Guidance

Guide to severity adjustments for reported road casualties Great Britain

Updated 28 September 2023

Introduction

The Office for National Statistics (ONS) Methodology Advisory Service have developed a methodology to quantify the effect of the introduction of injury based reporting systems (CRaSH and COPA) on the number of slight and serious injuries reported to the police, and to estimate the level of slight and serious injuries as if all police forces were using injury-based reporting systems. This is described in detail in the final ONS methodology report. The final report was published alongside our 2018 statistical release to set out how this methodology was finalised.

This methodology has allowed us to produce adjusted figures for the changes in injury severity as experimental statistics, and now we are aiming to upgrade them to official statistics. Therefore we would welcome any feedback from users by completing our feedback survey.

In 2018, we implemented the severity adjustment methodology for the first time and published adjusted figures in a limited number of our statistical tables. In 2019 we included adjusted figures in all of the statistical tables that were published alongside the annual report.

We also include probabilities of each casualty being serious under injury-based systems alongside the underlying dataset. Further guidance is given in the Annex to the adjustments method.

Definitions

CRaSH: Collision Recording and Sharing system. This is a centralised system used by some police forces to record road traffic collisions.

COPA: Case Overview Preparation Application. This is a system used by the Metropolitan Police Service to record road traffic collisions. ^

Sensitivity analysis

Forces joining Injury Based Reporting Systems

In 2021, we conducted sensitivity analysis to understand how adding an extra year of data affects the severity probabilities. When comparing adjusted figures between the 2019 and 2020 models, there was less than a 0.5% change on the annual trends for all seriously injured adjusted figures. When we looked at police forces breakdowns we noticed that the level of change is different depending on the police force. There is a higher percentage change for police forces that joined an injury based reporting system (IBRS) in 2019 (2 to 8%) with the highest percentage change for Police Scotland. Police forces that joined an IBRS in 2016 and the ones which are on a non-injury based reporting system (non-IBRS) had a percentage change of less than 2%.

Merging of forces

The model was updated in 2021 for use in producing 2020 adjustments. For this model run, all Scottish police forces have been grouped together into one police force called Police Scotland. The grouping of all Scottish police forces led to a slight increase in serious injuries, but had no impact on other police forces which have an IBRS. However, it affected police forces which have a non-IBRS due to the change in median police force.

Historical revisions to casualty severity

For the 2021 model, there was a revision to historic figures for slight and serious injuries impacting data as far back as 2012. Prior to 2020, whether a casualty was reported as ‘admitted to hospital’ factored into casualty severity calculations in the Collision Reporting and Sharing (CRaSH) system. Since 2020 this is no longer the case. Historic cases since the introduction of CRaSH have been updated and corrected to reflect this change. As a result some casualty severities have been downgraded between 2012 and 2019 compared to previously published numbers. Further information is available in the background guidance.

A sensitivity check was carried out to understand the effect of revising the severities on the model. This caused a decrease of 2 to 3% in the adjusted serious casualties compared to the model run using the pre-revision severities. These severity changes affected only casualties reported by police forces that have adopted CRaSH. The correction to historic casualty severities affected police force severity adjustments differently depending on their size and the number of revisions for that police force. But there was no substantive changes to trends over time.

Nevertheless, the severity revision affected Gloucestershire police force slightly differently compared to other police forces. The revision caused the historic adjusted serious time series (prior to the adoption of CRaSH) to be lower than unadjusted. While unusal, this is possible with the adjustment methodology used and reflects the fact that coding of injury severity is likely to have varied across forces prior to adoption of IBRS. It may be that Gloucestershire police were already recording a higher proportion of casualties as seriously injured, relative to other forces, so that the modelled impact of moving to IBRS is different.

In the next few years, we also expect to update the model as further forces move to an injury based reporting system.

Guidance on severity adjustment use

The flow diagram in Figure 1 shows an overview on when we consider that the adjusted and unadjusted figures are most appropriate to use, though all users should carefully considered their specific circumstances and we are happy to provide more detailed advice if helpful.

Figure 1: Guidance on when to use severity adjustments

Tables 1-4 below provide more in-depth guidance on when to use adjusted severities depending on the geographies and time series included in the analysis.

Comparing within the same geography or police force

Table 1: Use of adjustments when comparing within an area

Option selected	Result
Has used IBRS for the whole time period used in the analysis	No need to use adjusted severities
Has used IBRS for some of the time period used in the analysis	Severity adjustment needed
Uses IBRS but not for the time period I am examining	No need to use adjusted severities
Has not switched to using IBRS	No need to use adjusted severities

Comparing across geographies or police forces

Table 2: Use of adjustments when comparing across areas

Police force using IBRS	Time period	Result
All forces using injury based recording system	Whole time period is on injury based reporting system	No need to adjust, all the police forces analysed are using an injury based recording system over the time period we are analysing
All forces using injury based recording system	Some of the time period is on injury based recording system	Severity adjustment needed as some of the data analysed was recorded on IBRS and some was recorded on non-IBRS. There is a break in the series and comparisons are not reliable unless the adjustment is used to account for the change in the reporting system
All forces using injury based recording system	Whole time period in non_IBRS	No adjustment needed
Some forces using IBRS	Whole time period	Severity adjustment needed
Some forces using IBRS	Some of the time period	Severity adjustment needed
Some forces using IBRS	None of the time period	No severity adjustment needed
None of the police forces	Whole time period	No severity adjustment needed

Classification of injury severity using the CRaSH reporting system

Table 3: Classification of injury severity using the CRaSH reporting system

Injury in CRASH	Detailed severity	Severity classification
Deceased	Killed	Killed
Broken neck or back	Very Serious	Serious
Severe head injury, unconscious	Very Serious	Serious
Severe chest injury, any difficulty breathing	Very Serious	Serious
Internal injuries	Very Serious	Serious
Multiple severe injuries, unconscious	Very Serious	Serious
Loss of arm or leg (or part)	Moderately Serious	Serious
Fractured pelvis or upper leg	Moderately Serious	Serious
Other chest injury (not bruising)	Moderately Serious	Serious
Deep penetrating wound	Moderately Serious	Serious
Multiple severe injuries, conscious	Moderately Serious	Serious
Fractured lower leg / ankle / foot	Less Serious	Serious
Fractured arm / collarbone / hand	Less Serious	Serious
Deep cuts / lacerations	Less Serious	Serious
Other head injury	Less Serious	Serious
Whiplash or neck pain	Slight	Slight
Shallow cuts / lacerations / abrasions	Slight	Slight
Sprains and strains	Slight	Slight
Bruising	Slight	Slight
Shock	Slight	Slight

Adoption dates for CRaSH or COPA by police force

Table 4 shows the police forces which use or have used either CRaSH or COPA and the dates from which these systems have been used. Note that adoption dates are indicative as there can be phased introduction of new systems during transitions.

Table 4: Adoption dates for CRaSH or COPA by police force

Police Force	System Used	Adoption Date
Surrey	CRASH	November 2012
South Yorkshire	CRASH	January 2013
Staffordshire	CRASH	May 2015
West Midlands	CRASH	November 2015
Warwickshire	CRASH	November 2015
Essex	CRASH	November 2015
City of London	CRASH	November 2015
Gloucestershire	CRASH	November 2015
Metropolitan Police Service	COPA	November 2015
West Mercia	CRASH	December 2015
Devon and Cornwall	CRASH	December 2015
Cumbria	CRASH	January 2016
Humberside	CRASH	January 2016
Kent	CRASH	January 2016
Norfolk	CRASH	February 2016
Suffolk	CRASH	February 2016
Durham	CRASH	March 2016
Northumbria	CRASH	April 2016
Bedfordshire	CRASH	April 2016
Hertfordshire	CRASH	April 2016
Cambridgeshire	CRASH	May 2016
Lancashire	CRASH	December 2018
Sussex	CRASH	April 2019
Police Scotland	CRASH	April 2019
Greater Manchester	CRASH	February 2021
Nottinghamshire	CRASH	February 2021
West Yorkshire	CRASH	April 2021

Impact on trends

Following the introduction of CRaSH and COPA, the number of casualties recorded as serious has increased in Great Britain. Chart 1 shows the number of reported serious road casualties by police force over time, from two years before to two years after injury based reporting systems were introduced (year introduced, index = 100).

Chart 1: Reported serious road casualties by police force, two years before to two years after injury based reporting systems were introduced

Chart 1 shows that the size of the increase in serious casualties varies across police forces following the introduction of injury based reporting systems. A comparison line for non-IBRS forces has been included to show how reported serious casualties have changed in these forces. Some forces (such as the Metropolitan Police Service) show a more marked increase in serious casualties than others, while other forces (such as Durham) show a more stable trend in serious casualties over time. Devon and Cornwall and South Yorkshire have also been highlighted in Chart 1 to illustrate the variety of trends observed.

The differences in the impact of the introduction of injury based reporting systems is likely to depend on the practices within a police force that were in place before these new systems were introduced. For example, Durham have stated that having a relatively low number of casualties each year allows them to extensively validate how the severity of each casualty reflects the injury received, and that their previous system for severity recording was very similar to the CRaSH approach. Whereas, larger forces might not be able to carry out extensive severity reviews and there might be more differences in practices between the large numbers of officers recording accidents.

Background on the change

Approximately half of English police forces adopted the CRaSH (Collision Recording and Sharing) system for recording reported road traffic collisions at the end of 2015 or the first part of 2016, although Surrey has been using the system since November 2012. In addition, the Metropolitan Police Service (MPS) switched to a new reporting system called COPA (Case Overview Preparation Application), which went live to police officers from November 2016.

In 2019, 10 police forces adopted CRaSH. This includes all the Scottish police forces, Sussex and Lancashire.

In 2021 Greater Manchester, West Yorkshire and Nottinghamshire joined CRaSH, however only Greater Manchester and West Yorkshire have been included as CRaSH police forces in the 2021 model. For Nottinghamshire only 6% of the 2021 data was recorded using CRaSH.

The remaining forces use a wide variety of systems to report collisions, in which police officers use their own judgement and guidance to determine directly the severity of a casualty (‘slight’ or ‘serious’).

In contrast CRaSH and COPA are injury-based severity reporting systems where the officer records all injuries for the casualty (Table 1 shows the link between injury and severity as used in the CRaSH system). The injuries are then automatically converted to a severity level from ‘slight’ to ‘serious’ and the most serious severity is used to identify the casualty severity.

Eliminating the uncertainty in determining severity that arises from the officer having to make their own judgement means that the new severity level data observed from these systems using injury based methods is expected to be more accurate than the data from other systems.

Background on the introduction of severity adjustment

The Department for Transport (DfT) publishes road casualty statistics based on the Stats19 data collection system. This well-established system sets out the variables and data standards to which each of the police forces in Great Britain submit data relating to vehicle accidents in which an injury has occurred.

Historically each police force has operated its data collection system independently. In recent years, DfT and Home Office developed a standardised reporting tool (called CRASH - Collision Recording and SHaring) which is designed to provide a common way for police forces to collate and submit data. Further information on data collection systems and discussion on the development of severity reporting is given in Ward et al (2010). The CRASH system was first rolled out in 2012, being adopted by the Surrey police force. Apart from brief usage in one other police force, further adoption of CRASH did not begin until the end of 2015. By the end of 2016, however, CRASH was being used by nearly half of all English police forces. Separately, the Metropolitan Police Service (MPS) adopted a similar IBRS to CRASH during the latter part of 2016 – this is called COPA (Case Overview and Preparation Application).

The introduction of IBRS appears to have led to a change in the reported severity of road casualties. This can be explained by the change of reporting systems from Non-Injury-Based Reporting Systems (NIBRS), where judgment of the casualty severity is made by the reporting police officer, to IBRS, where the severity of the injury is determined automatically from the most severe type of injury suffered. It appears that some casualties that would have been categorised as ‘slight’ on NIBRS are recorded as ‘serious’ in IBRS. This became apparent from initial analysis of high level data suggesting that switching to CRASH and COPA added between 5 and 15% to the Great Britain total for ‘serious’ injuries (DfT, 2017).

Your feedback

We welcome your feedback on this approach, specifically how you are using these statistics and whether this meets your needs. Please contact the road safety statistics team

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