Definitions used in road accident statistics, and some other points to note
1. The definition of severity used in the Road Accident statistics
The classification of the severity of an accident (as fatal, serious or slight) is determined by the severity of the injury to the most severely injured casualty. The police usually record this information soon after the accident occurs. However, if further information becomes available which would alter the classification (for example, if a person dies within 30 days of the accident, as a result of the injuries sustained in the accident) the police change the initial classification of the severity.
For the purposes of the Road Accidents statistical returns:
a fatal injury is one which causes death less than 30 days after the accident;
a fatal accident is an accident in which at least one person is fatally injured;
a serious injury is one which does not cause death less than 30 days after the accident, and which is in one (or more) of the following categories:
(a) an injury for which a person is detained in hospital as an in-patient
or (b) any of the following injuries (whether or not the person is detained in hospital): fractures, concussion, internal injuries, crushings, severe cuts and lacerations, severe general shock requiring treatment
or (c) any injury causing death 30 or more days after the accident;
a serious accident is one in which at least one person is seriously injured, but no-one suffers a fatal injury;
a slight injury is any injury which is neither fatal nor serious - for example, a sprain, bruise or cut which is not judged to be severe, or slight shock requiring roadside attention;
a slight accident is one in which at least one person suffers slight injuries, but no-one is seriously injured, or fatally injured.
Over the years, improvements in vehicle design, and the provision and use of additional safety features, together with changes in the law (eg on the fitting and wearing of seat belts), will all have helped to reduce the severity of the injuries suffered in some accidents. Road safety measures should also have reduced the levels of injuries sustained. For example, if traffic calming schemes reduce average speeds, people may suffer only slight injury in collisions that previously would have taken place at higher speeds and so might previously have resulted in serious injury.
However, it is also possible that some of the changes shown in the statistics of serious injuries and slight injuries may be due to changes in administrative practices, which may have altered the proportion of accidents which is categorised as serious. For example, the distinction between serious and slight injuries could be affected by factors such as changes in hospitals' admission policies. All else being equal, the number of serious injury cases would rise, and the number of slight injury cases would fall, if it became standard procedure for a hospital to keep in overnight, for precautionary reasons, casualties with a particular type of injury. The increase in the number of serious injury accidents in 1994 was partly attributed to a change in the health boards' policies in admitting more child casualties for overnight observation, which in turn changed the classification of many injuries from slight to serious. The number of child casualties recorded as having serious injuries in 1994 was 35% higher than in the previous year.There could also be changes in hospitals' procedures that would reduce the numbers of serious injury cases.In addition, there is anecdotal evidence that changes in procedures for assigning severity codes may affect the categorisation of injuries. For example, different severity codes might be assigned by a police officer who was at the scene of an accident and by a clerk who bases the code on a police officer's written description of the accident.
2. Other definitions
Accident: The statistical returns include only those accidents which result in personal injury, which occur on roads (including footways), in which a vehicle is concerned, and which become known to the police. The vehicle need not be moving and it need not be in collision. The statistics are therefore of injury road accidents only: damage-only accidents are not included in the figures.
Adults: People aged 16 and over.
Built-up roads: accidents which occur on built-up roads are those which occur on roads which have speed limits of up to 40 miles per hour (ignoring temporary speed limits on roads for which the normal speed limit is over 40mph). Therefore, an accident on a motorway in an urban area would not be counted as occurring on a built-up road, because the speed limit on the motorway is 70mph. An accident on a stretch of motorway with a temporary speed limit of 30mph would not be counted as occurring on a built-up road, because the normal speed limit is 70mph.
Buses and coaches: Include works' buses and (in past years) trams and trolley buses. Vehicles are coded according to their construction, irrespective of their use at the time of the accident. Thus, vehicles of bus construction which are privately licensed are included under 'buses and coaches', while Public Service Vehicle licensed minibuses are included under minibuses.
Cars: Include estate cars and three-wheeled cars.
Casualty: A person killed or injured in an accident. One accident may give rise to several casualties.
Children: People under 16 years old.
Darkness: From half an hour after sunset to half an hour before sunrise, ie 'lighting-up time'.
Drivers: Persons in control of vehicles other than pedal cycles and two-wheeled motor vehicles.
Goods vehicles: Vans, lorries, tankers, milk floats, tractor units travelling without their trailer units.
Heavy goods vehicles: From 1994, heavy goods vehicles have been defined as goods vehicles with a maximum permissible gross vehicle weight of more than 3.5 tonnes. Prior to 1994, they were defined as those with an unladen weight of more than 1.5 tons (1.52 tonnes).
Junction: A place at which two or more roads meet, whatever the angle of the axes of the roads (including roundabouts), or within 20 metres of such a place.
Killed: Sustained injuries which caused death less than 30 days after the accident.
Light goods vehicles: From 1994, light goods vehicles have been defined as goods vehicles with a maximum permissible gross vehicle weight of up to 3.5 tonnes. Prior to 1994, they were defined as those with an unladen weight of 1.5 tons (1.52 tonnes) or less.
Major roads: Motorways and A roads.
Minor roads: B roads, C roads and unclassified roads.
Motor cycles: Includes all two wheeled motor vehicles.
Motorists: The drivers or riders of motor vehicles (including, for example, motorcyclists).
Motorways: Include A(M) roads.
Non built-up roads: Roads for which the normal speed limit (ignoring any temporary speed limits) is more than 40mph.
Other vehicles: Include ambulances, fire engines, pedestrian-controlled vehicles with motors, railway trains or engines, refuse vehicles, road rollers, tractors, excavators, mobile cranes, tower wagons, army tanks, etc - and from 1999, motor caravans. Other non-motor vehicles include those drawn by an animal, ridden horses, invalid carriages without motor, street barrows, etc.
Passengers: Occupants of vehicles, other than the person in control, including pillion passengers.
Pedal cycles: Including toy cycles ridden on the carriageway, tandems and tricycles. Pedal cyclists includes any passengers of pedal cycles.
Pedestrians: Includes people riding toy cycles on the footway, people pushing bicycles, people pushing or pulling other vehicles or operating pedestrian-controlled vehicles, those leading or herding animals, occupants of prams or wheelchairs, and people who alight safely from vehicles and are subsequently injured.
Riders: People in control of pedal cycles or two-wheeled motor vehicles.
Road users: Pedestrians and vehicle riders, drivers and passengers.
Trunk roads: Roads for whose upkeep Scottish Government Ministers are responsible.
Users of a vehicle: All occupants, ie driver (or rider) and passengers, including persons injured while boarding or alighting from the vehicle.
Vehicles involved in accidents: Any vehicle directly involved in an accident where at least one injury is sustained by a pedestrian or vehicle driver, rider or passenger. Vehicles which collide after the initial accident which caused injury are not included, unless they aggravate the degree of injury or lead to further casualties.
3. Some other points to note
Driver and casualty postcodes, and estimated distances between homes and the locations of accidents
Postcodes were added to the Stats 19 returns in 1999. It was accepted that their collection would have to be phased in, as they became readily available from police administrative systems. Indeed, the Stats 20 instructions state if the postcode is not immediately available, leave blank. As a result, blank (or the not known code) is used more often than should be the case in future. There are also codes for non-UK residents and for parked and unattended vehicles.
The straight line (or as the crow flies) distance between the location of the accident and the home of a driver, rider or casualty was estimated using the postcode of the person's home. The grid co-ordinates of the centre of the postcode were obtained from the General Register Office for Scotland's postcode directory file. These were taken as an approximation to the grid co-ordinates of the person's home, and used in conjunction with the grid co-ordinates of the location of the accident (as reported by the police) to estimate the distance. A similar approach was used in the small proportion of cases where there was only the start of a postcode (eg the police might record EH10 if they knew that someone lived in Edinburgh 10, but they could not provide the full postcode) or where only the postal district or postcode sector could be matched with the postcode directory. A distance could not be estimated if the postcode were blank, coded not known or non-UK resident, did not contain a valid postal district, or were for a place outwith Scotland.
Vehicle type: coding of motor caravans
The vehicle type code formerly used for 'Minibus/motor caravan' (code 10) was changed in 1999:
Minibus: the code 10 category now covers only minibuses;
Motor caravans are not identified as a separate category - they are now included with 'Other motor vehicles' (code 14)
As a result, the figures for the categories described in the tables as minibus and other are on different bases for (a) 1998 and earlier years and (b) 1999 and later years. The scale of the discontinuity is not known, because motor caravans have not been identified separately in the statistical returns. However, it is likely that this change has contributed to the fall in the minibus figures between 1998 and 1999, and the rise in the other figures.
Other changes to Stats 19 codes
Changes to the code lists for Stats 19 variables may affect the comparability of the data recorded for the detailed codes. However, they seldom affect the categories for which results are reported in Road Accidents Scotland. For example, when the Scottish Executive (SE) converted its data for 2004 and earlier years to be on the basis of the new (2005 onwards) code-lists:
- in some cases SE could determine the new code value from the old codes which had been recorded. This was straightforward in cases where only one new code corresponded to any particular old code (or combination of old codes). For example, with effect from the start of 2005, the old Road Type codes 3 (dual carriageway - 2 lanes) and 4 (dual carriageway - 3 or more lanes) were replaced by a single new code 3 (dual carriageway) - so the new code value had to be 3 whenever the old code was either 3 or 4.
- in other cases, it was impossible to deduce the new code value from data recorded on the old basis. For example, with effect from the start of 2005, the old Type of Vehicle code 04 (motorcycle over 125 cc) was replaced by two new codes (04 - motorcycle over 125 cc and up to 500 cc and 05 - motorcycle over 500 cc). In such a case, SE could not derive the correct 2005 code for every over 125 cc motorcycle involved in an accident in 2004 or earlier years, because it did not know their engine capacities. All that SE could do was to allocate whichever of the new codes was the more likely to be correct. DfT's vehicle licensing statistics show many more motorcycles over 500 cc than over 125 cc and up to 500 cc. Therefore, SE allocated a new code 05 (i.e. over 500 cc) whenever the old code was 04. However, the Road Accidents Scotland tables were unaffected because they grouped all types of motorcycle together (so it did not matter, for the purposes of those tables, which detailed motorcycle code had been allocated). For similar reasons, changes to other variables' code-lists in 1999 or 2005 should not affect the figures published in Road Accidents Scotland
4. Estimates of the total volume of road traffic
Some tables include estimates of traffic volumes, or accident or casualty rates calculated from them. The traffic estimates were provided by the Department for Transport (DfT), which produces estimates of the total volume of road traffic for Scotland and for other parts of Great Britain.
These estimates are based on data from a very small cross-section of the roads in Scotland: traffic counts taken at under 800 sites per year plus data from automatic traffic counters at about two dozen sites in Scotland (which are combined with data from similar sites in England and Wales).
DfT's estimates are based on an urban/rural classification of roads, not on the built-up/non built-up classification of roads used in the traffic estimates that were made up to 2002 (which is still used for the accident and casualty statistics). In general:
- an urban road is a road (other than a Motorway) that lies within the boundaries of an urban area with a population of 10,000 or more in 2001;
- a built-up road is one that has a speed limit of 40 m.p.h. or less
As traffic on a particular road can be classed as rural whilst accidents occurring on it classed as built-up, it would be incorrect to estimate an area's accident rate for built-up roads by dividing its number of accidents on built-up roads by its estimated volume of traffic on urban roads. Therefore, estimates of built-up and non built-up accident rates are provided in Table 5 only for Scotland as a whole - and these estimates may not be precise, due to the nature of the classifications.
The DfT traffic estimates provide only a rough indication of the likely total volume of traffic in each Council area. These are not National Statistics. For example, DfT believes that its estimates of the volume of traffic on minor roads (i.e. B, C and unclassified roads) for Scotland as a whole are of acceptable quality. However, the 320 or so counts now taken per year at minor road sites across Scotland represent an average of 10 per local authority per year - clearly too few to be the basis of reliable estimates for individual local authority areas for each year. DfT therefore estimate the total volume of traffic on minor roads in individual local authority areas in other ways (outlined in Scottish Transport Statistics). The resulting estimates, which are consistent with the overall totals for Scotland as a whole, provide only a broad indication of the likely total volume of traffic on minor roads in each local authority area. As a result:
- it is not possible for DfT to quantify the possible margins of error around them;
- they are not classed as National Statistics;
- more detailed breakdowns of the estimates for individual local authority areas (e.g. separately for B, C and unclassified roads; or for urban roads and rural roads) are not published
In addition, DfT's estimates of traffic on major roads in each local authority area are also not classed as National Statistics. They too are based on limited data: as manual traffic counts are taken on a rotating census basis, there may be several years between successive counts at a particular site. Therefore, DfT notes that there could be large errors in its traffic estimates for the major roads in some of the smaller local authority areas. Similar considerations apply to DfT's estimates of the total volume of traffic on all roads in each area, which are produced by adding together its estimates of traffic on major roads and on minor roads.
In conclusion: DfT provides its estimates of the volume of traffic in each local authority area as the best that it can produce from the limited amount of data available to it - rough indications of the likely volume of traffic in each area, for use with caution, as no better estimates are available.