SCOTTISH TRANSPORT STATISTICS No 22 - 2003 Edition
Chapter 12 PERSONAL AND CROSS-MODAL TRAVEL
1. Introduction
1.1 This chapter provides information which was collected from individual people in surveys like the National Travel Survey and the Scottish Household Survey. Such surveys provide "person-based" "cross-modal" information, in contrast to most of the earlier chapters, which tend to be based on particular modes of transport.
1.2 The following changes have been made for this edition:
- Tables 12.7, 12.8, 12.9, 12.13, 12.14 & 12.15 have been expanded to provide time-series and, in most cases, other some extra information.
1.3 The first six tables provide some statistics from the National Travel Survey. Because its Scottish sample is very small (see section 4.1), results can only be given for two or three years taken together. This is why the latest NTS figures that are given here relate to 1999/2001: they were produced by combining the samples for those three years - and, even then, may be subject to large percentage sampling errors, due to the small number of cases upon which they are based (see section 3.6). Therefore, the NTS-based statistics should be regarded as broad indications (rather than precise measures) of the relative use of different modes of transport.
2. Main Points
2.1 The average number of trips per person per year was 1,074 in 1999/2001, equivalent to an average of 2.9 trips per person per day. The average increased by 10% between 1985/86 and 1999/2001. Since 1985/86, the number of trips by car has risen by 60%, but there have been falls of 22% and 35% respectively in the numbers of trips for which "walking" or "local bus" is the main mode. ( Table 12.1)
2.2 Cars, vans and lorries accounted for over three-quarters (78%) of the average of around 7,200 miles which was travelled per year per person over the period 1999 to 2001. Almost half the distance (49%: about 3,500 miles) was covered as the driver, and a further 29% (almost 2,100 miles) as a passenger. No other mode of transport accounted for more than 6%: "local bus" had the next highest share, with 5.5% of the total distance travelled (396 miles). Surface Rail accounted for 5.4% (387 miles - the apparent fluctuations will reflect sampling variability, as discussed in Section 3.6), walking for only 2.7% (around 192 miles) and cycling for only 0.4% (30 miles). ( Table 12.2)
2.3 The average distance travelled per person per year has increased by 55% between 1985/86 (under 4,700 miles) and 1999/2001 (over 7,200 miles). Almost all the increase was accounted for by travel in a car as a driver (up from around 1,900 miles to just over 3,500 miles) or as a passenger (up from about 1,300 miles to just over 2,000 miles). ( Table 12.2)
2.4 Over the period since 1985/86, the average length of a car trip has remained around 8-10 miles, compared to around 4-5 miles for local bus trips and around 27-37 miles for train trips. ( Table 12.3)
2.5 In 1999/2001, shopping (22%) was the most frequent purpose of a trip, and three other purposes had large shares of the total: "commuting", "visiting friends at home" and "other personal business" each accounted for 13-18% of trips. ( Table 12.4)
2.6 Of the trip purposes, "commuting" had the largest share of the total distance travelled in 1999/2001 (19%: over 1,400 miles). "Visiting friends at home", "shopping" and "other personal business" each accounted for 14% of the distance travelled (in each case, just over 1,000 miles). "Holiday / day trip" accounted for 11% (about 810 miles) and "business" travel for 10% (725 miles). ( Table 12.5)
2.7 Between 1985/86 and 1999/2001, there were increases in the average length of trips for most purposes. For example, the average length of commuting trips rose from 5.4 miles to 8.6 miles, and the average length of shopping trips increased from 2.9 miles to 4.5 miles. ( Table 12.6)
2.8 The Scottish Household Survey (SHS) provides information about how often people aged 17 or over drive. In 2002, 56% of men, 36% of women and 46% of all people aged 17+ were said to drive "every day". A further 12% were said to drive at least once a week (but not every day), 3% drove less frequently, 4% had a full driving licence but "never" drove, and 36% did not have a full driving licence. Over the four years for which the survey has been running, the percentage said to drive "every day" has risen slightly from 44.4% in 1999 to 45.6% in 2002. ( Table 12.7)
2.9 The frequency of driving varied with age. In 2002, about three-fifths of people aged 30 to 49 were said to drive every day, compared with under a half of those aged 20-29 and just over a half of 50-59 year olds. By the time people reached their 80s, only 9% were still driving every day. The frequency of driving also varied with the annual net income of the household of which the person was a member. Around two thirds of people aged 17+ living in households with an annual net income of 25,000 or more were said to drive every day, compared with only about a fifth of those living in households with an annual net income of up to 10,000. As far as location was concerned, 38% of people aged 17+ in large urban areas drove every day compared to around 57% of those in "accessible" rural areas. ( Table 12.7)
2.10 The SHS asks adults (people aged 16 or over) on how many days, in the last seven days, they made a trip of more than a quarter of a mile by foot in order to go somewhere (e.g. to work, to the shops or to visit friends) - i.e. on how many days did they use walking as a means of transport (as distinct from walking solely for pleasure). In 2002, only 54% of adults said that they had made a journey of more than a quarter of a mile by foot to go somewhere in the last seven days - slightly more than in 1999 (52%). Young adults (aged 16-19) were the most likely to have walked to go somewhere, with 70% reporting this compared with only 50% of those in their 60s. The percentage of adults who walked to go somewhere did not vary greatly with household income. ( Table 12.8)
2.11 The SHS also asks adults how often, in the last seven days, they made a trip of more than a quarter of a mile by foot just for pleasure or to keep fit (jogging and walking a dog were counted under these purposes). In 2002, 41% of adults said that they had done so at least once - slightly more than in 1999, but less than in 2000 or 2001. Men were slightly more likely than women to report that they had walked for pleasure or to keep fit (men: 44%; women: 38%). There was some variation with age: the percentage was highest for those aged 50-59 (45%) and lowest for those aged over 80 (22%). There was also some variation with household income: the percentage was highest (51%) for adults living in households with an annual net income of over 30,000. ( Table 12.8)
2.12 Information about the frequency of cycling is also collected by the SHS. In 2002, only 3% of adults said that, in the previous seven days, they had made a trip of more than a quarter of a mile by bicycle in order to go somewhere. The percentage was slightly higher for men and for younger adults. Only 3% said that, in the previous seven days, they had made a trip of more than a quarter of a mile by bicycle for pleasure or to keep fit. The percentage was slightly higher for men, and for those from the highest household income band. The survey's results for the past four years suggest a fall in the percentage of adults who report that they had cycled. ( Table 12.9)
2.13 Labour Force Survey results show that, between 1992 and 2002, there has been an increase in the percentage for whom a car or a van is the usual means of travel to work (from 64% in 1992 to 70% in 2002) and decreases in the percentages using buses (from 15% to 11%) and walking (from 16% to 13%). People who work at home are excluded from these figures. ( Table 12.10)
2.14 There appears to have been little change in recent years in the average times taken to travel to work by the main modes of transport (in 2002: 23 minutes by car; 34 minutes by bus and 12 minutes by foot). The occasional fluctuations in the average times for 'rail' and 'other' may be due to sampling variability. ( Table 12.11)
2.15 The longer-term trends are shown by statistics from the population censuses, which have collected information about travel to work since 1966. Excluding those that worked at home, the percentage of the working population using cars to travel to work has increased from 21% in 1966 to 68% in 2001 and the percentage using buses has fallen from 43% in 1966 to 12% in 2001. There has also been a significant fall in the proportion of the working population who walk to work, from 24% in 1966 to 12% in 2001. ( Table 12.12)
2.16 SHS data can be used in more detailed analysis of travel to work patterns. The SHS shows that only 9.2% of employed adults worked from home in 2002, and that this percentage has increased in every year since 1999. Over half of self-employed people worked from home. ( Table 12.13)
2.17 Overall, the SHS found that the majority (68%) of adults who were employed and did not work at home used a car or van to travel to work in 2002. This percentage varied with sex (men: 72%, women: 64%), age (40-49 being highest, at 72%), type of employment (only 57% for those who work part-time), social class (ranging from 48% for those in unskilled occupations to 76% for those in managerial and technical occupations) and annual net household income (rising to 80% of those in the "30,000+" band). The other usual means of travel to work were: walking (13%); bus (12%); rail (3%); bicycle (1%); and other modes (2%). Use of such modes of transport also varied. For example: in general, the greater the income of the household, the less likely a person was to walk or use the bus to travel to work; the percentage who walked to work was highest in "remote" small towns (34%) and the percentage who commuted by bus was highest in large urban areas (19%). The survey's results do not suggest any marked trends between 1999 and 2002: some of the apparent year-to-year changes could be due to sampling variability. ( Table 12.14)
2.18 The SHS also collects information about the usual main method of travel to school. In 2002, 56% of children in full-time education at school usually walked to school, 22% usually went by bus, 19% by car or van, 1% cycled and about 2% used other means of transport (such as rail, taxi and ferry). There was little difference between the sexes, but the usual means of travel varied greatly with age: 64% of pupils of "primary school" age (those aged up to 11) usually walked to school compared with only 46% of those of "secondary school" age (those aged 12 and over); 24% of "primary" pupils went by car or van compared with only 12% of "secondary" pupils; and only 10% of "primary" pupils usually travelled by bus compared with 38% of those of "secondary" age. The percentage who usually travelled by car or van tended to rise with household income, to 27% of pupils from households with an annual net income of 30,000 or more. The percentage who walked to school was lowest (28%) for those living in "remote" rural areas. The survey's results for the four years do not show any clear trends: some of the apparent year-to-year changes may be due to sampling variability. ( Table 12.15)
2.19 According to the International Passenger Survey (IPS), Scottish residents made an estimated 3.7 million visits abroad in 2001 with about 3.4 million visits (90%) being made by air. Glasgow was the main airport used and accounted for about 1.7 million visits (46% of all visits abroad), followed by Edinburgh (some 587,000 or 16%) and London Heathrow (378,000 or 10%). Around 329,000 visits abroad (9%) were made by sea, and roughly 43,000 (1%) were made using the Channel Tunnel. ( Table 12.16)
2.20 Over three quarters of Scottish residents' visits abroad were made for holiday purposes. Of these, 1.8 million (49%) were on a package holiday whilst 1.0 million (27%) travelled independently. There were 343,000 visits abroad for business purposes (9%), and around 471,000 (13%) to visit friends or relatives. ( Table 12.16)
2.21 Over 70% (2.6 million) of Scottish residents' visits abroad were made to EU countries and visits to other European areas totalled 394,000 (11%). Visits to Canada and the USA together totalled about 458,000 (12%). ( Table 12.17)
2.22 The estimated number of visits abroad by Scottish residents increased from almost 2.3 million in 1993 to over 3.7 million in 2001, a rise of 65%. There were large percentage increases for each of the main purposes of visit, and for each of the main areas visited. One should not read too much into some of the apparent year-to-year changes, which may be due to sampling variability. ( Table 12.18)
2.23 Some information on travel between different parts of Scotland is available from the Central Scotland Transport Model (CSTM), which covers the area, broadly, from Perth and Dundee to the border. At present, such information relates to 1997, because that is the "base year" for the CSTM. The model is being developed and extended, in order to create a new model, which will be known as the Transport Model for Scotland (TMfS). This will include Aberdeen, and will have 2002 as its "base year". Information from the TMfS will become available around the end of 2003. In addition, some Scottish Household Survey figures for travel between parts of Scotland may be included in the next edition of the "SHS Travel Diary results" bulletin. However, until then, the information for the CSTM's "base year" is the latest that is available.
2.24 It is estimated that, on an average weekday in 1997, over 4 million person-trips were made by car, bus or train within the area covered by the CSTM. (The basis of these estimates is explained in the "Notes and Definitions" and "Sources" sections.) Over half of these trips were within Glasgow and Strathclyde (excluding Ayrshire), and a fifth were within Edinburgh and the Lothians. Only 12% of trips were between different CSTM "sectors", with the largest such flows being roughly 45,000 person trips in each direction between Glasgow/Strathclyde and Central; around 35-40,000 person-trips each way between Edinburgh/Lothians and Glasgow/Strathclyde; and about 35,000 person trips each way between Fife and Edinburgh/Lothians. Of the 4.2 million person trips per day it is estimated that 3.4 million (82%) were by car. ( Table 12.19)
2.25 It is estimated that there were nearly 0.8 million person-trips by bus or train per weekday which started or finished in, or passed through, the CSTM area. Almost three-fifths of these were within Glasgow/Strathclyde, and nearly a third within Edinburgh/Lothians. There were no cases of a flow between different CSTM sectors which involved as many as 10,000 passengers each way per weekday, and the numbers travelling between the CSTM area and elsewhere are also estimated to be fewer than 10,000 each way per weekday. ( Table 12.19)
2.26 There was an average of almost 3 million trips per weekday by cars and goods vehicles. Almost half were within Glasgow/Strathclyde, and a sixth were within Edinburgh/Lothian's: in total, 83% were within one CSTM sector. The largest flows between areas were 30-35,000 vehicles each way per weekday between Glasgow/Strathclyde and Central, and about 30,000 vehicles each way per weekday between Edinburgh/Lothian's and Glasgow/Strathclyde. Dundee was the sector, which had the most traffic with places outwith the CSTM area, averaging 26,000 vehicles each way per weekday, in part due to places such as Monifieth and Carnoustie being outwith the CSTM area. ( Table 12.19)
3. Notes and Definitions
Travel by Scottish residents - statistics from the National Travel Survey (NTS)
3.1 The averages given in the tables are averages per head of population, and they will vary greatly from person to person: for example, there will be many people who do not travel on business at all, and others who travel thousands of miles on business.
3.2 A trip is defined as a one-way course of travel having a single main purpose. Outward and return halves of a return trip are treated as two separate trips. If a single course of travel involves a mid-way change of purpose then it is split into two trips (but trivial subsidiary purposes, such as a stop en route to buy a newspaper, are disregarded).
3.3 Main mode of transport: the mode that was used for the longest stage of the trip, where a trip involves more than one mode of transport (e.g. a bus and then a train). In the text, references to car trips include a few by van and lorry.
3.4 Length of a trip: the distance actually covered by the traveller, as reported by the traveller and not the distance "as the crow flies".
3.5 Other personal business: includes - e.g. - trips to the bank, doctor, hairdresser, library and church.
3.6 Sampling variability: Because the NTS's Scottish sample is small (see section 4.1), its results may be affected by large percentage sampling errors. Thus, some of the apparent changes in some modes' figures in Table 12.2 may be due to sampling variability: for example, the apparent fluctuations in the "surface rail" figures (285 miles in 1985/86, 468 miles in 1989/91, 154 miles in 1992/94, 256 miles in 1995/97, 479 miles in 1998/2000 and 387 miles in 1999/2001) are inconsistent with the changes in the overall figures for rail passenger numbers for the same period. It is likely that the fluctuations in the NTS results reflect the inclusion (by random chance) in the sample of more rail users, or greater rail users, in some years than in other years. Similarly some of the NTS results in other tables may be affected noticeably by sampling variability.
Frequency of driving, walking and cycling; and usual main methods of travel to school and travel to work - statistics from the Scottish Household Survey (SHS)
3.7 These notes do not cover the statistics on travel to work in Tables 12.10 to 12.12, which are from the Labour Force Survey and the Census of Population.
3.8 Annual net household income: this is the net income (i.e. after taxation and other deductions) which is brought into the household by the highest income householder and/or his/her spouse or partner, if there is one. It includes any contributions to the household finances made by other members of the household (eg "dig money"). In the case of households for which any of the main components of income were not known (for example, because of refusal to answer a question), the SHS contractors imputed the missing amounts, using information that was obtained from other households that appeared similar.
3.9 SHS urban / rural classification: the urban / rural classification shown in some tables was developed for use in analysing the results of the SHS. It is based on settlement sizes, and (for the less-populated areas) the estimated time that would be taken to drive to a settlement with a population of over 10,000. Each postcode in Scotland was classed as either "urban" or "non-urban", then clumps of adjacent "urban" postcodes, which together contained more than a certain total number of addresses, were grouped together to form "settlements". Six categories were then defined:
Large urban areas - settlements with populations of 125,000 or more. These are around - but not the same as - Aberdeen, Dundee, Edinburgh and Glasgow. This category may (a) include areas outwith the boundaries of these four cities, in cases where a settlement extends into a neighbouring local authority, and (b) exclude some "non-urban" areas within the boundaries of these four cities.
Other urban areas - other settlements of population 10,000 or more.
"Accessible" small towns - settlements of between 3,000 and 9,999 people, which are within 30 minutes drive of a settlement of 10,000 + people.
"Remote" small towns - settlements of between 3,000 and 9,999 people, which are not within 30 minutes drive of a settlement of 10,000 + people.
"Accessible" rural areas - settlements of less than 3,000 people, which are within 30 minutes drive of a settlement of 10,000 + people.
"Remote" rural areas - settlements of less than 3,000 people, which are not within 30 minutes drive of a settlement of 10,000 + people.
3.10 Full driving licence and frequency of driving: the SHS asks whether the person currently holds "a full driving licence (car or motorcycle)". For those who are said to hold a licence, the SHS asks how often the person drives nowadays. The interviewer records whichever of the categories shown in the table is the most appropriate, in the light of the answer.
3.11 Frequency of walking: the SHS asks on how many of the last seven days the person made a trip of more than quarter of a mile by foot. The interviewer asks about walking for the purpose of going somewhere, such as work, shopping or to visit friends. The interviewer then asks about walking just for the pleasure of walking or to keep fit or to walk the dog.
3.12 Frequency of cycling: the SHS asks on how many of the last seven days the person made a trip of more than quarter of a mile by bicycle. The interviewer asks about cycling for the purpose of going somewhere, such as work, shopping or to visit friends. The interviewer then asks about cycling just for the pleasure or to keep fit.
3.13 Social class: the social class categories used in the SHS were developed for the analysis of the results of the 1991 Census of Population, to group together, as far as possible, people with similar levels of occupational skills. In general, each occupation group is assigned as a whole to one social class, and no account is taken of differences (eg in their education) between people within the same group, except that managers and foremen may be allocated to a higher class. The six occupational social classes are as follows, with examples of the occupations in each which are taken from Regional Trends (no. 35 / 2000 edition, pages 242-243):
- professional occupations - includes (e.g.) doctors, solicitors, chemists, professors and clergymen;
- managerial and technical occupations - includes (e.g.) school teachers, computer programmers, personnel managers, nurses, actors and laboratory technicians;
- skilled non-manual occupations - includes (e.g.) typists, clerical workers, photographers, sales representatives and shop assistants;
- skilled manual occupations - includes (e.g.) cooks, bus drivers, railway guards, plasterers, bricklayers, hairdressers and carpenters;
- partly-skilled occupations - includes (e.g.) bar staff, waitresses, gardeners and caretakers;
- unskilled occupations - includes (e.g.) refuse collectors, messengers, lift attendants, cleaners and labourers.
Scottish residents' visits abroad - statistics from the International Passenger Survey
3.14 The International Passenger Survey is designed to be representative of all people travelling in and out of the UK in terms of: the usage of air, sea and tunnel; UK residents going abroad and foreign residents coming to the UK; different types of traveller (eg holiday, business, etc); and travel to and from different parts of the world. However it is not designed to produce results which are representative for different regions of residence within the UK. While the survey's procedures should not lead to any major bias in the estimates for Scottish residents, the "luck of the draw" inherent in the sampling process may result in their being over-represented in the survey in some years, and under-represented in other years..
3.15 Visits abroad: The figures include all tourists who make trips which last no more than a year, those travelling to Eire have been included in the IPS since 1999.
3.16 Miscellaneous and other purposes: includes visits for study, to attend sporting events, for shopping, health, religious or other purposes, and multi-purpose visits for which no one purpose predominates.
3.17 Area visited: in cases where two or more countries are visited, a person is counted on the basis of the one country in which he or she stayed for the longest time.
Trips made on an average weekday - estimates from the Central Scotland Transport Model (CSTM)
3.18 These are the estimated annual average numbers of trips made per weekday between the areas shown, using the specified modes of transport (for example, they do not include trips made by foot, by bicycle, or by motorcycle). The figures represent the estimated total flows over the whole 24 hours of an average weekday. A return journey, from A to B and back again, on the same day, would be counted as two trips: one from A to B and one from B to A.
3.19 'Person trips' relate to the number of people travelling by the specified modes of transport, and 'vehicle trips' to the numbers of vehicles going between the specified areas. Thus, for example, if a car containing two people goes from A to B, it is counted as two person trips and one vehicle trip.
3.20 The areas identified in the table are "sectors" within the Central Scotland Transport Model (CSTM). These correspond broadly (but not necessarily exactly) to the areas of the similarly-named former Regions and/or current Councils.
3.21 "Elsewhere" defines those areas outwith the CSTM model area, including England, and all other parts of Scotland. CSTM does not hold information regarding the movement of people for trips wholly outwith the model area.
3.22 In some cases, the estimated average number of trips originating in an area differs markedly from the estimated number with a destination in that area - for example, compare the estimates of 954,000 person trips with a destination in Edinburgh & Lothians and 930,000 trips originating in Edinburgh & Lothians. This is because the estimation process (which is described in section 4) is mainly based upon survey data covering the 7 a.m. to 7 p.m. period, and cannot take full account of trips which involve returning later in the evening. Therefore, the CSTM-based estimates indicate broadly the levels of flows within Scotland, but do not provide precise measures.
4. Sources
4.1 Travel (within GB) by Scottish residents (Tables 12.1 to 12.6)
4.1.1 Information about travel (within Great Britain) by Scottish residents is obtained from the National Travel Survey (NTS). This collects "travel diary" details from a sample of households across Great Britain. Travel for all private purposes is included. Trips in the course of work are included if they fulfil the requirement that the main reason for the journey is for the traveller to reach the destination. However, travel in the course of work to convey passengers or to deliver goods is excluded, such as travel in the course of their work by bus drivers, lorry drivers and postmen. Trips off the public highway, such as country walks, are excluded.
4.1.2 The NTS is not designed to provide reliable estimates for Scotland for single years: the sample includes only a few hundred Scottish households each year. Therefore, the samples for a number of years must be combined in order to produce Scottish results, and even they will be subject to sampling variability.
4.2 Frequencies of driving, walking and cycling; and usual main method of travel to school (Tables 12.7 to 12.9 and 12.15)
4.2.1 Information on these and some other transport-related topics is collected by the Scottish Household Survey, which started in February 1999. The SHS collects information on a wide range of topics, to allow exploration of the relationships between different sets of variables. Interviewing takes place throughout the year.
4.2.2 The SHS is a survey of private households. For the purposes of the survey, a household is defined as one person or a group of people living in accommodation as their only or main residence and either sharing at least one meal a day or sharing the living accommodation. A student's term-time address is taken as his/her "main residence", in order that he/she is counted where he/she lives for most of the year. The sample was drawn from the Small User file of the Postcode Address File (PAF) which does not include (eg) many nurses homes, student halls of residence, hostels for the homeless, other communal establishments, mobile homes, and sites for travelling people. Therefore, the SHS cannot produce figures completely representative of people living in Scotland: its target population includes only a proportion of the student population, for example. People living in bed and breakfast accommodation may be included, if it is listed in the Small User file of the PAF and if it is their sole or main residence. Prisons, hospitals and military bases are excluded.
4.2.3 Each year, SHS interviews are conducted with a randomly-chosen sample of (on average) over 15,000 households across Scotland. Within each Council area, the sample is stratified using a geo-demographic indicator in order that it will be representative across that Council's area. A higher sampling fraction is used in the areas of the Councils with the smallest populations, in order that (in each two-year period) there is a minimum of 550 household interviews per Council. The results are then reweighted so that they will be representative of Scotland as a whole.
4.2.4 The SHS interview is conducted in two parts. The first part is with the highest income householder, or his/her spouse/partner (if any), who answers questions about the household and its members. This part of the interview provides the information about the ages and sexes of household members, about the annual net household income, about the type of driving licence (if any) held by each adult member of the household, and about the frequency of driving of those who hold a full driving licence. It also provides the information about the usual main method of travel to school for one randomly-chosen schoolchild member of the household (if there is one). Because the information is collected for at most one schoolchild per household, in the analysis proportionately greater weight is given to cases where there are greater numbers of schoolchildren in the household.
4.2.5 The second part of the SHS aims to obtain results which are representative of Scottish adults by interviewing a randomly-chosen adult (aged 16+) member of the household (who may happen to be the person who answered the questions in the first part of the interview - for example, this is always the case for single pensioner households). The second part of the survey has fewer respondents because some of the randomly-chosen adults cannot be interviewed, perhaps because they choose not to take part in the survey or perhaps because the interviewer cannot contact them. This part of the interview provides the information about the frequency of walking and the frequency of cycling and, in cases where the person is employed, about the place of work and the usual means of travel to work. Because at most one adult is interviewed per household, in the analysis proportionately greater weight is given to cases where there are greater numbers of adults in the household.
4.2.6 Although the SHS's sample is chosen at random, the people who take part in the survey will not necessarily be a representative cross-section of the people of Scotland. For example, purely by chance, the sample could include disproportionate numbers of certain types of people, in which case the survey's results would be affected. In general, the smaller the sample from which an estimate is produced, the greater the likelihood that the estimate could be misleading. As an example, Table 12.14 shows that the percentages of people aged 16-19 who usually travelled to work in various ways were calculated from data for only 121 commuters of that age in the SHS in 2002. It was estimated that 3% of them cycled to work - but if, purely by chance, the sample had contained just two more 16-19 year olds who cycled to work, the estimate would have been 5% rather than 3%. Results produced from a small sample could therefore be greatly affected by sampling variability. The larger the sample, the less likely it is that the results will be affected greatly by sampling variability. The Transport Statistics publications of SHS results and the SHS Annual Reports (see sections 5.3 and 5.4) provide examples of the "95% confidence limits" for estimates of a range of percentages calculated from sub-samples of a range of sizes.
4.2.7 The above information relates only to sampling variability. The survey's results could also be affected by non-contact / non-response bias: the characteristics of the (roughly) one-third of households who should have been in the survey but who could not be contacted, or who refused to take part, could differ markedly from those of the people who were interviewed. If that is the case, the SHS's results will not be representative of the whole population. Without knowing the true values (for the population as a whole) of some quantities, one cannot be sure about the extent of any such biases in the SHS. However, comparison of SHS results with information from other sources suggests that they are broadly representative of the overall Scottish population, and therefore that any non-contact or non-response biases are not large overall. Of course, such biases could be more significant for certain sub-groups of the population. In addition, because it is a survey of private households, the SHS does not cover some sections of the population - for example, it does not collect information about many students in halls of residence. The SHS Technical Report (see section 5.4) provides more information on these matters.
4.3 Travel to work (Tables 12.10 to 12.14)
4.3.1 The information about the usual means of travel to work and the time taken to travel to the usual place of work shown in tables 12.10 and 12.11 is obtained from the Labour Force Survey using questions which have been included in those survey interviews which have been conducted in the Autumn each year since 1992. The tables include the self-employed, those on Government training schemes and unpaid family workers as well as employees, but exclude those working at home, and those whose workplace or mode of transport to work was not known. The LFS is a household survey covering 60,000 households each quarter in GB, and about 6,000 households per quarter in Scotland.
4.3.2 Table 12.12 provides some Census of Population information about travel to work. There have been some changes in the categories used - for example, the 1966 Census had a category described as "none" which was included in the 1971 Census under its "On foot and none" category; the 1971 Census had a category described as "Public Transport" which was separate from the categories for "Train" and "Bus"; and the 1966 and 1971 Census "travel to work" figures did not identify separately those who were working at home, so they are included in the figures for those years. However, the effect of such differences on the statistics will be small compared to the scale of the changes in the shares of the main modes of travel.
4.3.3 Information about travel to work is also collected by the SHS (see section 4.2 above), which is the source for tables 12.13 and 12.14.
4.4 Scottish residents' visits abroad (Tables 12.16 to 12.18)
4.4.1 This information is collected by the International Passenger Survey (IPS), from a sample of passengers returning to the UK by the principal air, sea and tunnel routes (excluding some routes which are to small in volume or which are too expensive to be covered). Travellers passing through passport control during the day are randomly selected for interview (interviewing is suspended at night). A weighting procedure takes account of the non-sampled routes and time periods. Edinburgh and Glasgow are the only Scottish airports at which interviewing takes place, so it is not possible to produce estimates for other Scottish airports.
4.4.2 The figures in the tables are based on interviews with Scottish residents who returned to the UK. This is the Office for National Statistics' standard practice for producing such estimates, as it can then also analyse other information that is collected in the interviews (such as the amount that people say that they spent while on holiday).
4.4.3 The survey covers both adults and children, and is voluntary - for example, the response rate was 81% in 1998, and the results reported in these tables for that year are based upon interviews with about 2,300 Scottish residents.
4.4.5 The IPS data used in the tables are adjusted to take account of the fact that not all people respond to questions regarding area of residence. This means that tables produced by area of residence will not always exactly match other published data regarding trips abroad by UK residents.
4.5 Trips made on an average weekday (Table 12.19)
4.5.1 These figures were provided using the Central Scotland Transport Model (CSTM). This covers the area from Perth and Dundee to the border, in which lives roughly 80% of the population of Scotland.
4.5.2 The area covered by the model is divided into 1181 zones. The model uses planning data for each zone (e.g. population, number of households, car ownership, employment, number of employed residents) to calculate the number of trips that would be expected to be generated. It also uses information collected by traffic counts, roadside interviews and surveys of passengers on public transport. Some of the information was collected specifically for the CSTM, and some was obtained from other 'donor' models (such as the Strathclyde Integrated Transport Model). While some of the data were collected in other years, the programme of surveys to collect data for the CSTM started in 1997, and 1997 is the base year for the model.
4.5.3 The pattern of travel movements is held in a series of trip matrices covering the morning peak period, the evening peak period and the intervening off-peak period. Taken together, these matrices can be combined to provide a matrix reflecting trip movements during the period 7 a.m. to 7 p.m. on a typical weekday. Daily, monthly and annual averages can then be derived by grossing-up these figures. The resulting expected flows around the transport network are then "calibrated" and "validated" using information about the actual numbers of trips that were made on particular routes.
4.5.4 Applying the calibration and validation process to the "expected numbers of generated trips" calculated by the model produces estimates of the numbers of trips which are consistent with the observed traffic counts and the results of surveys and interviews. The estimated numbers of trips for the areas shown in the table were then produced by aggregating the estimated numbers of trips for the relevant zones.
4.5.5 The quality and coverage of the data that are held within the CSTM vary between different areas and different parts of the transport network. This is the result of the historical interest in the movement of people and goods between various points on the transport network, and the resultant availability of data. The model is being developed and extended, in order to create a new model, which will be known as the Transport Model for Scotland (TMfS). This will include Aberdeen, and its "base year" will be 2002. The TMfS will use information from the 2001 Census of Population and the Scottish Household Survey (which started in 1999), as well as the same kinds of data as the CSTM uses (see paragraph 4.5.2: the data collection for the TMfS continued into spring/summer of 2003). As a result, the base information used for the TMfS will be more robust and comprehensive than the information used for the CSTM.
5. Further Information
5.1 The Scottish Executive statistical bulletin "Travel by Scottish residents: some National Travel Survey results for 1998/2000 and earlier years" provides a range of National Travel Survey statistics for Scotland, and some information about the survey. More details of this publication are given under "Scottish Executive Transport Statistics publications". The National Travel Survey is also described in the Department for Transport publications " Focus on Personal Travel" and "National Travel Survey 1999-2001 update", and in the " National Travel Survey Technical Report", which is produced by the Office for National Statistics.
5.2 Enquiries regarding National Travel Survey statistics, and transport statistics from the Labour Force Survey, should be directed to Spencer Broadley of the Department for Transport (tel: 020 7944 3097)
5.3 There are a number of publications on the Scottish Household Survey. The main transport-related results are given in three series of bulletins:
- "Household Transport…: some SHS results";
- "SHS Travel Diary results…"; and
- "Transport across Scotland: some SHS results for parts of Scotland".
More details of these publications appear under "Scottish Executive Transport Statistics Publications".
5.4 There are also a number of SHS publications. The main one is "Scotland's People", which consists of two volumes per year: a detailed Annual Report, which provides many tables of results on a wide range of the topics covered by the survey; and an accompanying Technical Report, which contains information about the survey's procedures (such as the sample design and the method of reweighting) and an edited version of the questionnaire. In addition, between 1999 and 2002, a series of SHS bulletins, provided a selection of SHS results, including some on transport-related topics, together with a brief description of the survey. All the SHS publications are available from the Stationery Office bookshop.
5.5 Enquiries regarding the Scottish Household Survey should be directed to the SHS Project Manager: Katherine Hudson of the Scottish Executive Central Research Unit (tel: 0131 244 8420).
5.6 The annual report on the International Passenger Survey is called " Travel Trends", and is published by the Stationery Office. Enquiries regarding the International Passenger Survey should be directed to Josh Lovegrove of the Office for National Statistics (tel: 020 7533 5765).
5.7 Further information or guidance on the detailed application of the Central Scotland Transport Model and the Transport Model for Scotland can be obtained from Diarmid Lindsay, the Scottish Executive Transport Division 1 (tel: 0131 244 7260) or Kevin Lumsden, MVA (tel: 0131 220 6966).