Scottish Government

Article 3: Article 3. Contributory factors to reported road accidents

Summary

This article 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 fifth year of collection.

• Driver/rider errors or reactions were reported in 92 per cent 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 15% of all reported accidents and 31% of fatal accidents.
• Pedestrian only factors were reported in 41% of fatal accidents whilst travelling too fast for the conditions and failed to look properly were the most frequently reported driver/rider factors (both involved in 19% of fatal accidents).

1. Introduction

1.1 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's opinion changing).

1.2 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).

1.3 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).

1.4 This article presents analysis from accidents in Scotland reported to the police in 2009, with the following background note describing the collection of the contributory factor system in more detail.

2. Accidents

Categories

2.2 Each of the 77 contributory factors fits into one of nine categories. Chart M shows the percentage of accidents reported to the police with associated contributory factors in each these categories.

• It was the most frequently reported category for each type of severity of accident and was reported in 99 per cent of fatal accidents reported to the police).
• Injudicious action (including travelling too fast for conditions, following too close or exceeding speed limit) was the second most frequently reported category, involved in 26 per cent of all reported accidents, increasing to 35 per cent of fatal accidents.
• Pedestrian contributoryfactors (where the factor has been attributed to an injured or uninjured pedestrian involved in the accident), were reported in 25 per cent of reported accidents , rising to 41 per cent of fatal accidents.
• Road environment factorswere reported in 23 per cent of reported accidents.

Chart M: Contributory factor type: Reported accidents by severity, 2009

Factors

2.3 On average there were more than two 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 per cent of all reported accidents. This was followed by loss of control (20%) and failed to judge other person's path/speed (17%). S lippery road (15%) and careless/reckless or in a hurry (12%) were also in the top five.
• Travelling too fast for the conditions or excessive speed was reported in 15% of all reported accidents and 31% of fatal accidents.
• For fatal accidents, travelling too fast for the conditions and failed to look properly were the most frequently reported driver/rider factors (both involved in 19%). Careless, reckless or in a hurry was involved in 17 per cent of accidents.

2.4 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 20% of all accidents for which CFs were recorded but 47% of fatal accidents; slippery road due to weather is cited in 15% of all accidents but 13% of fatal ones; travelling too fast for the conditions is cited in 11% of all accidents but 20% of fatal ones and exceeding speed limit is cited in 4% of all accidents but 12% of fatal ones.

2.5 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

2.6 Table N compares the top 10 contributory factors listed in 2009 against previous years. The ten factors remained the same in all five years, though the order and frequency changed over the 5 years of collection. The 2 most frequently recorded factors, failed to look properly and loss of control, are associated with a larger proportion of accidents in 2009 than when the CF system was introduced in 2005.

2.7 It's not currently possible to identify whether changes are a result of reporting officers developing their understanding of the new system or a genuine change in the kinds of factors contributing to accidents reported to the police.

3. Vehicle & pedestrians

3.1 Tables 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.

3.2 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 bus or coaches and motorcyclists.
• Sudden braking was the most frequently reported factor for bus or coaches (13%) whereas loss of control (12%) was the most commonly reported factor for motorcyclists.
• Slippery road was the second most common factor reported for cars or taxis (10%).
• Cyclist entering the road from pavement was the second most common factor associated with cyclists (associated with 8% of bicycles).
• Failed to judge other person's speed/path was the second most common factor reported for good vehicles (reported in 13%).
• Travelling too fast for the conditions or excessive speed were associated with a total of 10% 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 49% of all pedestrians), followed by careless/reckless or in a hurry (18%), impaired by alcohol (13%) and failed to judge vehicle speed/path (also 13%).

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

• loss of control was recorded for 26% of motorcycles but only 2% of vehicles in the bus/coach/minibus grouping;
• sudden braking was recorded for 13% of buses but for only 4% of all vehicles involved.

3.4 On average, fewer contributory factors were recorded for pedal cycles (an average of 0.68 per cycle involved in a reported accident) and bus or coaches (an average e of 0.76), compared to an overall average of 1.31 factors per vehicles.

3.5 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

3.5 Table Q shows the most frequent pairs of contributory factors assigned to the same reported road accident participant in 2009.

• 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 768 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 and loss of control each occur in three of the ten most frequently-occurring combinations.

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

4 Casualties

4.1 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.

4.2 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

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

• loss of control - 107 deaths (representing 50% of all deaths in accidents for which CFs were recorded);
• travelling too fast for the conditions and exceeding speed limit together - 75 (35% of all fatalities);
• (driver/rider) failed to look properly - 51 (18%);
• (driver/rider) careless / reckless /in a hurry - 37 (17%);
• slippery road (due to weather) - 29 deaths (13%)

Seriously injured

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

• loss of control - 601 serious injuries (representing 26% of all serious injuries in accidents for which CFs were recorded);
• (driver/rider) failed to look properly - 523 serious injuries (23%);
• (driver/rider) careless / reckless / in a hurry - 355 (16%);
• slippery road (due to weather) - 294 (13%)
• pedestrian failed to look properly - 293 (13%);

5 Overall frequencies of recording

5.1 In 2009 at least one contributory factor was recorded in 99.8% of reported accidents (11,528). A total of 25,027 factors were recorded, resulting in an average of 2.12 factors per accident.

5.3 Around 90% (21,884) of all factors listed were related to vehicles (and their drivers/rider) and the road environment). Around 10% (2,826) were related to pedestrians who were casualties. Relatively few were passengers (68 or 0.3%) or uninjured pedestrians (194 or 0.8%).

5.2 Table T presents a ranking of all 77 factors by the frequency of reporting in 2009. (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 - for example, many were used fewer than 100 times.

5.3 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 differs from Tables M & N (which exclude repeats of the same CF within an accident).

Possible vs. Very likely

5.4 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.

5.5 Overall, almost three-quarters of CFs (70%) 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 92% of occasions on which they were used:

• Vehicle door opened or closed negligently (93%);
• Stolen vehicle (93%)
• Vehicle in course of crime (92%)
• Vehicle travelling along pavement (92%);

and the following were described as very likely on fewer than 60% of the occasions on which they were used:

• Pedestrian failed to judge vehicles path or speed (59%)
• Dazzling sun (59%)
• Road layout (e.g. bend, hill, narrow carriageway) (58%)
• Following too close (57%)
• Poor or defective road surface (57%)
• Rain, sleet, snow or fog (55%)
• Inexperience with type of vehicle (54%)

Conclusion

The collection of contributory factors has been part of the GB wide police reporting system for 5 years. It's clear that the 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 that 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.

Telephone: 0131 244 7281
Email: Transtat@transportscotland.gsi.gov.uk

Background: The collection of Contributory Factor data

B1. 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.

B2. 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.

B3. Regardless of the number of vehicles that were 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 that 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.

B4. 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

B5. 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:

This accident has three participants and sixCFs, 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

B6. 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.

B7. 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).

B8. 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.

Table M: Contributory Factors: Reported accidents ¹ by severity, 2009

1 Includes only accidents where a police officer attended the scene and in which a contributory factor was reported
2 Includes only one count of a CF per accident
3 Columns won't sum to 100 per cent as accidents can have more than one CF

Table N: Contributory factors: Reported Accidents: 2005-2009 comparison ¹

1 Includes only accidents where a police officer attended the scene and in which a contributory factor was reported.
2 Includes only the ten most frequently reported contributory factor citied in 2009. Factors not shown may also have been reported.
3 Columns won't sum to 100 per cent as accidents can have more than one CF

Table O: Contributory factors: vehicles, 2009

1. Excludes invalid codes or pedestrian only factors incorrectly assigned to a vehicle.
2. Includes those without any CFs.

Table P: Contributory factors: pedestrians ¹, 2009

1. Includes pedestrians injured and non injured in the accident
2. Excludes pedestrians incorrectly attributed a vehicle factor or special code

Table Q: Most common pairs of contributory factors reported together, 2009

NOTE: the basis upon which the combinations are produced is described in the text.
However, an additional example may be helpful.
Suppose that the "defective brakes" CF has been allocated to participant A,
the "failed to look properly" CF has been allocated to two participants A and B, and
the "failed to judge other person's path/speed" CF has been allocated to participants A, B and C,
The following combinations of CFs would be allocated to the same participant:
A defective brakes + A failed to look …
A defective brakes + A failed to judge …
A failed to look ... + A failed to judge …
B failed to look ... + B failed to judge …

Table R: Contributory factors: Casualties in reported accidents - fatalities, 2009

1 Includes one accident where no CF was recorded.
NB: As described in the text, an accident will be counted once for each combination of CF (excluding "repeats") and death.
For example, an accident with four different CFs and three deaths would be counted twelve times in this table - each death would be counted against the first CF, then against the second CF, and so on. As a result, the percentages would total far more than 100%.
However, "repeats" are excluded: if the same CF applies to two different participants, each death will be counted only once against that CF.

Table S: Contributory factors: Casualties in reported accidents - seriously injured, 2009