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Contributory Factors

Contributory Factors ( CFs ) - 2006
Summary Tables

( a ) Overall summary

( b ) Most often-recorded CFs - all road users(from Table One)

( c ) Most often-recorded CFs - main types of road user(from Table Two)

( d ) Most often-recorded CFs - by the severity of the accident (#)(from Table Four)

(*) most of these numbers do not appear in the other tables
(D/R) indicates "Driver / Rider" (to distinguish this Contributory Factor from a similar one which applies to Pedestrians)
( $ ) including some "pedestrian only" Contributory Factors that were allocated to a vehicle
( # ) excluding any "repeats" of the same Contributory Factor within an accident - e.g. if two of the participants in an accident were thought to be "exceeding speed limit", that Contributory Factor is only counted once for the purpose of producing this table

1. Introduction

1.1 The "Stats 19" road accident statistics system has always collected a good deal of information about the circumstances in which accidents occur, the vehicles that were involved and their manoeuvres, and the resulting casualties. However, such data primarily answer questions about the "where", "when" and "who" of accidents. Insight into the "why" and "how" may be provided by the new Contributory Factor ( CF) data, which were added to the "Stats 19" system in 2005.

1.2 The aim of collecting the CFs is to help to identify the key actions, failures and occurrences that led directly to the actual impact: the data may show why the accident occurred, and may give clues about how it might have been prevented (but this cannot always be the case). The factors recorded in the "Stats 19" returns reflect the reporting officer's opinion at the time of reporting the accident (or, in some cases, the opinion of a person whose duties include deciding which CFs should be recorded in the light of the information contained in the officer's report). The factors 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). 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.

1.3 Section 2 describes briefly the injury road accident CF data, the collection of which was the biggest single change made to the "Stats 19" road accident statistics returns at the start of 2005. Sections 3 to 5 then provide some of the results which have been obtained from the CF data for 2006:

• section 3 - numbers of CFs reported in total and by type of participant;
• section 4 - numbers of accidents (by severity) for which each CF was reported;
• section 5 - numbers of deaths and serious injuries (by road user type) resulting from accidents in which each CF was reported.

1.4 The main points shown by the CF data for 2006 include:

• a "typical" accident does not have a single "cause" - on average, there were more than two CFs per accident (the figure varies between Police Forces - see paragraph 2.5);
• on average, the number of CFs reported was lowest for "slight" accidents and highest for "fatal" accidents;
• overall, for all the types of accident reported in the "Stats 19" returns, the most frequently-used CFs were:
  • "(driver/rider) failed to look properly";
  • "loss of control";
  • "(driver/rider) failed to judge other person's path/speed";
  • "(driver/rider) careless / reckless / in a hurry";
  • "slippery road (due to weather)";
  • "pedestrian failed to look properly";
  • "poor turn or manoeuvre"; and
  • "travelling too fast for the conditions".
• the CFs most often associated with fatal accidents were:
  • "loss of control";
  • "(driver/rider) failed to look properly";
  • "travelling too fast for the conditions";
  • "exceeding speed limit";
  • "(driver/rider) careless / reckless / in a hurry";
  • "poor turn or manoeuvre"; and
  • "(driver/rider) failed to judge other person's path/speed ".

More detailed information about the results appears in subsequent sections of this note.

2. The collection of the Contributory Factor data

2.1 The "Instructions for the Completion of Road Accident Reports" (the so-called "Stats 20" document) make a number of points about the CFs. In summary, these include:,

• the CFs reflect the reporting officer's opinion at the time of reporting, and may not be the result of extensive investigation;
• it is recognised that 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.

Notes on such points appear on page 10 and on pages 84 to 101 of "Stats 20".

2.2 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 a code-list with 77 entries, 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.

As the example below indicates:

• 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 only to the overall limit of a maximum of six sets of CF data per accident.

2.3 The illustrative "Stats 19" form that appears in Annex B includes a page that shows all the CF codes and their descriptions, along with a brief set of instructions for the completion of the CF part of the "Stats 19" returns. More detailed information is available in "Stats 20": pages 84 to 101 contain detailed information about the individual CF codes, and page 10 sets out basis on which they should be recorded (some points from which were mentioned above, in paragraph 2.1) and the procedure for allocating them - for example:

• the CFs may be recorded in any order (so one cannot infer anything from the order in which they appear - e.g. the first one is not necessarily the "most important" one);
• 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.

2.4 Clearly, there could be a lot of CF information in the case of (say) 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. Suppose that Car 1 is going rapidly along a straight road and that 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.

2.5 Because 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, some points from which were mentioned in the corresponding article in the previous edition of this publication. It also remains the case that the extent to which CFs are used seems to vary between Police Forces. In 2006, excluding the few accidents for which there were no CFs, there were, for example 2.1 CFs (on average) per accident for Scotland as a whole - but this varied between 1.5 and 2.6 for different 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, who read the officers' reports and then allocated the CFs. It was suggested that the data might be more accurate for fatal and serious accidents than for slight accidents, because the former would generally be attended by more experienced road policing officers.

2.6 The meeting also discussed the fact that there was inconsistency between the CF code and the Type of Participant code in (at that time) about 3-4% of cases. 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 general, in such cases, one cannot deduce from the data whether it is the CF code that is wrong (i.e. the code for "driver/rider failed to look" should have been used) or the Type of Participant code that is wrong (i.e. the code for a pedestrian Casualty or an Uninjured pedestrian should have been used). The then Scottish Executive therefore introduced additional computer cross-checks of the data submitted by Police Forces, in order that such cases would be queried as and when they arose, which led to an improvement in the quality of this aspect of the CF data (there appears to be inconsistencies in only about 1% of cases in the data for 2006), and should produce further improvements in future years.

2.7 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.

3. Numbers of Contributory Factors reported: Summary Tables (a) to (c), and Tables One to Three

3.1 The basis of the figures

3.1.1 Data relating to all reported CFs were used to produce Summary Tables (a) to (c), and Tables One to Three. In cases where the same CF applies to more than one vehicle in the same accident, it is counted once for each of them. As a result, the example described in paragraph 2.4 would be counted six times in Summary Tables (a) to (c) and in Tables One and Two. For example, in Table Two, it would be counted:

• twice under "exceeding speed limit" - once under "Car" and once under "Motorcycle"
• once under "impaired by alcohol" - under "Car"
• once under "failed to look properly" - under "Car"
• once under "sudden braking" - under "Car"
• once under "following too close" - under "Motorcycle"

The example does not have any CFs associated with a Casualty or with an Uninjured pedestrian - but, if it did, they would be counted in the "Other persons" column in Table Two.

3.1.2 The basis of these tables therefore differs from that of the later tables (those analysing the numbers of accidents and casualties, which were produced from data which exclude "repeats" of the same CF within an accident)

3.1.3 Table Two shows that (as mentioned in paragraph 2.6) some "pedestrian only" CFs have been allocated to vehicles, and some "vehicle" CFs have been allocated to casualties or to uninjured pedestrians. As mentioned there, inconsistent cases represent only a small percentage of the total, and it is hoped that they will be reduced further in future by the continued cross-checking of the codes for the CF and the Type of Participant.

3.1.4 Table Three shows the combinations of CFs that were most often recorded for the same participant. A combination of two CFs (e.g. "X" and "Y") could be reported in two ways - either as "X+Y" or as "Y+X". The convention adopted for the purpose of producing this table is that the CF with the lower "Stats 19" code value should appear as the firstCF in the combination, regardless of the order in which the CFs were reported. The example in paragraph 2.4 would produce three combinations of CFs, each of which could be counted in Table Three:

• "exceeding speed limit" + "sudden braking", as both these CFs were allocated to Car 1;
• "failed to look properly" + "impaired by alcohol", as both these CFs apply to Car 2;
• "exceeding speed limit" + "following too close", as both were allocated to Motorcycle.

Following the convention adopted for the purposes of this table, the combination for Car 2 would be counted as "failed to look properly" + "impaired by alcohol" even though the two CFs appeared in a different order in paragraph 2.4.

3.2 Summary Tables (a) to (c)

3.2.1 Summary Table (a) shows that 13,109 accidents were reported in 2006. CFs were recorded for 13,066 accidents, and 43 accidents did not have any CFs recorded. In total, 27,282 CFs were recorded, so there were more than twice as many CFs as accidents for which CFs were recorded. The average of 2.09 CFs per accident for which CFs were recorded provides a reminder that a "typical" accident is the result of the combination of two or more CFs, so does not have a single "cause".

3.2.2 The remainder of Summary Table (a) shows the types of participant for whom the CFs were recorded. Over four-fifths (23,112) were vehicles (drivers/riders or the road environment), and about a seventh (3,643) were pedestrians who were casualties. Relatively few were passengers (183 or 0.7%) or uninjured pedestrians (138 or 0.5%).

3.2.3 Summary Table (b) lists the ten CFs which were recorded most often, overall. The first six were:

• "(driver/rider) failed to look properly" - 3,303 cases, or 12% of all the CFs used;
• "loss of control" (2,266 or 8%);
• "(driver/rider) failed to judge other person's path/speed" (2,057 or 8%);
• "(driver/rider) careless, reckless or in a hurry" (1,709 or 6%);
• "slippery road (due to weather)" (1,585 or 6%);
• "pedestrian failed to look properly" (1,543 or 6%);
• "poor turn or manoeuvre" (1,456 or 5%)

Together, these seven CFs accounted for 51% of all the CFs that were recorded. None of the other CFs accounted for more than 5% of the total.

3.2.4 Summary Table (c) lists the most often-recorded CFs for each of the main types of road user. The total of the figures for a particular CF (e.g. "failed to judge other person's path/speed") will be less than that given in part (b) for two reasons: first, some types of road user are not shown in part (c); and, second, some of the CFs do not appear among the most often-recorded for each of the types of road user. The figures show some differences between types of road user - for example, "failed to look properly" is the most often-recorded CF overall, and for most of the main types of road user, but "loss of control" is recorded most often for motorcyclists and "sudden braking" for bus drivers.

3.3 Contributory Factors listed in order of their frequency of use - Table One

3.3.1 Table One lists all 77 CFs in descending order of their frequency of use. The most often-used have already been described, as they appeared in Summary Table (b). Table One shows that some CFs are not used often - for example, many were used fewer than 100 times.

3.3.2 This table also shows how often each CF was described as "very likely", and how often as "possible". Overall, almost three-quarters of CFs (72%) were described as "very likely", but the percentage varied markedly between different CFs. Leaving aside those which were used fewer than 100 times, the following were described as "very likely" on at least 90% of occasions on which they were used:

• "junction restart" (92%);
• "stolen vehicle" (91%);
• "crossed road masked by stationary / parked vehicle" (90%)

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

• "road layout (e.g. bend, winding road, hill crest)" (58%)
• "exceeding speed limit" (57%)
• "travelling too fast for the conditions" (55%)
• "distraction in vehicle" (53%)
• "fatigue" (53%)
• "failed to signal / misleading signal" (53%)
• "nervous / uncertain / panic" (51%)
• "inexperience with type of vehicle" (50%)
• "distraction outside vehicle" (49%)
• "rain, sleet, snow or fog" (47%)

3.4 Contributory Factors recorded - by type of participant - Table Two

3.4.1 This table shows all the CFs, grouped in the way in which they appear in the illustrative "Stats 19" form. The penultimate row gives the total number of vehicles of each type that were involved in all the accidents that were reported in the "Stats 19" returns (including accidents for which no CFs were recorded, and vehicles to which no CFs were assigned), and the final row shows the average number of CFs reported per vehicle of each type. The most often-recorded CFs for the main types of road user have already been identified in Summary Table (c).

3.4.2 Table Two shows that many of the CFs were recorded very rarely for most of the types of road user, and some marked differences between the CFs that were assigned to different types of vehicle - such as:

• "loss of control" was recorded for 22% of the motorcycles involved in injury road accidents (235 out of 1,091) but for only 1% of vehicles in the bus/coach/minibus grouping (12 out of 1,067); and
• "sudden braking" was recorded for 15% of buses (165 out of 1,067) but for only 4% of all vehicles involved (837 out of 21,962).

The final row shows that, on average, relatively few CFs were recorded per vehicle involved in injury accidents (including those without CFs) for pedal cycles (0.78) and the bus/coach/minibus grouping (0.65), and that this average was highest for motorcycles (1.29).

3.5 The combinations of Contributory Factors that were recorded most often for the same participant - Table Three

3.5.1 This table lists the combinations of CFs for the same participant that were recorded at least 100 times, produced on the basis described in paragraph 3.1.4. 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 606 occasions. As would be expected, the CFs that were identified earlier as being used most often tend 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. However, the numbers indicate that even the most frequently-occurring combination of CFs arose in only a small percentage of all accidents.

4. Numbers of accidents in which each Contributory Factor was reported: Summary Table (d) and Table Four

4.1 Table Four shows the numbers of accidents to which each CF was thought to have contributed. For this purpose, each accident is counted at most once for each CF - even in cases where a particular CF applies to more than one of the participants in the accident. For example, the accident described in paragraph 2.4 is counted only once against "exceeding speed limit", even though "exceeding speed limit" was recorded for two of the participants (counting the "repeats" of "exceeding speed limit" within that accident would exaggerate the number of accidents to which that CF contributed). Therefore, "repeats" of the same CF within an accident were excluded from the data used to prepare this table.

4.2 On this basis, the example accident would be counted five times:

• only once under "exceeding speed limit" (even though it was recorded twice);
• once under "impaired by alcohol";
• once under "failed to look properly";
• once under "sudden braking";
• once under "following too close".

4.3 As mentioned earlier, a typical accident (for which there is no obvious single "cause") has more than one CF, and (overall) there are an average of over two CFs per accident which has any CFs recorded. As a result, the total of all the percentages in the column for accidents of a particular severity would be much greater than 100%. The final row of Table Four shows that the average number of CFs per accident (excluding "repeats") is highest for fatal accidents (2.41) and lowest for "slight" accidents (2.01).

4.4 It should be noted that one cannot simply add together the figures for any two CFs to get the total number of accidents to which those factors were thought to have contributed, because there could be a number of accidents for which both factors were recorded (and adding together the two figures would "double-count" such accidents). For example, one cannot add together the figures for "exceeding speed limit" and "travelling too fast for the conditions" to get the total number of accidents for which one (or both) of these CFs was reported, because there will be some accidents for which both CFs were reported, and which are therefore counted in Table Four against both CFs.

4.5 Summary Table (d) lists the most often-used CFs for each category of accident severity. For fatal accidents, these were:

• "loss of control" - believed to be a factor in 114 fatal accidents (39% of all the fatal accidents for which CFs were recorded);
• "(driver/rider) failed to look properly - 61 (21%);
• "travelling too fast for the conditions" - 51 (17%);
• "exceeding speed limit" - 46 (16%);
• "(driver/rider) careless / reckless / in a hurry" - 45 (15%);
• "poor turn or manoeuvre" - 35 (12%);
• "(driver/rider) failed to judge other person's path/speed" - 32 (11%); and
• "pedestrian failed to look properly" - 31 (11%).

The total of all the percentages would be well over 100% because, as indicated above, the table counts every CF (apart from "repeats"), and there was an average of 2.41 such CFs per fatal accident.

4.6 The other parts of the table provide the corresponding figures for the other categories of severity of accident. The CFs used most often for all accidents were:

• "(driver/rider) failed to look properly - 3,248 accidents (25% of all the accidents for which CFs were recorded);
• "loss of control" - 2,260 accidents (17%);
• "(driver/rider) failed to judge other person's path/speed" - 1,980 (15%);
• "(driver/rider) careless / reckless / in a hurry" - 1,696 (13%);
• "pedestrian failed to look properly" - 1,527 (12%);
• "slippery road (due to weather)" - 1,523 (12%);
• "poor turn or manoeuvre" - 1,432 (11%).
• "travelling too fast for the conditions" - 1,338 (10%);

These figures are slightly lower than those which appear in Summary Table (b) and Table One, due to the exclusion of any "repeats" of the same CF within an accident.

4.7 Table Four shows how the incidence of some CFs varies with the severity of the accident. For example: "loss of control" is cited in 17% of all accidents for which CFs were recorded but 39% of fatal accidents; "slippery road due to weather" is cited in 12% of all accidents but 5% of fatal ones; "travelling too fast for the conditions" is cited in 10% of all accidents but 17% of fatal ones and "exceeding speed limit" is cited in 4% of all accidents but 16% of fatal ones.

5. Numbers of deaths and serious injuries resulting from accidents in which each Contributory Factor was reported: Tables Five and Six

5.1 Table Five shows the numbers of deaths resulting from accidents in which each CF was reported. For this purpose:

• as with Table Four, each accident is counted at most once for each CF - even in cases where a particular CF applies to more than one of the participants in that accident. For example, the accident described in paragraph 2.4 would be counted only once against "exceeding speed limit" (even though "exceeding speed limit" was recorded for two of the participants) because counting the "repeats" of "exceeding speed limit" within that accident would exaggerate the number of deaths to which it contributed. Therefore, "repeats" of the same CF within an accident were excluded from the data used to prepare this table.
• every death should be counted against each of the CFs reported for the accident (apart from any "repeats" of CFs), because each of those CFs was thought to have contributed to the occurrence of the accident which caused those deaths.

5.2 The accident which was described in paragraph 2.4 involved two deaths (the motorcycle rider and passenger). For the purpose of producing Table Five, counting all the deaths against each of the CFs (apart from the "repeats") means that:

• two deaths are counted against "exceeding speed limit";
• two deaths are counted against "impaired by alcohol";
• two deaths are counted against "failed to look properly";
• two deaths are counted against "sudden braking";
• two deaths are counted against "following too close";

- so an accident in which two people died contributes ten to the grand total. The final row of Table Five shows that the result is that the total of the numbers reported in the table is considerably greater than the actual number of deaths.

5.3 It should be noted that one cannot simply add together the figures for any two CFs to get the total number of deaths to which one (or both) of those CFs were thought to have contributed. This is because there may be a number of accidents for which bothCFs were recorded, and adding together the two figures would "double-count" the deaths resulting from such accidents.

5.4 Table Five shows that the CFs which were associated with the largest numbers of deaths were:

• "loss of control" - 122 deaths (representing 39% of all deaths in accidents for which CFs were recorded);
• "(driver/rider) failed to look properly" - 67 (21%);
• "travelling too fast for the conditions" - 57 (18%).
• "exceeding speed limit" - 53 deaths (17%);
• "(driver/rider) careless/reckless/in a hurry" - 47 deaths (15%).

The total of all the percentages would be well over 100% because, as indicated above, the table counts every death against everyCF (apart from "repeats").

5.5 Table Six was produced in a similar manner, and shows the numbers of serious injuries which were associated with each of the CFs. The most often-cited CFs were:

• "loss of control" - 638 serious injuries (representing 24% of all serious injuries in accidents for which CFs were recorded);
• "(driver/rider) failed to look properly" - 516 serious injuries (20%);
• "(driver/rider) careless / reckless / in a hurry" - 394 (15%);
• "pedestrian failed to look properly" - 392 (15%);
• "travelling too fast for the conditions" - 306 (12%).

Again, the total of all the percentages would be well over 100%.

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