4. Comparisons with NTS GB Results and SHS Data

4.1. This section compares some key figures from the NTS and travel diary for Scotland with GB and with the Scotland results from the SHS and travel diary.

4.2. Some data is collected of all respondents in both the NTS and SHS which provides estimates for the whole population. The question on driving licence possession is compared below. Other data is collected regarding journeys made. Trips, distance and time are compared below.

4.3. Some key methodology differences between the NTS and SHS are described in the box below.

Travel Diary:

NTS: Asks respondents to keep a travel diary over the next 7 days. Diary is completed by respondent.
SHS: In a face to face interview, respondents are asked about their travel on the previous day.

Short walks:

NTS: Has always included short walks, though a change in form design led to some under recording of short journeys in 2007.
SHS: Includes short walks from 2007 of less than 5 minutes or under a quarter of a mile. Previously these were excluded. Short walks between other modes of transport are not included.

Journey Duration

NTS: Self reported estimate
SHS: Self reported estimate

Journey Distance:

NTS: Respondent estimates
SHS: Straight line distance calculated as part of data processing

Return journeys:

SHS: Questions changed in 2007 impacting on time series analysis.

4.4. As both the NTS and SHS are survey data sources, any results calculated from the datasets will have confidence intervals around them, ie a margin of error. Tables of standard errors for selected key statistics derived from the 2009 National Travel Survey are published at: http://assets.dft.gov.uk/statistics/series/national-travel-survey/stderror2009.xls

4.5. Tables of confidence intervals for the Scottish Household Survey are included in Table 30 of Scottish Household Survey: Travel Diary 2009/2010 (http://www.transportscotland.gov.uk/strategy-and-research/publications-and-consultations/j205156-00) with more information in the appendix on page 44.

Comparison with GB figures and the SHS

Driving Licence possession.

4.6. Table 2 shows driving licence possession as reported in the SHS and the NTS. Both the SHS and NTS show similar proportions of the population holding a driving licence (around 68%) and similar proportions of males and females holding one (around 60% for women and 75-80% for men).

4.7. Chart 1 shows the proportion of people in Scotland reporting having a driving licence by age for the two surveys for the most recent year available, 2010 for the SHS and 2009/2010 for the NTS. The chart shows a very similar pattern for both surveys. The SHS line is slightly lower for younger people, however the NTS groups 20 year olds in this category who have a higher proportion of licence possession than the younger ages which will pull the average up.

4.8. The NTS is below the SHS line for older people. The NTS combines all people aged over 70 where as the SHS groups these people into two. Combining those aged 70-79 with those aged 80+ will pull down the average as those aged 80+ are less likely to be in possession of a driving licence.

Chart 1: Percentage of the adult population with a driving licence by age.

Source: National Travel Survey and Scottish Household Survey

Number of trips

4.9. Comparing Scotland results from the NTS with the results for the whole of GB for 2009 (Table i) shows no significant differences as the confidence interval for the GB data lies wholly within the confidence interval for the Scotland figures.

Table i: Number of trips per person per year (2009)
	Observations		Mean (trips)	Standard Error	95% Confidence Interval
	Unweighted	Weighted	Mean (trips)	Standard Error	95% Confidence Interval
Great Britain	19,914	19,918	973	7	959	987
Scotland	1,615	1,730	961	20	921	1,000

4.10. Converting the number of trips per person per year as reported in the National Travel Survey to a number of trips per day gives an average of 2.6 trips per person per day in 2009 / 2010. Historically the SHS Travel Diary has been presented as proportions eg a proportion of trips by mode, however by using the sample sizes, it is possible to estimate the number of trips made per day. These are shown In Table ii. Those people that reported travelling on the previous day made a similar average number of trips to that reported through the NTS 7 day travel diary. This assumes that those who didn't make a trip the previous day (ie didn't respond to the SHS TD) would have made trips on other days.

Table ii: Journeys per person per day
	SHS			NTS
Survey Years¹	Approximate numbers travelling previous day²	Number of journeys	Journeys per person	Average number of journeys per person per year³	Journeys per person per day
2009/2010	13,476	34,975	2.60	957	2.62
2008/2009				978	2.68
2007/2008	14,612	40,440	2.77	965	2.64
2006/2007				969	2.66
2005/2006	19,402	49,279	2.54	1,014
2004/2005				1,014	2.77

Source: National Travel Survey and Scottish Household Survey Travel Diary
1. Survey years are the two years of survey data combined ie 2009/2010 refers to 2009 and 2010 surveys.
2. Number of respondents multiplied by percentage of people travelling on previous day eg Table 1 of SHS Travel Diary 2009/2010 publication.
3. Figures taken from National Travel Survey Table nts9903

Distance travelled.

4.11. Comparing Scotland results from the NTS with the results for the whole of GB for 2009 (Table iii) shows no significant differences as the confidence interval for the GB data lies wholly within the confidence interval for the Scotland figures.

Table iii: Miles travelled per person per year (2009)
	Observations		Mean (distance / miles)	Standard Error	95% Confidence Interval
	Unweighted	Weighted	Mean (distance / miles)	Standard Error	95% Confidence Interval
Great Britain	19,914	19,918	6,771	93	6,590	6,953
Scotland	1,615	1,730	6,979	338	6,317	7,641

4.12. Chart 2 shows a comparison of the results from the National Travel Survey and the Scottish Household Survey for the average (mean) trip distance in miles (note that the SHS publications report distance in kilometres). Distance travelled reported in the SHS is consistently below that reported in the NTS. This will partly reflect the different methodologies.

Chart 2: Average trip distance (miles) in the NTS and SHS

Source: National Travel Survey and Scottish Household Survey

4.13. The National Travel Survey asks respondents to estimate the distance travelled for each journey (and runs some checks on the responses as part of the quality assurance process). The SHS currently asks respondents for start and end location and then a crow fly (ie straight line) distance between the two points is calculated as part of the post interview data processing which in some cases will provide an underestimate of the distance travelled.

4.14. A separate analysis paper has been produced and published on the Transport Scotland Statistics pages: http://www.transportscotland.gov.uk/analysis/statistics/about/data-sources providing analysis of the impact of straight line distance compared to actual road distance. This methodology is being looked at as part of the introduction of a new contract for the survey.

4.15. This difference in average distance travelled is also seen across the modes of transport as Chart 3 shows.

Chart 3: Trip lengths by main mode of transport (2009/2010)

Source: National Travel Survey and Scottish Household Survey

Trip duration

4.16. Chart 4 shows average (mean) trip duration from the SHS compared to the average trip duration from the NTS. Trip duration will be directly related to trip distance and mode ie the further someone travels the longer it will take.

Chart 4: Average trip duration (minutes) in the NTS and SHS

Source: National Travel Survey and Scottish Household Survey

4.17. Average trip duration reported in the SHS is much higher than the NTS prior to 2007. This is likely to be caused by the exclusion of short journeys (1/4 mile or 5 minutes on foot) in the SHS Travel Diary. From 2007 these were included.

4.18. Since 2007, the mean journey duration in the SHS and NTS has been comparable suggesting that differences in distance shown in Chart 2 are to do with straight line distances used in the SHS Travel Diary. This implies that it is the methodology that explains the small differences between the surveys.

4.19. Chart 5 shows average journey duration by purpose of journey for 2009/2010 (2010 for GB figures). As would be expected from Chart 4, some reported journey durations are greater in the SHS, in particular the business travel figure. The two data sources provide similar estimates for most other purposes.

Chart 5: Comparisons of average journey time by purpose of journey (2009/2010)

Source: National Travel Survey and Scottish Household Survey

Note: The above chart compares similar purposes across the two surveys. The NTS purposes are used as the base. SHS purposes with not direct match have been excluded so conclusions need to be treated with caution.