Guidance

Guide to contributory factors for reported road casualties Great Britain

Updated 28 September 2023

Overview of contributory factors

Contributory factors (CFs for short) provide some insight into why and how road collisions occur. They are designed to give the key actions and failures that led directly to the actual impact to aid investigation of how collisions might be prevented.

When police officers attend the scene of a collision, they are able to select up to 6 factors they believe contributed to the collision. These can be assigned to vehicles, casualties or uninjured pedestrians involved. Please note that this does not assign blame for the collision to any specific road user but gives an indication of which factors the attending officer thought contributed to the collision.

The police officer also indicates whether the factor was ‘very likely’ to have contributed to the collision or only have a ‘possible’ link to the collision. More than one factor can be linked to a single road user, and the same factor can be linked to a series of road users.

Officers do not need to carry out a full investigation of the incident before allocating CFs; they usually use professional judgement about what they can see at the scene. Some CFs, such as exceeding the speed limit, may not be obvious to the officer and are therefore likely to be underreported.

Strengths and weaknesses

Please note that not all collisions are included in the contributory factor data. Only collisions where the police attended the scene and reported at least one contributory factor are included. A total of 67% of collisions reported to the police in 2022 met these criteria.

It is also important to note that collisions can have multiple contributory factors attributed to them. As a result, it is advised that you do not add the results of individual factors in our published tables because collisions could be counted multiple times.

The contributory factors are different in nature from the remainder of the STATS19 data which is based on the reporting of factual information. CFs are largely subjective and depend on the skill and experience of the investigating officer to reconstruct the events which led directly to the collision. This should be kept in mind when interpreting the data.

Guidance to police (STATS20) states that the contributory factors reflect the reporting officer’s opinion at the time of reporting and are not necessarily the result of extensive investigation. Furthermore, it is recognised that subsequent enquiries could lead to the reporting officer changing their opinion, for example because something was not obvious at the time or is revealed following a more detailed investigation (such as the driver using a mobile phone, or a hidden vehicle defect).

Factors should be identified on the basis of evidence rather than guesses about what may have happened. This evidence can come from various sources such as witness statements, vehicle and site inspections. It can be of variable quality, which is the reason for recording the assessment of the reliability of the contributory factors.

Data access and confidentiality

The contributory factor data are considered sensensitive as it may be considered that they imply blame and responsibility for a collision. If the factors can be matched with specific collisions (and specific drivers, vehicles or casualties) they may be sub judice to a prosecution, or may lead to individuals or media reports laying accusations at participants in the collision.

As a result, CF data is not made available at detailed record level or in our data download tool. Data are available via our published spreadhsheet data tables, or a data extract can be requested from the road safety team. This is subject to completion of an end user agreement setting out the use of the data and how the sensitive data will be protected.

The department has previously published research looking at appropriate release practices for contributory factors as carried out by the Methodology Advisory Unit at the Office for National Statistics. This outlined the main risks for identifying contributory factors with individuals and what action would be required to take to minimise this risk. In practice, the main change this brought about was allowing the release of a table containing the total number of contributory factors by local authority. This table was published for the first time in 2013 and is now table RAS0706.

Further information and data collection form

The form used by the police to report contributory factors includes a list of 78 contributory factors. These 78 factors fall into nine categories and these are: Road environment contributed, vehicle defects, injudicious action, driver or rider error or reaction, impairment or distraction, behaviour or inexperience, vision affected by external factors, pedestrian only factors (casualty or uninjured) and special codes.

A copy of the form can be found on our STATS19 forms page, as part of the overall STATS19 form. A description of all the CFs used can be found in the STATS20 guidance which can be found at the same link.

Analysis of CF reporting

CF reporting can vary from force to force depending on operational practices, experience of road traffic collisions, systems used and requirements of the force.

Tables 1-4 give more insight about underlying reporting practices.

Table 1: Percentage of reported collisions included in the contributory factor analysis, by severity

Severity 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022
Fatal 92 93 91 85 87 87 86 84 86 90
Fatal or Serious Collisions (unadjusted) 87 88 86 81 81 81 80 79 80 82
Fatal or Serious Collisions (adjusted) 87 88 86 82 82 81 80 78 80 82
All collisions 79 79 77 73 72 69 67 66 66 67

Table 2: Percentage of reported collisions included in the contributory factor analysis, by road class

Road class 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022
Motorways 88 88 84 80 81 78 76 75 74 77
A Roads 80 81 80 77 75 73 69 68 68 69
B Roads 80 80 79 74 73 71 69 67 68 69
Other roads 75 75 73 69 67 64 63 62 63 62

Table 3: Percentage of vehicles in reported collisions directly allocated a contributory factor, by vehicle type

Vehicle type 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022
Pedal cycles 67 67 65 62 58 55 52 53 52 50
Motorcycles 86 87 86 82 81 80 77 72 71 70
Cars 80 80 78 74 73 70 68 67 68 69
Buses or coaches 66 67 70 68 70 66 64 65 66 69
Light goods vehicles 80 80 78 76 76 73 70 68 68 69
Heavy goods vehicles 85 86 83 79 81 78 77 76 76 79
Other or unknown vehicles 76 78 71 63 62 58 58 62 58 58

Table 4: Percentage of reported collisions included in the contributory factor analysis, by police force

Police force 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022
Avon and Somerset 61 69 63 67 69 59 58 52 54 58
Bedfordshire 73 71 70 71 69 70 73 65 68 59
Cambridgeshire 70 71 73 71 69 68 66 56 66 57
Cheshire 77 79 80 70 75 75 74 75 70 66
City of London 82 76 74 76 71 73 66 70 53 69
Cleveland 78 75 75 75 80 77 76 73 76 76
Cumbria 60 60 58 63 62 58 72 75 67 68
Derbyshire 77 75 74 73 70 65 64 75 68 69
Devon and Cornwall 82 82 81 77 76 80 81 80 80 79
Dorset 75 73 81 86 80 80 84 77 77 76
Durham 77 73 69 43 27 54 19 18 36 51
Essex 78 80 74 36 36 32 65 60 57 62
Gloucestershire 87 84 86 77 71 58 69 73 76 81
Greater Manchester 79 82 77 60 60 54 48 49 35 32
Hampshire 82 81 81 83 81 80 77 77 74 77
Hertfordshire 81 80 75 70 67 75 75 68 69 61
Humberside 75 75 73 60 57 63 62 64 64 62
Kent 81 82 83 64 54 51 67 71 71 72
Lancashire 100 100 100 100 100 100 76 75 72 74
Leicestershire 76 72 69 73 70 70 73 79 87 94
Lincolnshire 84 84 81 80 81 79 80 84 84 87
Merseyside 82 81 80 81 76 70 70 68 66 64
Metropolitan Police 82 82 83 82 77 71 65 61 60 60
Norfolk 85 85 82 61 76 78 71 45 73 70
North Yorkshire 84 82 82 82 81 80 80 81 74 80
Northamptonshire 59 53 46 52 55 64 70 76 78 76
Northumbria 72 72 71 58 46 34 28 25 30 29
Nottinghamshire 75 74 73 74 74 71 72 73 74 57
South Yorkshire 77 80 81 74 70 73 70 74 72 72
Staffordshire 75 77 46 36 36 34 32 41 54 54
Suffolk 83 86 88 60 73 74 72 46 71 68
Surrey 64 57 56 81 85 83 76 74 73 74
Sussex 73 77 77 75 74 73 73 75 73 74
Thames Valley 80 80 78 81 84 74 68 68 67 66
Warwickshire 86 86 84 77 78 80 75 81 81 79
West Mercia 79 78 79 78 78 78 78 76 79 76
West Midlands 76 74 64 51 45 40 40 51 53 49
West Yorkshire 64 75 73 69 69 69 69 68 73 76
Wiltshire 85 87 76 86 87 88 88 84 84 82
Dyfed-Powys 87 87 81 81 81 80 81 80 76 86
Gwent 68 82 84 82 89 86 88 90 91 84
North Wales 76 79 78 82 83 83 84 83 83 83
South Wales 96 97 97 97 97 95 96 98 94 96
Police Scotland 84 82 84 85 85 86 69 61 76 82

Future developments

The recent STATS19 review recommended replacing contributory factors with a new category: Road Safety Factors (RSF). The aim of RSF is to ease data collection. RSF intends to provide greater focus by reducing the list of possible factors from 79 to 36, and reorganising them to align to the Safe Systems approach. This information is being collected to create actions that can be used to improve road safety.

The findings of the review recommended that:

  • at least one factor should be completed for each participant (vehicle or pedestrian) in a collision
  • three to six factors should be recorded overall

It is expected that the new codes will be implemented in 2024.

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