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Abstract
Background Road traffic casualties show some of the widest socioeconomic differentials of any cause of morbidity or mortality, and as yet there is little evidence on what works to reduce them. This study quantified the current and potential future impact of the introduction of 20 mph zones on socioeconomic inequalities in road casualties in London.
Methods An observational study based on analysis of geographically coded police road casualties data, 1987–2006. Changes in counts of casualties from road collisions, those killed and seriously injured and pedestrian injuries by quintile of deprivation were calculated.
Results The effect of 20 mph zones was similar across quintiles of socioeconomic deprivation, being associated with a 41.8% (95% CI 21.0% to 62.6%) decline in casualties in areas in the least deprived quintile versus 38.3% (31.5% to 45.0%) in the most deprived quintile. Because of the greater number of road casualties in deprived areas and the targeting of zones to such areas, the number of casualties prevented by zones was substantially larger in areas of greater socioeconomic deprivation. However, the underlying decline in road casualties on all roads was appreciably greater in less deprived areas (p<0.001 for trend) so that despite the targeting of 20 mph zones, socioeconomic inequalities in road injuries in London have widened over time. Extending 20 mph schemes has only limited the potential to reduce differentials further.
Conclusions The implementation of 20 mph zones targeted at deprived areas has mitigated widening socioeconomic differentials in road injury in London and to some degree narrowed them, but there is limited potential for further gain.
- Injury
- road traffic crashes
- social inequalities
- socioeconomic variations
- traffic
- traffic calming
- 20 mile-an-hour zones
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- Injury
- road traffic crashes
- social inequalities
- socioeconomic variations
- traffic
- traffic calming
- 20 mile-an-hour zones
Injuries and deaths from road collisions show some of the widest socioeconomic differentials of any cause of morbidity or mortality.1 2 For pedestrians in London, the risk of injury is over twice as high in the most deprived as in the least deprived areas,3 but while such inequalities are well documented,1 4 there is little evidence on what works to reduce them.5 Current policy approaches aim to reduce inequalities primarily through targeting interventions at those areas or populations known to be at highest risk.6
There is good evidence that reducing the speed and volume of traffic reduces casualty rates,7 and therefore it is plausible that prioritising the implementation of traffic calming measures in areas of socioeconomic deprivation might reduce not only overall casualty rates, but also inequalities in casualties.8 9 However, few empirical studies have tested this. An ecological study found a narrowing of inequalities in one UK city that had traffic calming concentrated in more deprived areas, compared with another city, but the authors note the limitations of such ‘natural experiments’, which cannot control for all potential confounding factors and the need for further research to replicate these findings.10
We have recently provided evidence that, in London, 20 mph zones have been associated with a 42% decline in road injury within the zones.11 Twenty mile per hour zones are a form of area-wide traffic calming in which physical engineering measures (such as speed humps and chicanes) are placed every 100 metres to help reduce traffic speeds to 20 mph (for a more detailed description of the 20 mph zone intervention see Grundy et al).11 In this paper we assess the effect these zones have had on socioeconomic inequalities in road casualties in London as a whole, and the potential for further reductions in inequalities from future expansion of the number of 20 mph zones.
Methods
Analysis was based on Police Stats19 data, 1986 to 2006, which record the date, location and number and type of casualties for all injury-related road collisions (damage-only collisions were excluded). In London, the commissioning of 20 mph zones occurs on a financial year calendar (April to March), therefore each casualty was first assigned an appropriate financial year.
Using a geographical information system, these data were then linked to a detailed road segment database (2005), which included the characteristics of all classified and unclassified roads in London. For each financial year, each segment of road between junctions was classified according to the type of road, and whether or not it was in a 20 mph zone or adjacent to a 20 mph zone. Each segment was further classified by quintile of deprivation (Q1–Q5) using the 2004 index of multiple deprivation12 for the lower super output area (LSOA) in which each road segment was located. Q1 indicates the least deprived areas of London, whereas Q5 represents the most deprived areas (figure 1). LSOA are geographical areas containing an average of 1500 people, defined by the Office of National Statistics using measures of population size, mutual proximity and homogeneity. For these analyses we applied LSOA boundaries defined in 2001 to our network of road segments. There are 4765 LSOA in London, within 33 boroughs. The index of multiple deprivation 2004 was chosen because it is available at the level of LSOA, which has advantages over ward-based measures in densely populated urban areas where LSOA are smaller than wards.13
Characteristics of roads and casualty numbers 1987–2006, by quintile of socioeconomic deprivation. Q1, least deprived quintile; Q5, most deprived quintile. (A) Kilometers of road. (B) Casualties 1987–2006.
Each road segment was further classified by the date engineering works started on the 20 mph zone (when relevant), and the date it commenced in operation, which may have been several years from the start of engineering works. Therefore, using these dates, each road segment was classified as pre-intervention, under construction, or post-implementation. The implementation status was assumed to change only at the beginning of each financial year, so that a change from ‘under construction’ to ‘post-implementation’ status, for example, occurred on 1 April following the implementation date.
A geographical information system was also used to generate adjacent areas around 20 mph zones. In this way, we defined three types of roads according to their intervention status: (1) those that were within or would become part of a 20 mph zone; (2) those that were part of an area adjacent to a 20 mph zone and (3) all other roads. For more information on methods see Grundy et al.11
From the combined dataset, counts of casualties were generated for each road segment and year. The road segments enable stratification of the results by intervention status, deprivation quintile and by borough.
Statistical methods
We followed analytical methods described in more detail elsewhere.11 In brief, our first focus was to characterise the influence of the 20 mph zones on casualties by quintile of socioeconomic deprivation of the area where the collisions occurred after allowing for underlying trends over time. Analyses were based on the patterns of change in annual counts within each road segment where there had been one or more casualties over the period of analysis. We compared the pre–post change in injury counts within 20 mph zones relative to the change seen on other roads within each deprivation quintile. To examine the effect of 20 mph zones by deprivation quintile, we fitted a model that included interaction terms for deprivation quintile and intervention status. Robust SE were obtained using jackknife procedures, clustering on the borough (n=33).
The estimated number of casualties avoided as a result of 20 mph zones in 2006 by quintile of deprivation was calculated as:
To estimate the potential for further change in injury inequalities from extending 20 mph zones, we confined analysis to minor residential road segments outside existing 20 mph zones where there had been 0.7 or more casualties per km/year over 2004–6 (0.7 casualties per km/year is a ‘benchmark’ for cost-effectiveness of 20 mph zones).14 The potential number of casualties preventable within each deprivation quintile was then calculated using a similar approach to that described above using as a baseline the average number of casualties on each road segment for 2004–6. For these calculations we used a more conservative estimate of the 20 mph zone effect based on analysis of data for 2000–6 only, as there was evidence that the casualty reduction effects of 20 mph zones have been smaller in more recent years.11
Results
Since 2001, the kilometres of road within 20 mph zones increased rapidly, particularly in the most deprived areas of London (figure 2A). By 2006, 20 mph zones had been implemented on over a quarter of road kilometres in the most deprived areas of London, compared with less than 3% of road kilometres in the least deprived areas of London (table 1).
Trends in road casualty numbers and kilometres of road inside 20 mph zones, London, 1987–2006, by quintile of socioeconomic deprivation. Q1, least deprived quintile; Q5, most deprived quintile. (A) Kilometres of road inside zones. (B) Total casualties.
Characteristics of 20 mph zones by deprivation group
Whereas the number of casualties decreased in all deprivation quintiles between 1987 and 2006, casualties decreased at a greater rate in relative terms in areas of low socioeconomic deprivation (figure 2B), particularly in the 1987–2001 period. For instance, in Q1 there were 7372 casualties in 1987 compared with 5577 casualties in 2001, an overall reduction of 24%. In Q5 there were 11 094 casualties in 1987 compared with 11 096 casualties in 2006, an overall reduction of 0%. In the 2001–6 period there appears to be a modest narrowing of inequalities. In 2006, there were 4154 casualties in Q1, corresponding to a 26% reduction in casualties in the 2001–6 period in the least deprived areas of London. There were 7253 casualties in Q5 in 2006, corresponding to a 35% reduction in casualties in the 2001–6 period in the most deprived areas of London.
Effect of 20 mph zones
The effects of the 20 mph zones on casualties in each deprivation quintile are summarised in table 2. The models used to derive these estimates allow for the (generally) downward trend over time in the annual number of casualties in London by deprivation quintile.
Effect (percentage reduction) of introducing 20 mph zones on casualties in 20 mph zones and in adjacent areas, and the annual average decline in casualties on all roads, 1987–2006, by deprivation quintile
There was no clear evidence that the 20 mph zone effect varied with socioeconomic deprivation in relation to all casualties, killed or seriously injured (KSI) or pedestrian casualties (table 2), although the point estimate of effect was smallest in the most deprived areas for all outcomes. Twenty mile per hour zones were generally associated with reductions in casualties in adjacent areas, but there was again no evidence that the degree of protection varied with socioeconomic status (table 2).
However, there was evidence that the underlying annual rate of decline in casualties on all roads was appreciably faster on road segments in the least deprived areas for all casualties (p<0.001) and pedestrian casualties (p=0.02), but not for KSI casualties (p=0.3) (table 2). Estimates on the effect of 20 mph zones on cyclist, powered two-wheeler and car occupant casualties are available in an appendix (available online only).
Estimate of casualties avoided
The estimated number of casualties prevented by 20 mph zones in 2006 is shown in figure 3. We calculate that 510 casualties were prevented in 2006 within 20 mph zone boundaries (against 31 202 road casualties overall), the majority within the two most deprived quintiles (figure 3). Per kilometre of road, 20 mph zones have prevented substantially more casualties in the most deprived areas of London compared with the least deprived areas (0.22 casualties per kilometre in the most deprived vs 0.01 casualties per kilometre in the least deprived areas).
Road casualties 2006 by quintile of the 2004 index of multiple deprivation.
We estimate that without 20 mph zones the difference in the number of casualties between the most and least deprived quintile of socioeconomic deprivation would have been 3619, compared with the observed difference of 3099, a relative reduction of 14.4%. Despite this socioeconomic differentials overall have widened over time; in 1987 50% more casualties occurred on roads in the most deprived quintile compared with roads in the least deprived quintile; by 2001 that percentage had risen to 99%. Twenty mile per hour zones, the majority of which were introduced after 2001, appear to have mitigated the widening of socioeconomic differentials; by 2006, the percentage difference in the number of casualties on roads in the most deprived quintile compared with the least deprived quintile had narrowed to 75%.
Potential benefit from extending zones in London
The potential additional benefit of extending 20 mph zones to all suitable roads in London that have not yet been included in 20 mph zones is shown in table 3. We estimate that road casualties overall could be reduced by 699 a year. However, relatively few casualties occur on roads eligible for future 20 mph zones in the most deprived areas of London, and in consequence, future extension of 20 mph zones in appropriate areas would decrease the difference in the number of casualties in the most deprived quintile compared with the least deprived quintile by only 1%.
Potential further reduction in casualties following 20 mph zone implementation
Discussion
Reducing inequalities in health is an important policy goal. There is, however, limited evidence on how to do this and the possibility that interventions can lead to overall health gain but exacerbate inequalities, often because their effectiveness is greater in more advantaged groups.15 16 This study suggests that although there is an effective measure to reduce road injuries, namely 20 mph zones,11 and that those zones have been effectively targeted at deprived areas of London, the effect of such targeting on socioeconomic differentials has been more than offset by the underlying downward trend in road casualties that still favours less socioeconomically deprived areas. Remarkably, overall casualty numbers in less deprived areas have thus fallen faster over time, so that the impact of targeted 20 mph zones has been to mitigate the widening of differentials, rather than to reduce them.
The effect of 20 mph zones on socioeconomic differentials has nonetheless been substantial, reducing the gap in the number of casualties between the most and least deprived quintiles of deprivation by approximately 14% compared with what it would otherwise have been. However, given that historical targeting has left fewer remaining suitable roads for 20 mph zone interventions in deprived areas, and the fact that the majority of collisions now occur on roads not currently considered eligible for 20 mph zones, we estimate that extending 20 mph zones to appropriate areas has the potential to reduce further the gap between the number of casualties in the most and least deprived quintiles of socioeconomic deprivation by only 1%. Future efforts to reduce socioeconomic inequalities in casualties may therefore have to address major roads, on which the majority of injuries occur.
It should be noted that, because this study was based on an analysis of road injuries by place of occurrence, the results reflect changes in casualty numbers (by area type) rather than in casualty rates calculated by reference to a population denominator. (No such denominator exists for place of injury, as there are no readily obtainable data on population movements throughout the day.) However, much of the published evidence on inequalities in road injury also utilises collision location, and there is evidence that the majority of child pedestrian injuries occur close to home. This analysis could not take into account other changes to the road environment or the impact of other road safety measures such as road safety cameras. If, like 20 mph zones, they were introduced more frequently in deprived areas and were successful in reducing casualties, we have potentially underestimated the impact of 20 mph zones on inequality reduction.
A further uncertainty is the known underreporting of road injuries in the Stats19 data that we used, and this might plausibly vary by deprivation. However, reporting in London is relatively good compared with the rest of the UK,17 and for such underreporting to affect the results of our analysis one would have to invoke selective changes over time and deprivation quintile in recording of injuries in 20 mph zones compared with other road types. National evidence suggests that the rate of underreporting overall has not substantially changed over time.17
This analysis did not account for any changes in exposure over time. Road user behaviour is complex and likely to change in response to engineering interventions. As 20 mph zones are introduced, pedestrian behaviour in particular is likely to change, as residents may feel safer and more confident in travelling around and playing in their local environment, thus increasing their exposure to road injury risk. There is no consensus as yet from the limited available evidence on the impact of traffic calming on pedestrian behaviour,18 19 but potential changes in behaviour due to traffic calming might plausibly differ by area deprivation. We have identified evidence that 20 mph zones not only reduce the burden of injury, but play a role in reducing injury inequality. Challenges remain in identifying why background trends in injury decline have been faster in less deprived areas. Further research is needed to explore any potential changes in travel patterns by socioeconomic status.
What is already known on this subject
There are steep socioeconomic gradients in road injury.
Twenty mile per hour zones are an effective way to reduce road injury risk.
What this study adds
Over the past 20 years, casualty rates have fallen fastest in less deprived areas.
Twenty mile per hour zones are equally effective at reducing injury in deprived and affluent areas.
In London, the targeting of 20 mph zones in more deprived areas has helped mitigate widening inequalities in road injury.
Acknowledgments
The STATS19 and 20 mph zone data were supplied by Peter Sadler at the London Road Safety Unit. The road network used was Ordnance Survey Integrated Transport Network layer supplied by Transport for London under licence and is copyright Ordnance Survey. The authors would like to thank John Cairns and Ben Armstrong for their contributions to the design of the study, and are grateful to referees for comments on earlier versions of this paper.
Supplementary materials
Web Only Data jech.2010.112193
Files in this Data Supplement:
Footnotes
Funding This work was undertaken by the London School of Hygiene and Tropical Medicine who received funding from Transport for London. The views expressed in the publication are those of the authors and not necessarily those of Transport for London.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.