Mode Share Considerations

Targets, as we all know, should always be "smart" - Specific, Measurable, Achievable, Relevant and Time-Based.

The adoption of targets for public transport mode share is a popular policy in the strategic transport plans for many metropolitan areas. Leaving aside their measurability, their achievability and their relevance, about all of which much more can be said, how specific are they ?

• It makes a difference whether you are talking about the share of trips or the share of travel (person-kilometres). Walk trips tend to be very local, bike trips short, bus trips short-ish, car trips medium and rail trips long (at least at peak times and in Sydney).

• It makes a difference whether you are talking about all trips or just motorised trips. Walking will tend to account for 20-40% of all trips and leaving them out distorts the picture considerably, more so for trips than for travel.

• It makes a difference whether you are talking about linked trips or unlinked trips. Surveys that record multi-legged trips often report on a "priority mode" basis, so for instance a trip that involves a bus leg and a train leg would be reported as a train trip. The role of buses tends to be under-reported under such arrangements. So, and even more, is the role of walking : all public transport trips and many car trips have a walk component, which disappears under "priority mode" reporting.

• It makes a difference whether you are talking about an average day, an average weekday or a typical weekday. An average day reports annual travel divided by 365, and therefore mixes up weekdays and weekends (which are quite different) and school terms and school holiday periods (which are somewhat different). An average weekday will exclude the weekend factor, and a "typical" weekday will exclude the school holiday factor as well.

• It makes a difference whether you are talking about the mode share over 24 hours or during peak periods only. In Greater Sydney in 1991 47% of trips to work took place outside the period 0700-0930, although this did include people returning to work eg after a lunch break.

• It makes a difference whether you are talking about trips for all purposes or only trips by workers (or for any other specified purpose). Less than half the population are employed, and even employed people make daily trips, including at peak times, other than their journey to work and back.

• It makes a difference whether you are talking about the mode share for trips which originate in an area or for trips for which the area is the destination. Over 24 hours these should balance out, though with no sense of trip purpose remaining. Purpose is an important distinction because some types of trip are harder to influence than others. For a specific time of day, the mode shares for originating and terminating trips will be the same only at the broadest level of spatial aggregation (the whole city).

Planners in Sydney are fortunate in that the NSW Government now collects travel data in a continuous rolling survey program that covers all purposes, all day types and all modes. Elsewhere in Australia, there tends to be heavier reliance for travel monitoring on data from the five-yearly census mounted by the Australian Bureau of Statistics. This has at least three great advantages, as well as the not insignificant feature that it is funded by Commonwealth rather than State government. These are that :

• the geographic coverage is universal and the data is eventually made available at least for every local government area in the country.

• it is as close to a 100% sample as it is possible to get.

• every employed person is asked how they got to work on census day, and therefore the exercise generates a wealth of data about travel to work.

The main disadvantage stems from these advantages. Planners in the past have tended to concentrate on the journey to work issue because they think they know reasonably well what is going on there. This can be very distorting - in Greater Sydney on an average weekday, trips to work account for about 10% of trips, the reverse trips from work another 10% and trips at work, ie people travelling as part of their work, also about 10%. So around 70% of trips on a weekday are not work-related. The most prevalent of non-work travel purposes, also accounting for about 10% of weekday trips, is "serve passenger", or in other words taking someone else (often children) somewhere.

The Census data also requires some care in interpretation, in particular :

• It is a snapshot of work travel that happened on a single specific day. Census Day in Australia is in winter. This means, for instance, that daily bicycle trips will be at their minimum throughout the country at such a time. Census Day has often been within a week or so of the shortest day of the year in the Southern Hemisphere, meaning that many will eschew a bike trip to work because of aversion to riding home in the dark. Similarly, any abnormal weather on Census Day will reflect in abnormal travel patterns.

• Australians are asked for the location of their employer, and by what mode or modes they got to work on Census Day. It is assumed that when they did go to work, they went to their employer's address. This is not necessarily so.

• The Census results relate to a 24-hour period : there is no information about the timing of the trip.

Does any of this matter ? Well, in practice, there are usually two different objectives which lead to the acceptance of mode split targets.

• One is operational/economic - the desire to reduce congestion on the roads by increasing the use of public transport.

  For this objective, the most appropriate measure would probably be the ratio of all car driver trips to all trips by all modes, in the morning or afternoon peak (for all purposes).

  A non-working parent taking a kid to or from school contributes to peak congestion as much as the worker - and for some workers, e g tradesmen needing to carry ladders, paintpots or whatever with them, there is far less choice about when and how to travel than for many non-workers.

• The other is environmental - the desire to either reduce travel or to make sure that what travel takes place does so as much as possible by environmentally benign means.

  The main concerns are probably public health impacts of deteriorating air quality, and climate change being induced by accumulation of greenhouse gases.

  Both are related to emissions from transport.

  Weekend emissions are as damaging as weekday ones, and any environmental targets should probably relate to the average day rather than the average weekday or the weekday peak, and to travel rather than trips.

As a case study, the following data for travel in 1991 in Greater Sydney shows how wide the latitude to interpret travel behaviour is.

The first table presents some findings from the Road and Traffic Authority's massive Home Interview Survey or HIS (the "Big Bang" predecessor of today's "Steady State" rolling program of surveys), and from the ABS Census of 1991 for the same area.

The second table shows various mode shares than can be derived from the first table. We show two different statistics. Consider the data in the first table and think how you would define mode share targets for transport planning.