Vehicle Speed Compliance Statistics: Methodology, Notes and Definitions, Background Quality Report
Updated 24 August 2023
© Crown copyright 2023
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This publication is available at https://www.gov.uk/government/publications/free-flow-vehicle-speed-statistics-guidance/vehicle-speed-compliance-statistics-methodology-notes-and-definitions-background-quality-report
Introduction
Vehicle Speed Compliance is an annual publication that looks at how different vehicles comply with speed limits across the different road classes.
All Speed Compliance Statistics are published through the Speed compliance statistics series on the Department for Transport’s (DfT) statistics web page.
These statistics are designated as National Statistics, but the release may also include other statistics to provide additional context, not all of which are National Statistics.
Section 1: Vehicle Speed Tables (SPE01)
These statistics mainly provide insight into the speeds at which drivers choose to travel when free to do so and their compliance with speed limits, but should not be taken as estimates of actual average speed across the road network, which are available separately for both local authority managed ‘A’ roads and the Strategic Road Network, these are available online as part of a suite of new travel time measures from the Road Congestion and Reliability Statistics series. Note that this series also uses the term “free flow speed” but this is calculated in a different way (for example, ‘capped’ at national speed limits) and using a different data source to the statistics presented in this release.
Methodology
Free flow vehicle speeds are based on traffic speed data collected from a sample of Automatic Traffic Counters (ATCs) operated by DfT. ATC sites count traffic continuously as well as recording the speed at which the vehicles travel and the physical properties of passing vehicles, which are used to classify traffic.
Data collection
A subset of ATC sites are selected for use in producing speed compliance statistics, the sample being chosen to exclude ATC locations where external factors might restrict driver behaviour (for example, junctions, hills, sharp bends and speed enforcement cameras), so that estimates of the speeds of individual vehicles in free flow conditions can be produced. Although every effort has been made to remove sites with traffic calming measures, due to their prevalence in 20mph zones it has not been entirely possible for this road class and so some of these sites may include limited traffic calming measures.
Changes to the number of sites used are due to re-evaluations of sites and the method of counting.
| Data year | ATCs used [note 1] | Total Vehicle observed (millions) |
|---|---|---|
| 2011 | 99 | 685 |
| 2012 | 99 | 702 |
| 2013 | 98 | 605 |
| 2014 | 105 | 567 |
| 2015 | 107 | 743 |
| 2016 | 107 | 725 |
| 2017 | 92 | 637 |
| 2018 | 102 | 655 |
| 2019 | 79 | 510 |
| 2020 | 79 | 371 |
| 2021 | 76 | 479 |
| 2022 | 77 | 524 |
Note 1: Counters on separate carriageways (for example, on a motorway) are counted as separate sites.
The sample provides a general picture for Great Britain but cannot be used to determine average free flow speeds for regions, local authorities or specific roads.
The aggregated observations are used to produce estimates by 4 different road types.
| Road Type | ATCs used (in 2022) | Number of locations (in 2022) |
|---|---|---|
| Motorways | 30 | 20 |
| National speed limit carriageways | 19 | 19 |
| 30mph roads | 19 | 19 |
| 20mph roads | 9 | 9 |
Where possible, statistics are published for the following vehicle types, as classified by the ATCs:
- Cars
- Short buses and coaches (under 12 metres)
- Long buses and coaches (over 12 metres)
- Light commercial vehicles
- Rigid 2-axle heavy goods vehicles (HGVs)
- Rigid 3 or more axle heavy goods vehicles
- Articulated 3 or 4 axle heavy goods vehicles
- Articulated 5 or more axle heavy goods vehicles
- Motorcycles
Speed limits differ by vehicle type and road type. The most typical applicable speed limits are used to determine if vehicles are exceeding the speed limit. For the purpose of these statistics, rigid 2-axle HGVs do not have a definitive national speed limit. All other observed HGVs are considered to have more than 7.5 tonnes maximum laden weight and therefore have been assigned the national speed limit that apply to such vehicles. More information can be found on the GOV.UK speed limits webpage.
Calculation of the estimates
The raw speed data for each observed vehicle is checked for periods where the ATCs recorded information is likely to be erroneous and such data are excluded from the calculations. In addition, we identify and remove periods of very low speeds on some faster roads, as these are indicative of congestion. These relate to recorded speeds of under 25mph on motorways and under 11mph on national speed limit single carriageways.
From this validated data, the average speed for each vehicle type is calculated by summing the speeds of each vehicle and dividing this by the number of vehicles observed. Similarly, the percentage of vehicles exceeding the speed limit is calculated from this data by summing the number of vehicles exceeding the speed limit and dividing this by the number of vehicles observed.
To calculate the number of cars exceeding the speed limit at each hour of the day we take the number of cars observed exceeding the speed limit in an hour, and then divide this by the total observed travelling in that hour.
Headway research
Vehicle headway data (SPE0114) is collected from the free flow statistics motorway sites and are only based on traffic in the inside lane. The ATCs record the time between 2 vehicles passing over the sensors, which we categorise into less than 2 seconds, between 2 and 4 seconds, 4 to 6 seconds, and over 6 seconds. The Highway Code (rule 126) states that all drivers “should allow at least a two-second gap between you and the vehicle in front on roads carrying faster-moving traffic”, and “this should be at least doubled” in inclement conditions.
Definitions
| Terminology | Definition |
|---|---|
| Free flow speed | Free flow speeds are observed in locations where external factors which might restrict driver behaviour (for example, junctions, hills, sharp bends and speed enforcement cameras) are not present. |
| Exceeding the speed limit | Vehicles travelling at a speed higher than their applicable speed limit are defined as “exceeding the speed limit”. |
| Headway | The measurement of time between 2 vehicles. |
Types of vehicle: the definitions for vehicle types included in the release are as follows:
| Vehicle type | Definition |
|---|---|
| Cars | Includes passenger vehicles with 9 or fewer seats, 3-wheeled cars, 4-wheel drive ‘sports utility vehicles’, taxis, car-derived vans and dual-purpose vehicles. |
| Buses and coaches | Includes all public service vehicles and works buses which have a gross weight greater than 3.5 tonnes. For the purposes of these statistics, these buses and coaches are further split into those estimated by the ATC to be under or over 12 metres long (short buses or long buses). |
| Light commercial vehicles (LCVs or vans) | Goods vehicles not exceeding 3.5 tonnes gross vehicle weight. Includes transit vans, ambulances, pickups and milk floats. Does not include car-derived vans and dual-purpose vehicles. This category has previously been referred to as Light Goods Vehicles (LGVs). |
| Heavy goods vehicles (HGVs) | Includes all goods vehicles more than 3.5 tonnes gross vehicle weight. When a HGV is travelling with 1 or more axles raised from the road (sleeping axles) then the vehicle is classified by the number of axles on the road, and not by the total number of axles. |
| Rigid 2-axle HGVs | Includes all non-articulated HGVs with 2 axles. Includes tractors (without trailers), road rollers, box vans and similar large vans. A 2-axle motor tractive unit without trailer is also included. |
| Rigid 3 or more axle HGVs | Includes all non-articulated HGVs with 3 or more axles irrespective of the position of the axles. Excludes 2 axle rigid vehicles towing a single axle caravan or trailer. 3 axle motor tractive units without a trailer are also included (Note 2). |
| Articulated 3-axle HGVs | Includes all articulated HGVs with 3 axles. The tractor will have 2 axles and the trailer 1 (Note 2). |
| Articulated 4-axle HGVs | Includes all articulated vehicles with a total of 4 axles regardless of the position of the axles, such as 2 on tractor with 2 on the trailer, or 3 on the tractor with 1 on the trailer (Note 2). |
| Articulated 5 or more axle HGVs | This includes all articulated vehicles with a total of 5 or more axles regardless of the position of the axles (Note 2). |
| Motorcycles | This includes all road-legal 2-wheel motor vehicles including mopeds, motorcycles and scooters. |
Note 2: For the purposes of assigning speed limits in these statistics, these vehicles are all assumed to have a gross vehicle weight over 7.5 tonnes.
Section 2: Contextual Statistics (SPE02)
These previously published statistics provide a summary of speeding offences and reported accidents involving speeding. They are collected from:
- Home Office (HO)
- Ministry of Justice (MoJ)
- National Driver Offender Retraining Scheme (NDORS)
- Road accidents and safety statistics team at the Department for Transport (DfT)
Definitions
| Terminology | Definition |
|---|---|
| Fixed Penalty Notices (FPNs) | Offered to motorists to avoid prosecution for various motoring offences (for example speeding, neglect of traffic directions and using a mobile phone while driving) by paying a prescribed financial penalty. |
| Speed awareness course | NNDORS is a set of schemes unique to the UK, where a motorist who has been caught committing a ‘low level traffic offence’, such as speeding, is given an opportunity to attend a course focusing on re-education designed to achieve a greater compliance with the Road Traffic legislation, rather than receiving a FPN or attending court and face possible conviction. |
| Conviction Ratio | The number of convictions as a proportion of the number of proceedings. These are calculated on a principle offence basis. |
| Collision | Involves personal injury occurring on a public highway (including footways) in which at least 1 road vehicle or vehicle is in a collision with a pedestrian which becomes known to the police within 30 days of its occurrence. Damage-only collisions with no human casualties or collisions on private roads or car parks are not included. |
| Fatal collision | An collision in which at least 1 person is killed. |
| Serious collision | One in which at least 1 person is seriously injured but no person (other than a confirmed suicide) is killed. |
| Slight collision | One in which at least 1 person is slightly injured, but no person is killed or seriously injured. |
| All collisions | Means all fatal, serious, or slight collisions. |
| Contributory factor | Contributory factors 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. |
For further assistance with these tables, use the following resources:
- DfT’s reported accidents statistics
- Home Office’s police powers and procedures in England and Wales statistics
- Ministry of Justice’s quarterly criminal justice system statistics
- Speed awareness courses
- RAC Report on Motoring
The 2022 release also quotes non-governmental data published by the RAC Foundation from an annual quota sample of around 4,235 responses obtained through an online survey.
Section 3: Quality assessment of the statistics
Relevance
Relevance is the degree to which a statistical product meets user needs in terms of content and coverage.
Published information
The statistical outputs presented within the annual ‘Speed Compliance Statistics for Great Britain’ includes:
- a statistical release summarising vehicle speeds in free flow conditions by vehicle type and road class
- ODS data tables summarising the underlying data used in the release
- user guidance including definitions and methodology notes, in the release itself and in this note
The statistical release also includes information from other sources, mostly National Statistics, to round out the picture of driver behaviour in relation to speed, such as speeding offences, road accidents in which speed was identified as a contributory factor, and public attitudes to speeding
Users of the statistics
Within DfT, these statistics and the underlying data on which they are based are used to inform roads policy, for example studying the impact of changes to speed limit legislation, and to inform communications campaigns.
These statistics are also used by specialist researchers and lobby groups, to develop policy positions, the provision of advice and educational fact sheets, and to respond to media queries.
Developments to improve relevance
During 2020 the department carried out additional analyses on this data set, when it seemed likely that speed compliance behaviour may be impacted by the substantial reduction in road traffic occasioned by the coronavirus (COVID-19) pandemic.
As a result to the additional analyses, a temporary quarterly publication was introduced, featuring daily summary statistics, and was maintained through the remainder of 2020 and 2021. Publication reverted to the usual annual basis for 2022, after any discernible coronavirus impact had subsided, but retaining some of the daily summary data in the annual publication.
Accuracy
Accuracy refers to how close the estimated value in the output is to the true result.
The DfT automatic traffic counter network consists of counters at just over 300 locations, which are intended to provide a broadly representative coverage of road traffic in Great Britain. A subset of these is used for producing speed compliance statistics, excluding those at locations where external factors might restrict driver behaviour (for example, junctions, hills, sharp bends and speed enforcement cameras), so that estimates of the speeds of individual vehicles in free flow conditions can be produced. The resulting set of sites is then used to provide data on vehicle speeds, for those combinations of road and vehicle types where the sample is considered adequate to give robust results at the GB level (that is road types for which a reasonable number of counters are available, with a substantial flow of the relevant vehicle types on them).
For road types where the number and variety of sites available is not considered adequate to give robust estimates, no statistics are produced. This means, for example, that regional and subnational statistics are not produced.
No further attempt to weight the results to be representative of traffic is made. It is considered that given the lack of robust information available for constructing a more elaborate methodology, such attempts would carry a high risk of producing spurious results. For example, reliable nationally consistent information is not yet available on speed limits across the whole road network, and no data is available on the extent of the road network on which ‘free flow’ conditions exist.
As noted in the published report, a particular caveat should be put on the results for 20mph roads, as the 20mph roads on which ATCs are installed are likely to be untypical of the majority of 20mph roads.
Timeliness and Punctuality
Timeliness describes the time between the date of publication and the date to which the data refers, and punctuality describes the time between the actual publication and the planned publication of a statistic.
Speed Compliance Statistics is published annually. While the review of sites to be included can begin earlier, the production of the report commences as soon as possible after the production of the final quarterly traffic estimates for the year in question. This is because the speed compliance statistics rely on data quality measures produced as part of the traffic statistics production process. It is also advantageous to include as many updates as possible from the external sources which are used. Production also relies on the availability of staff resources – as the production of the speed compliance statistics is a by-product of the road traffic statistics production process, in cases of limited resource it can be necessary for road traffic projects to take priority over the speed compliance statistics publication. In general therefore, the speed compliance statistics are published not before June in the year after the statistics cover, and may be later. Dates are published on the GOV.UK release calendar and forthcoming publications schedule. To date, all statistics have been published to the scheduled pre-announced date.
Accessibility and Clarity
Accessibility is the ease with which users are able to access the data, also reflecting the format in which the data are available and the availability of supporting information. Clarity refers to the quality and sufficiency of the metadata, illustrations and accompanying advice.
The outputs are published on the GOV.UK DfT statistics page in accessible formats. Statistical releases are now available in HTML format, which are made accessible for those who use assistive technologies. Data tables are available in ODS file format which can be accessed by using freely available software.
The statistical release aims to use plain language, in which technical terms, acronyms and definitions are defined where appropriate. The main findings are presented using a combination of text, charts and infographics. The report includes background information detailing strengths and weaknesses of the data, and also information on definitions and methodology where these are considered important to understanding the statistics. Further information on definitions and methodology is included in the earlier sections of this document.
Comparability and Coherence
Comparability is the degree to which data can be compared over time and domain. Coherence is the degree to which data that are derived from different sources or methods, but refer to the same topic, are similar.
Comparability
The report and data tables in this publication include data from 2011 onwards. Changes resulting from a major review in 2016 were applied retrospectively in the annual publication relating to 2016 to give consistent data back to 2011. There have not been substantial methodological changes since then, and generally data for previous years have not been further revised. Note however, that the suitability of traffic counter sites for use in the statistics is usually reviewed every year, and this may result in newly available ‘free flow’ sites being added, or occasionally existing sites being removed. Also, sites with not valid data for part or all of the year will not be used. This will create differences in the sample from year to year, so comparisons from year to year need to be treated with caution, as discussed in the main report.
Coherence
As well as analysing available data on traffic speeds, this report presents secondary data analyses on related topics from other sources, with the aim of giving a more rounded picture of driver behaviour in relation to speed.
In addition, the department produces also produces statistics on the following topics:
- Road congestion and travel time: statistics and data about road congestion and travel times on the Strategic Road Network and local ‘A’ roads
- Journey time: statistics and data of journey times to key services, covering food stores, education, health care, town centres and employment centres
Output / Quality Trade-offs
Trade-offs between output quality components describes the extent to which different aspects of quality are balanced against each other.
A relatively small network of less than 100 automatic traffic counters is used to produce these statistics. This is a trade-off that balances cost and feasibility against the level of detail which can be provided. The economic case for the existence of the traffic counters is based on their use to produce National Statistics on road traffic – it would not be viable to commission such a network solely for producing speed compliance statistics, or to significantly increase its coverage.
The use of the DfT counter network provides a relatively high degree of confidence around the accuracy and consistency of data used. Many highway authorities may also collect data on vehicle speeds, but to their own varying definitions and standards, so even if such data could be collated we could have little confidence in the comparability of different data sets across the country, and no control over the coverage of such data.
Assessment of User Needs and Perceptions
Assessment of user needs and perception covers the processes for finding out about users and uses, and their views on the statistical products.
DfT regularly engages with users by social media, email, and face to face methods when possible. This includes requesting feedback on the statistics series, with contact details provided in every statistical release.
Performance, Cost and Respondent Burden
Performance, cost and respondent burden describes the effectiveness, efficiency, and economy of the statistical output.
The data source for this publication is the pre-existing DfT automatic traffic counter network, which is maintained for the purposes of producing National Statistics on road traffic. The cost of the speed compliance statistics is therefore just the marginal cost of the additional staff time required to carry out the additional analysis and presentation of existing data.
The data referenced in this statistical release is collected by observation. As such there is no respondent burden.
Confidentiality, Transparency and Security
Confidentiality, transparency and security refers to the procedures and policy used to ensure sound confidentiality, security and transparent practices.
All data are stored, accessed and analysed in a secure SQL environment where access to data is controlled in accordance with departmental policy.
The automatic traffic counters do not collect any personal data, nor do any of the secondary sources used in the publication include any personal data at the point they are acquired by the production team. No statistical disclosure control methods are applied to the outputs, as the variables included in the statistics are not considered to be sensitive.
All of the guidance material published with this release may be found either in the statistical release itself or in the free flow vehicle speed statistics guidance.
Summary and conclusions
This Background Quality Report presents information for users of ‘Speed Compliance Statistics for Great Britain’ covering different aspects of its quality. The department concludes, on the basis of the assessment outline above, that the statistics are of a quality which is considered fit for the purposes for which the statistics are being used. Comments and feedback on this report, or any other aspect of these statistics are welcome, and can be provided by email to the road traffic statistics team.
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