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Monitoring and Data Management Protocol: Environmental Indicators for Transport

Acknowledgements

1 Introduction

2 Why Monitor?

3 What to Monitor

4 Vehicle Fleet Composition Indicator

5 Travel Activity Indicator

6 Mode of Journey to Work Indicator

7 Travel Time Indicator

8 Glossary

Appendix A: Sampling Framework

Appendix B: Estimated Precision In Estimating Average AADT

Appendix C: Identifying the Road Use Category (or Traffic Pattern Control Group)

Appendix D: Week Multipliers and Relative Error for Each Road Use Category

References

6 Mode of Journey to Work Indicator

As noted in the introduction, the journey to work is a particularly important journey undertaken regularly by a large proportion of the population. As such it may be more effectively targeted by a wide range of policies aimed at reducing the nation's dependence on travel in private motor vehicles. The mode of journey to work will provide a measure of how successful such policies are. However, it is not just the proportion of people travelling by a particular mode that is important but also the number of people travelling and the average distance travelled.

Data on the mode of journey to work can only be obtained through surveys (or questionnaires) of the population in which participants are asked about individual travel behaviour.

Two such surveys are currently undertaken on a regular basis. These are the National Census undertaken by Statistics New Zealand, and the Continuous Household Travel Survey undertaken by the Land Transport Safety Authority.

While both provide information regarding people's mode of travel to work, each has particular advantages and disadvantages. When reporting at a national or regional level the Household Travel Survey information should be used. When reporting at a local TLA level national census data is sufficient and should be used.

6.1 Mode of journey to work indicator from Census

6.1.1 How to monitor

This indicator is based on data collected in the New Zealand Census undertaken nationally every five years. One of the major benefits of the census is that the sample is close to 100%, so the resulting data is a measure rather than an estimate of peoples transport activity. More importantly the sample size allows the analysis to be reported at varying levels of geographic disaggregation either nationally or regionally or at TLA level.

Having a robust sample at this level means that the indicator can potentially be used to identify high impact areas and to assess policies aimed at reducing the environmental impacts of transport by people travelling to work.

Against these advantages are the disadvantages associated with the relative infrequency of the census, which is undertaken only once every five years, and the fact that since 1991 the Census question has asked only about one particular journey to work, i.e. that which occurred on the Tuesday before the census. Furthermore the question used in the Census has changed over time and a further change has been proposed for the next (2006) Census.

6.1.2 What to monitor

The Census monitors responses to the question:

"On Tuesday 6 March what was the one main way you travelled to your work - that is, the one you used for the greatest distance" and on this basis the "main means of travel to work for usually resident employed persons aged 15 years and over."

The response categories include:

• worked at home
• did not go to work
• drove private car, truck or van
• drove private car, truck or van
• passenger in car, truck, van, or company bus
• public bus
• train
• motorcycle or powercycle
• bicycle
• walked or jogged
• other (such as taxi, ferry, plane)
• not stated.

6.1.3 Data sources

The most recent data is available from the website www.stats.govt.nz/census.htm under the title "main means of travel to work for usually resident employed persons aged 15 years and over" and can be supplied at national, regional, or TLA level.

Historic data can also be obtained though Statistics New Zealand or from one of the accredited agents. However the current data is only comparable to that collected in the 1991, 1996 and 2001 Census as prior to 1991 the question was not date specific and simply asked what was the main mode of transport to work, rather than that used on a specific day.

6.1.4 Reporting

Although the Census data records some 12 different categories of response, it is necessary to report only the nine primary indicator values shown in Table 10. Both the number and proportions (%) should be reported. Examples for presenting the data are shown on the following page.

Table 10: Mode of journey to work (Census) primary and secondary indicators

6.2 Mode of journey to work - household travel survey

This indicator is based on data collected in the Land Transport Safety Authority Continuous Household Travel Survey. This survey records data on the travel undertaken by the members of a sample of households. Two one-off surveys of household travel have been undertaken to date. The first in 1989/90 (Ministry of Transport, undated) surveyed 4,123 households (total target sample), and the second in 1997/98 (LTSA, 2000) surveyed 7,164 households. In each case all members of the household were interviewed regarding their travel on two specified days of the year. This provided data on 72,830 and 129,414 person trips for 1989/90 and 1997/98 respectively.

While the infrequent nature of the surveys to date has limited the use in the later months of 2002 the LTSA were provided with funding to continuously survey household travel on an annual basis. The proposal is to survey a sample of approximately 1,700 households drawn from approximately 281 census mesh blocks. The mesh block sampling has been designed to provide reliable estimates of travel at a regional government level.

One of the key advantages of using the household travel survey data is the ability not only to monitor trends in mode choice but also in the distance travelled by mode and purpose.

6.2.1 How to monitor

As with the previous surveys, each household provides data on all travel undertaken by all household members over a two-day period, the two consecutive travel days. These travel days will be allocated to the sample households by the LTSA in a fashion that ensures a reasonable spread of interviews throughout the year. As with similar surveys overseas, the process is continuous over many years with annual reporting based on the data collected in the previous X years, where X is the period of some running average that provides the necessary sample size for the analysis.

Based on a stratified sampling methodology the survey provides estimates of travel at a national and regional level. Although the sample size is modest, the survey will run continuously (the idea being to have surveys every day of the year) from 2002. Improved estimates may be obtained by reporting the results of analyses based on the last two or three years' data.

The recommendation is that national estimates be based on reporting the data collected over the previous two years. It is expected that regional reporting will be on the basis of the previous four years data, although it must be recognised that some the initial estimates will necessarily be over a shorter time frame until a suitable history is established.

6.2.2 What to monitor

The survey collects data on the trips made by household members where a trip is essentially continuous travel using a single mode for a single purpose. The trip mode and purpose are described in Table 11. It is important to note that trip purpose describes the 'activity' undertaken at the destination. Each household in the sample is allocated two days on which the household members record details of their personal travel.

Table 11: Key variables describing travel in the continuous household travel survey

6.2.3 Data source

Trip data may be obtained by contacting:

The Manager
Research and Statistics
Land Transport Safety Authority
PO Box 2840
Wellington.
info@ltsa.govt.nz

It is expected that data for each annual survey for the period from 1 July to 30 June will be available in December of that year.

A request should be made for trip data file with the following variables included (a description of the available variables is contained in Land Transport Safety Authority publication Travel Survey Report: Increasing our understanding of New Zealanders' travel behaviour 1997/98):

REGION The regional authority in which the household is located

SAMNO Sample number of household

PERSON The person number within the household

TRIPDAY The travel day on which the trip occurs

JSTART Destination/purpose of previous of previous journey

JDEST Destination/purpose of previous of current journey

DAYWK Day of week the trip was undertaken

TRIP NO The sequential number of the trips by PERSON on TDAY

TRLEAVE Time of start of trip

TRARRIV Time of end of trip

TRMODE Trip mode

TRACTIV Destination purpose of trip

PURPOSE Reason for journey

VTYPE Vehicle type used for trip

BESTDIST Digitised trip distance, if available and credible; other wise reported distance

TRWEIGHT Trip weighting factor

ANNWGT1 The annual weighting factor

6.2.4 Data processing

The household travel survey collects data about trips. Trips are defined as continuous travel by a single mode for a single purpose. However we are interested in journeys. It is quite possible that the journey to work will involve more than a single trip as shown below.

It is also important to understand that the purpose of a trip, and therefore a journey, is based on the purpose or activity at the destination. For example, a parent drives from home to the shop and on to school to drop off a child, and then to the railway station to catch the train to work as shown below.

The Leg A is a single trip journey for the purpose of shopping. Similarly Leg B is a single trip journey for the purpose of accompanying (by the driver), while Legs C, D and E are a multi trip journey for the purpose of attending work.

Within the data the variable PURPOSE provides information on the journey purpose, which allows the analyst to identify those CHANGE MODE trips that make up a journey to WORK.

However, when considering the journey, which ends at work, no distinction is made between the first trip to work and those that returned to work. For example having left for work at lunchtime on personal business the return journey will also have WORK (main job) as the PURPOSE.

To overcome this problem it is necessary to identify the first trip to work each day and consider only these trips, recognising that such an assumption disadvantages people working split shifts or those whose employment consists of multiple part-time jobs.

Step 1 Loading the data into a suitable computer package, such as Microsoft Excel, sort the data in ascending order (note Excel should be capable of handling regional or TLA data but the maximum number of lines available in Excel is 65,000 and as a consequence it is unlikely to be suitable for large datasets in which case Access or statistical packages such as SAS or SPSS should be used):

First level SAMNO ascending

Second level PERSON ascending

Third level TRIPNO ascending

Step 2 Save the data and run a suitable macro or programme that undertakes the following process expressed here as a series of conditional (if) statements.

Step 3 The process of Step 2 identifies whether the trip to work is the first for that person on that day. Selecting only the first trips to work it is then possible to identify the main mode of transport for the journey to work, that used over the greatest distance. However, because the distance of walk trips is not recorded in the survey, the assumption must be made that wherever another mode is used, the distance travelled by that mode is greater than the walk component. Although trip distance is not available for walk trips one can, if desired, used trip time to estimate whether the assumption is correct.

Step 4 Using the travel survey data it is also possible to identify the total distance travelled by each mode (except walk) and the number of trips. This data is obtained by simply multiplying the distance travelled (BESTDIST) in the sample of trips (for all purposes) by the annual weighting factor (ANNWGT1), to obtain estimates of the total annual travel, which may be tabulated by mode (TRMODE) and journey purpose (PURPOSE). Dividing the annual distance by the WEIGHTED number of trips provides the average trip length by journey and purpose.

6.2.5 Reporting

The main mode of journey to work should be reported as the percentage use of each mode as in Figure 6. It is recommended that irrespective of whether graphical presentation is used the data should be tabulated as some categories have very small values and will be lost if reported solely in graphical form. Figure 6 uses data from the National Household Travel Survey (NHTS) 1998/99.

The estimated total distance by mode and purpose should also be reported as shown in Table 12.

Table 12: National estimates of distances travelled by mode and purpose

View national estimates of distances travelled by mode and purpose (large table)