Contributory factors to reported road accidents

Summary

This section describes the scope and limitations of the information on contributory factors collected as part of the road accident reporting system and presents Scottish results from the fourteenth year of collection.

Driver/rider errors or reactions were reported in 56% of all reported accidents with failed to look properly the most common type (involved in 29%).
Travelling too fast for the conditions or excessive speed was reported in 10% of all reported accidents and 21% of fatal accidents.
Pedestrian only factors were reported in 23% of fatal accidents whilst failed to look properly and loss of control were the most frequently reported driver/rider factors (involved in 31% and 20% of fatal accidents respectively).

Introduction

From 2005, all police forces across Great Britain reported contributory factors as part of the stats19 collection. These were developed to provide insight into why and how road accidents occur. Their aim is to help identify the key actions and failures that led directly to the actual impact, to aid investigation of how it might have been prevented. Care should always be taken when interpreting the factors as they:

reflect the reporting officer’s opinion at the time of reporting the accident (or the opinion of a person whose duties include deciding which CFs should be recorded based on the officer’s report).
are based on the information which was available at that time, so may not be the result of subsequent extensive investigation (indeed, subsequent enquiries could result in the reporting officer opinion changing).

A reporting office attending the scene of a road accident may select up to 6 contributory factors (from a list of 77) to assign to that accident. Multiple factors may be listed against any participant or vehicles in the accident, (therefore percentages in the tables provided may not sum to 100).

Because of this, analysis of contributory factor information requires careful consideration; figures will differ depending on the focus of the analysis. Care should be taken when interpreting tables provided here which consider different aspects of the data (i.e. accidents, vehicles/participants, casualties and frequencies).

This section presents analysis from accidents in Scotland reported to the police in 2020, with the following background note describing the collection of the contributory factor system in more detail.

Note that most tables are by individual contributory factor so care needs to be taken when carrying out analysis. Adding together numbers for individual contributory factors will result in some double counting e.g. some accidents will have 'exceeding speed limit' and 'driving too fast for the conditions' recorded as a factor.

Accidents

Factors

On average there were 1.7 contributory factors listed per reported accident with more factors recorded for fatal accidents and fewer for slight accidents. Table M shows the numbers (and percentages) of reported accidents in which each contributory factor was reported.

Failed to look properly was the most frequently reported contributory factor, involved in 29 % of all reported accidents. This was followed loss of control (13%), by failed to judge other person’s path/speed (12%), poor turn/manoeuvre and Slippery road (both 10%) and Careless/reckless or in a hurry (8%), were also in the top six.
Travelling too fast for the conditions or excessive speed was reported in 10% of all reported accidents and 21% of fatal accidents (Note that the individual percentages for each of these factors cannot simply be added together to obtain combined totals.)
For fatal accidents, failed to look properly was the most frequently reported driver/rider factor involved in 31% of accidents. Failed to judge other persons path/speed was reported in 16%, travelling too fast for conditions and careless / reckless /in a hurry (both in 13%) and exceeding the speed limit in 12%. Pedestrian failed to look properly and Pedestrian wearing dark clothing at night were involved in 9% and 8% of fatal accidents respectively.

Table M also shows how the incidence of some CFs varies with the severity of the accident. For example: loss of control is cited in 13% of all accidents for which CFs were recorded but 20% of fatal accidents; following too close is cited in 4% of all accidents but 2% of fatal ones and exceeding speed limit is cited in 4% of all accidents but 12% of fatal ones.

Note that repeats of the same contributory factor within an accident are excluded from the table, however an accident will appear more than once if more than one different contributory factor is reported.

Changes over time

Table N compares the top ten contributory factors listed in 2020 against previous years. These top ten factors remained the same in all five years, though the order and frequency changed over the 14 years of collection.

Vehicle & pedestrians

Table O shows the number and percentage of vehicles assigned each type of contributory factor (for each vehicle involved in an accident reported to the police). Table P shows this for pedestrians only.

Tables O & P show that:

Failed to look properly was the most frequently reported factor both overall (reported in 17% of all vehicles’ factors), and for every vehicle except motorcyclists.
Loss of control (19%) was the most commonly reported factor for motorcyclists.
Failed to judge other person’s path/speed was the second most common factor reported for cars or taxis (7%).
Failed to judge other person’s speed and loss of control were the second most common factors associated with cyclists (associated with 5% of bicycles).
Failed to judge other person’s speed/path was the second most common factor reported for good vehicles (reported in 10%).
Travelling too fast for the conditions was associated with a total of 4% of all vehicles involved in reported accidents.
Pedestrians involved in accidents were most likely to have failed to look properly as an associated contributory factor (recorded in 38% of all pedestrian accidents), followed by failed to judge vehicle speed/path and crossed road masked by stationary/parked vehicle (both 9%), impaired by alcohol (8%) and careless/reckless or in a hurry (7%).

Table O also shows that many contributory factors were rarely recorded for most vehicles, for example:

loss of control was recorded for 19% of motorcycles but only 0% of vehicles in the bus/coach/minibus grouping;
sudden braking was recorded for 6% of buses but for only 1% of all vehicles involved.

On average, fewer contributory factors were recorded for goods vehicles (an average of 0.16 per goods vehicle involved in a reported accident) and bus or coaches (an average of 0.22), compared to an overall average of 0.70 factors per vehicles.

Note that percentages differ from Tables M & N which presents the percentage of accidents with each contributory factor. As more than one vehicle may be involved in an accident, the average number of factors associated with an individual vehicle is generally lower.

Pairing of factors

Table Q shows the most frequent pairs of contributory factors assigned to the same reported road accident participant in 2020.

The most frequently-occurring combination is driver/rider failed to look properly + (driver/rider) failed to judge other person’s path/speed, which was recorded on 110 occasions.
As would be expected, the CFs identified (earlier) as most frequent to appear in several of the most frequently-occurring combinations – for example, (driver/rider) failed to look properly occurs in the first three of the most frequently-occurring combinations.

However, the numbers indicate that even the most frequently-occurring combination of CFs arose in only a small proportion of all accidents.

Casualties

Tables R & S show the number (and percentage) of fatal and seriously injured casualties involved in accidents where each contributory factor was reported. Unsurprisingly the pattern is similar to that seen in Tables M & N showing the number of accidents with each factor reported. Comparison shows that accidents with pedestrian only factors reported had lower numbers of casualties per accident.

Note a casualty will appear in the tables against each (unique) factor associated with the accident (resulting in the casualty) and therefore may appear more than once. As with the accident tables, repeats of the same contributory factor within an accident are excluded.

Fatalities

Table R shows the Contributory Factors associated with the largest numbers of deaths were:

(driver/rider) failed to look properly– 31 deaths (representing 13% of all deaths in accidents for which CFs were recorded);
loss of control – 20 deaths (9%);
(driver/rider) failed to judge other persons path/speed – 16 deaths (7%);
(driver/rider) careless / reckless /in a hurry - 15 deaths (7% of fatalities) ;
Exceeding the speed limit – 14 deaths (6%);
Travelling too fast for the conditions – 13 deaths (6%)
Poor turn or manoeuvre – 11 deaths (5%);

Seriously injured

Table S shows the CFs associated with the largest numbers of serious injured were:

(driver/rider) failed to look properly – 254 (representing 15% of all serious injuries in accidents for which CFs were recorded);
loss of control – 165 serious injuries (9%);
(driver/rider) failed to judge other persons path/speed –109 serious injuries (6%) ;
(driver/rider) careless / reckless / in a hurry – 102 (6%);
Slippery road (due to weather) – 94 (5%)
Travelling too fast for the conditions – 94 (5%)
Poor turn or manoeuvre – 94 (5%)

Overall frequencies of recording

In 2020 at least one contributory factor was recorded in 100% of reported accidents where a police officer attended the scene (2,324). A total of 3,936 factors were recorded, resulting in an average of 1.7 factors per accident.

Around 87% (3,417) of all factors listed related to vehicles (and their drivers/rider) and the road environment. Around 10% (400) related to pedestrians who were casualties. Relatively few related to uninjured pedestrians (35 or 0.9%).

Table T presents a ranking of all 77 factors by the frequency of reporting in 2020. (Note that figures differ from earlier tables as repeats of factors within the same accident are counted). It is apparent that some CFs are not used often – many were used fewer than 100 times.

Note that data relating to all reported CFs were used to produce Tables O to T. In cases where the same CF applies to more than one vehicle in the same accident, it is counted once for each of them. These tables therefore differ from Tables M & N (which exclude repeats of the same CF within an accident).

Possible vs. Very likely

Reporting officers record whether it was thought very likely or just possible that a factor contributed to the occurrence of the accident. Table T also shows how often each CF was described as very likely, and how often as possible.

Overall, just over three quarters of CFs (77%) were described as very likely, but the percentage varied markedly between different CFs. Excluding those used fewer than 100 times, the following were described as very likely on at least 76% of occasions on which they were used:

Pedestrian failed to look properly (88%)
Loss of control (88%)
(driver/rider) failed to look properly (83%)
Poor turn or manoeuvre (82%)
(driver/rider) Careless / reckless /in a hurry (76%)

and the following were described as very likely between 59 and 73 of the occasions on which they were used:

Disobeyed Give Way or Stop sign or markings (94%)
Impaired by alcohol (92%)
Following too close (76%)
Exceeding speed limit (64%)

Conclusion

The collection of contributory factors has been part of the GB wide police reporting system for 14 years. It is clear contributory factor information can provide useful indications of the circumstances that may have led to a reported road accident. These can also be attributed to the different participants within the accident, which can help build a picture of how the accident may have occurred.

However, there are limitations to the system and care should be taken when both analysing and interpreting the results. This should help ensure the data is used in the correct manner and that consistent messages/results are achieved by users.

We welcome comments on the analysis presented here or any questions regarding the contributory factor system.

Transport Statistics
Transport Scotland
Victoria Quay
Edinburgh EH6 6QQ

Telephone: 0131 244 7254

Email: Transtat@transport.gov.scot

Background: The collection of Contributory Factor data

Guidance on recording road accidents is provided in the Department for Transport’s Stats20 document which includes the following points on CFs:

CFs reflect the reporting officer’s opinion at the time of reporting, and may not be the result of extensive investigation;
subsequent enquiries could result in a change in the reporting officer’s opinion;
the CFs are largely subjective, and depend upon the skill and experience of the investigating officer to reconstruct the events which led directly to the accident;
the need to exercise judgement when recording CFs is unavoidable;
CFs should be identified on the basis of evidence from sources such as witness statements and vehicle and site inspections;
the evidence may be of variable quality, so the officer should record very likely or possible for each CF;
when there is conflicting evidence (e.g. conflicting witness statements), the reporting officer should decide on the most credible account of the accident and base the codes on this, taking into account all other available evidence.

Some CFs may be less likely than others to be recorded, since clear evidence of them may not be available, or may be very difficult to obtain, after an accident has occurred (e.g. in the case of the nervous, uncertain or panic factor). Participants and witnesses may provide incomplete or conflicting accounts of what happened. The CF data therefore depend upon the skill and experience of the reporting officer to reconstruct the events which led directly to the accident, and so are more subjective in nature than other Stats 19 data. This should be kept in mind when using these results.

Regardless of the number of vehicles involved in the accident, at most six sets of CF data can be recorded per accident. Each set contains three pieces of information:

a factor which is thought to have contributed to the occurrence of the accident – selected from list of 77, such as:
- exceeding speed limit (CF code 306);
- travelling too fast for the conditions (307);
- failed to look properly (405);
- impaired by alcohol (501);
- impaired by drugs (illicit or medicinal) (502)
the participant in the accident to whom the factor is related:
- whether this is a:
  - Vehicle – in which case the factor may relate to the driver/rider or to the road environment;
  - Casualty – a pedestrian or a passenger in a vehicle; or
  - Uninjured pedestrian.
- if a Vehicle or a Casualty, the relevant Stats 19 reference
whether it was thought very likely or just possible this factor contributed to the occurrence of the accident

Therefore more than one factor may be recorded for the same participant and any given factor may be recorded for two or more different participants, subject to the limit of a maximum of six sets of CF data per accident.

Appendix B of this publication illustrates the CF codes and their descriptions, including a brief set of completion instructions for the reporting officer. More detailed information is available in the DfT’s Stats 20 document (pages 10; 84 -101) and the procedure for allocating them – for example:

the CFs may be recorded in any order (so nothing can be inferred from the order in which they appear);
more than one CF may be related to the same road user; and
the same CF may be related to more than one road user.

Worked example

Clearly, there could be a lot of CF information in the case of an accident which involved several vehicles, if it was thought that several of them contributed to its occurrence. The following is an example of the potential complexity of the CF data. Car 1 is rapidly travelling along a straight road when Car 2 suddenly appears in front of it, having emerged from a pub car park. The driver of Car 1 brakes sharply, to avoid a collision. As Car 2 drives off, Car 1 is hit from behind by a motorcycle, whose rider and passenger are both killed. The following might be recorded as the CF data for this accident:

CF no.	Participant	Contributory Factor	How likely?
1	Car 1	Exceeding speed limit	Possible
2	Car 2	Impaired by alcohol	Possible
3	Car 2	Failed to look properly	Very likely
4	Car 1	Sudden braking	Very likely
5	Motorcycle	Following too close	Very likely
6	Motorcycle	Exceeding speed limit	Possible

This accident has three participants and six CFs, two of which are the same (exceeding speed limit) but apply to different participants (Car 1 and Motorcycle). This example will be referred to from time to time, when describing some of the CF results.

Quality

As the CFs were added to the Stats 19 data specification at the start of 2005, the results for 2005 could have been affected by teething troubles. In June 2006, the Liaison Group on Road Accident Statistics (LGRAS) discussed a paper on aspects of the quality of the data. It also remains the case the recording of CFs varies between Police Forces. In 2009, there were around 2.1 CFs per accident for Scotland; varying between 1.5 and 2.6 between Forces. In addition, while most Police Forces’ CFs are allocated by the reporting officer, in one Force they are allocated by a small team of specialist CRASH investigators. It may be that a higher degree of accuracy exists for fatal and serious accidents than for slight accidents, as the former may be attended by more experienced road policing officers.

On introduction inconsistencies arose between the CF code and the Type of Participant code (around 3-4% in 2005). The most frequent problem was the combination of the CF code for pedestrian failed to look properly with the Type of Participant code for a Vehicle. In such cases, it wasn’t possible to deduce (from the data) which was incorrect. Since then additional quality assurance was introduced leading to an improvement in quality (currently around 1% of cases).

There may be other changes in some of the patterns of the reporting of CFs, as a result of such discussions, the introduction of additional computer cross-checks of the data, Police Forces’ increasing experience of the collection and recording of such information, and the use of the data by the Police, local authorities and central government.

< Previous | Contents | Next >

Contributory factors to reported road accidents