Chapter 1 Introduction and Background

The National Travel Survey (NTS) provides up-to-date and regular information about personal travel within Great Britain and monitors trends in travel behaviour. The Ministry of Transport commissioned the first NTS in 1965 to 1966, and it was repeated on an ad-hoc basis in 1972 to 1973, 1975 to 1976, 1978 to 1979 and 1985 to 1986. In July 1988 the NTS became a continuous survey (that is, fieldwork was conducted on a monthly basis starting in January and ending in December) with an annual set sample size of 5,040 addresses. This increased to 5,796 by 2001 and to 15,048 addresses in 2002.

Since January 2002, the Department for Transport (DfT) has commissioned the National Centre for Social Research (NatCen), an independent social research institute, as the contractor for the NTS. NatCen is responsible for questionnaire development, sample selection, data collection and editing, data file production and building the database. The DfT is responsible for data analysis, publication, and archiving.

During 2011, DfT undertook a consultation exercise to review the methodology and content of the NTS for 2013 onwards. The review explored sample coverage, data collection methodology, interview question content and items recorded as part of the travel diary. As a result of the review, in 2013 a number of items were removed from the interview questionnaire and the travel diary. In addition, the sample was restricted to cover England only with Scotland and Wales no longer included. The sample size for England remained at its previous level, meaning that the overall sample size for the survey was reduced.

In 2023 the NTS sample was increased significantly from the annual sample size of 12,852 addresses issued in previous years to 21,758 addresses. This was done to facilitate more granular analysis of the data (see section 1.2 below for further details).

From its inception, the NTS has operated using a face-to-face (F2F) design. Under the F2F approach, initial contact with the selected addresses was made by an advance letter, sent through the post. This was then followed up with a visit to the address by a trained interviewer who attempted to encourage participation and for the household to complete a F2F in-home survey and 7-day travel diary, which was then collected by the interviewer from the respondent’s home.

Following the onset of the COVID-19 pandemic in March 2020, F2F fieldwork was paused. The survey switched initially to a phone-based interview, then later resumed doorstep recruitment but with the interview remaining phone-based. Please refer to the 2020 and 2021 Technical Reports for more information. F2F was subsequently reintroduced from April 2022. For 2023 the NTS design used F2F from the start of fieldwork.

This chapter introduces the reinstated traditional F2F methodology for sample design, fieldwork procedures, data preparation and data provision for the 2023 NTS.

1.1 Uses of the NTS data

The NTS is one of DfT’s main sources of data on personal travel patterns in Great Britain. The survey collects detailed information on the key characteristics of each participating household and any vehicles to which they have access. In addition, everyone within the household is interviewed and then asked to complete a 7-day travel diary. The survey therefore produces a rich dataset for analysis with information recorded at several different levels (household, individual, vehicle, long-distance journey, day, trip, and stage).

Data from the NTS is used extensively by DfT to monitor changes in travel patterns and to inform the development of policy. The findings and data are also used by a variety of other organisations, including:

• other government departments (such as HM Revenue and Customs, HM Treasury, the Department for Environment, Food and Rural Affairs)
• university academics and students
• transport consultants, local authorities and voluntary sector organisations representing a wide range of interests including motorists, cyclists, older people, rural communities and children

Table 1.1 below gives examples of the uses of NTS data with links to the relevant webpages.

Key results from the 2023 NTS are published by DfT in the statistical release National Travel Survey: 2023.

DfT deposit a non-disclosive version of the NTS dataset at the UK Data Service, which is available at the UK Data Service.

Table 1.1: Examples of the uses of NTS data

Uses
To help forecast future trends in road traffic as part of the National Transport Model in Road use statistics Great Britain 2016
To monitor the number of cycle stages per person per year for an indicator in the Department’s Single Departmental Plan
As an input into the Cycling and Walking Investment Strategy
To answer Parliamentary Questions and other Ministerial Correspondence, for example Roads: Accidents Bus Services: Commuters
As a possible supplement to rail demand forecasting models in DfT
In the development of the National Cycling Propensity Tool for DfT
For monitoring road accident rates amongst different road users, especially pedestrians in the Road accidents and safety statistics
To understand how people travel to the shops and the impact of home deliveries in the Why people travel: Shopping factsheet
To examine travel among different groups, such as older people and people with mobility difficulties in the Disability and travel: 2007 to 2014 factsheet
To get information about users of modes of transport, for example in the Motorcycle use in England factsheet
To produce free annual reports that allow analysis of changes in personal travel over time
To study how children travel to school and how this has changed over time, in the Travel to school factsheet
To provide analysis and advice for around 300 direct requests to the DfT statistics team per year

1.2 Overview of sample selection

The 2023 NTS was based on a stratified, clustered random sample of private households in England, drawn from the Postcode Address File (PAF), as was the case in 2022 (see Chapter 2 for further details).

The initial aim for 2023 was broadly to double the issued sample size to 25,608 addresses. This was to facilitate more granular analysis of the data both geographically and demographically, such as allowing more analysis by specific protected characteristics.

Due to continuing fieldwork delivery challenges following the COVID-19 pandemic it was not possible to issue the full sample, so this sample increase was introduced in 2 stages. Firstly, from January to June the issued sample size was 8,954 addresses (a reduction of 30% from the sample that was initially drawn). From July to December 2023 the issued sample size increased further to 12,804 addresses (100% of the initially drawn sample), giving a final annual issued sample size of 21,758 addresses for NTS 2023.

From the 2024 survey the issued sample size will increase to the full 25,608 addresses.

1.3 Overview of fieldwork sequence

Historically, the NTS has used 2 data collection methods:

• F2F interviewing using computer assisted personal interviewing (CAPI) on interviewer laptops

• self-completion of a 7-day travel record (issued as a paper diary)

However, due to fieldwork restrictions in response to the COVID-19 pandemic, during 2020 to 2022 the traditional F2F interview and diary self-complete methodology was replaced with alternative phone-based approaches with interviewers completing the diaries on the respondent’s behalf (please see the relevant Technical Reports for details). F2F was reintroduced for quarter 2 onwards of 2022 and was used throughout 2023.

The overall sequence of tasks, from the initial sample selection to data analysis and reporting was as follows:

1. Sample selection from the Postcode Address File (PAF).

2. Questionnaire development and fieldwork preparation.

3. Fieldwork.

4. Data transmission and documents returned by interviewers.

5. In-house data input and editing.

6. Quality checking.

7. Data checking using SPSS.

8. Data file production, variable derivation and imputation and delivery to NTS.

9. Data analysis and reporting by NTS.

In 2023, the phone back-up option (PB) was used alongside F2F. PB was only used when F2F was not possible, primarily for households where participants were unable to participate due to COVID-19 or where COVID-19 remained a concern. Of the 7,593 productive households, 624 were PB cases (figures are unweighted and include cases with missing weighting information).

This PB fieldwork approach broadly followed the same fieldwork sequence as F2F with the exception of diary completion and carrying out the placement and pick-up interviews over the phone. For PB cases the diaries were populated by interviewers rather than the individuals in the household, and they were required to make 2 mid-week phone call checks rather than one.

Fieldwork sequence for F2F:

1. Sampled addresses issued to interviewers.

2. Advance letters posted (by interviewers).

3. Make contact at the address and organise interview.

4. Placement interview conducted in person, and travel diary placed with household.

5. Travel week starts, as per ‘fixed travel week’ allocation method.

6. One mid-week check (in person or by telephone).

7. Travel week ends.

8. Pick-up interview in person.

9. Return work and transmit data.

Fieldwork sequence for PB:

1. Sampled addresses issued to interviewers.

2. Advance letters posted (by interviewers).

3. Make contact at the address with the intention of arranging a F2F interview, but if concerns around COVID-19 were present then a telephone interview would be arranged instead.

4. Placement interview conducted via telephone, and travel diary task introduced to the household.

5. Travel week starts, as per ‘fixed travel week’ allocation method.

6. 2 mid-week checks via telephone, with interviewer populating a couple of days of the travel diary at each check.

7. Travel week ends.

8. Pick-up interview via telephone including populating remaining days of the travel diary.

9. Return work and transmit data.

1.4 Overview of fieldwork procedures

Interviewer points (also known as assignments) were distributed evenly across a quarter, whilst the individual start dates for each point were spread out across each month. The aim of this approach was to obtain a balanced and representative response to the survey across the whole year (see sections 2.7 and 2.8 for further information).

The fieldwork sequence is outlined above in section 1.3 and began with the interviewer sending advance letters to their sampled addresses. These letters briefly explained the purpose of the NTS and mentioned that an interviewer would contact them. It also stated that each respondent would receive a shopping voucher as a thank you, if all household members completed the survey. From June 2004, a book of first-class stamps has also been included with the advance letter as an unconditional incentive and remained in place for the 2023 survey. In 2023 the book of stamps contained 4 stamps, compared with 6 stamps in previous years (this was due to a change in format of stamps, introduced by the Royal Mail, which meant that books of 6 stamps were no longer available).

Interviewers followed up the advance letter by making in-person contact with the household to arrange a placement interview. The start date of their point determined when they could first go into field to make contact. The placement interview gathered information about the household, its individual members, household vehicles and long-distance journeys that the household members had made in the last week. At the end of the placement interview, the interviewer explained and placed the 7-day travel diaries, allocating a travel week using the ‘fixed travel week’ method.

The ‘rolling travel week’ approach (whereby the travel week always started on the day before the placement interview) was introduced in 2020 to replace fixed travel weeks as it was not possible to control for the timing of the placement interview as part of the push-to-telephone methodology. It has been retained in 2023 as a fallback option, only used in exceptional cases where the interviewer has been allocated their point late into the fieldwork period, and permission must be sought to use them.

If respondents agreed to it, they were sent an automated text reminder on the morning of the first day of the travel week to remind them to start completing their diary. If there was a gap of more than a few days between the placement interview and the start of the travel week, interviewers were expected to make a reminder call or visit or delivered a reminder card to the household to remind them that their travel week was about to start. This was followed by a mid-week check call (either by telephone or F2F) during the travel week to check on the household’s progress in completing their diaries. For F2F cases the respondent completed the diary themselves, whereas in PB cases the interviewer populated the diaries on behalf of respondents over the phone.

After the travel week (and ideally within 6 days) a pick-up interview was conducted. The diaries were collected and checked in the case of F2F interviews, and the remaining days of the diary populated for PB cases. The pick-up interview was also used to complete any outstanding sections of the placement interview and to check whether any key factors had changed since the placement, such as the purchase of a new car.

1.5 Overview of data input and editing

The CAPI data was transmitted back to the NatCen operations department, usually on the day after the pick-up call, and all paper documents were returned by interviewers by post. Once the documents had been received, a team of NTS coders booked the diaries into the system. They then coded, keyed and edited the travel diary information using the Diary Entry System. The contents of the CAPI questionnaire were edited and checked, and all interviewer notes examined. The interviewers were contacted if there were any queries that could not be resolved by the coders. If necessary, the interviewer re-contacted respondents to resolve any issues, although this would be rare given the burden on the respondents.

Quality checks were also made on selected interviewers on a rota basis and 10% of addresses were back-checked (please see section 3.17 for more information).

1.5.1 Overview of data file structure

The data were organised into different levels, namely:

• households

• individuals

• vehicles

• long-distance journeys (made in the 7 days before the placement interview or before the travel week which ever date was earliest)

• journeys made during the travel week and days within the travel week

• stages of these journeys

Lastly, NatCen provided DfT with Primary Sampling Unit (PSU) level variables associated with each household, but which were not collected directly from households.

1.6 Impact of the return to traditional F2F fieldwork

To monitor and measure the quality of the data collected, NatCen produce regular, standardised metrics. For 3 years during 2020 to 2022, the change in methodology and impact of the pandemic meant care was needed when comparing data to previous years. NTS 2023 is the first year since 2019 which has operated full F2F fieldwork, and a return to pre-pandemic standards is apparent for most fieldwork quality indicators.

The proportion of in-person interviews was 59% in 2023, level with the 2019 benchmark. This is a key improvement on the 2020 to 2022 NTS survey years during which the rules around proxy respondents were relaxed out of necessity. The spread of travel week start dates across days of the week and days of the month is much more even in 2023 than in the 2020 to 2022 period, reflecting the profile seen in 2019. This demonstrates the benefits of returning to fixed travel weeks (as opposed to using rolling travel weeks during 2020, 2021 and some of 2022).

The rate of interviewers making reminder calls or leaving a reminder card was 59% in 2023 compared to 66% in 2019. Additionally, 73% of interviewers made mid-week checks in 2023 compared to 77% in 2019.

The trip rate for all cases was 13.67 in 2023 compared to 14.36 in 2019, close to pre-pandemic levels. The no travel rate was 4.4% in 2023, very similar to the 2019 rate of 4.3%.

The last quarter of 2023 had a much higher proportion on average of interviewers who had worked on NTS in the last year (84%) compared to the first quarter of the year (70%), showing growing experience across the fieldwork team. For some indicators which are very similar to pre-pandemic levels this is likely supported by this greater interviewer experience.

1.6.1 Overview of survey response

Only households classed as ‘fully co-operating’ are included in the response calculations. Based on this, a national response rate of 32% was achieved in 2023. See section 3.14 for a definition of ‘fully co-operating’ and for full response details.

Overall, this is equivalent to an achieved sample rate (ASR) of 29%. The ASR includes those households classified as ineligible in the denominator.

As seen across all large-scale random probability F2F surveys, response rates in 2023 remained significantly below pre-pandemic levels. This is driven by 2 main factors. Firstly, refusal rates are higher now than they were pre-pandemic. Before 2020 around a third of selected households declined to take part in the survey. Since F2F fieldwork returned in April 2020 this has increased to around half. Secondly, on-going challenges that survey organisations face in re-building their fieldworker capacity since the pandemic mean that it is not always possible to issue all selected sample points to interviewers. In 2023, 7% of selected sample points were unworked.