100% which was not statistically significant at the 95% level. The average before severity ratio (proportion of total injury casualties that are fatal and serious) was The after severity ratio (KSI casualties/all reported casualties) decreased to zero All child casualties Table 4.11 contains details of the before to after changes in all child casualties within 20 mph zones in London. TRL Limited 21 PPR243
Braking point 20mph speed limits in London
Transport Committee Braking point 20mph speed limits in London April 2009 Copyright Greater London Authority April 2009 Published by Greater London Authority City Hall The Queen s Walk More London London
20 mph zones and Road Safety in London
20 mph zones and Road Safety in London A report to the London Road Safety Unit Chris Grundy, Rebecca Steinbach, Phil Edwards, Paul Wilkinson, Judith Green London School of Hygiene and Tropical Medicine
Supplementary Report for St. Thomas Hospital Central London.
Reporting of road traffic accidents in London: Matching police STATS19 with hospital accident and emergency department data. Supplementary Report for St. Thomas Hospital Central London. by Heather Ward,