by Kay Axhausen

The image shows a daily schedule with 2 tours including 1 subtour, 8 trips and 18 stages.

No surprise, but professional and everyday language overlap in their vocabulary, while not identical in their meanings. In many fields this is not a big problem, as the technical and professional discourse do not overlap, as laymen rarely read or hear the technical discourse, but for the students at the beginning of their training. In transportation it is a problem, as the professionals have to address the public as voters, decision makers or respondents in surveys. They talk with each other continuously.

As expected, transport planners and engineers have developed a detailed vocabulary to talk about movement, unfortunately even differing ones for the different modes of transportation. Roget’s Thesaurus give as synonyms trip, journey, excursion, cruise, expedition, foray, jaunt, outing, run, swing, tour, travel, trek, errand, hop, junket, peregrination, ramble. They clearly have connotations, which make one more suitable for certain occasions, but they can be used interchangeably for a movement from A to B and back.

Observing and thinking about movements for a moment it becomes clear, that a ‘trip’ will have smaller building blocks and might belong to something bigger, say a vacation. The professionals have given names and a structure to these, so that they can measure and talk about them clearly. The layman might want to appreciate the differences when he or she listens to a policy proposal arguing with them.

The key terms are stage, trip, tour, daily schedule with their variants in different countries and industries. A stage is the smallest unit, the movement from A to B with one mode ore one vehicle of that mode: walking from home to the bus stop or flying from the first airport to the hub airport are stages. The airline industry talks about legs and means the same thing, as do American planners when they talk about ‘unlinked trips’. In logistics the stage with the longest distance is generally called the ‘main haul’.

A sequence of stages from one activity to the next is a trip, which now requires a definition of activity. Following the example of time use studies and sociology, the activity is defined as a meaning interaction with another person or task. In transportation a trip is always one way.

A sequence of trips from A via various other locations back to A form a tour. Journey is used to specify tours starting and ending at home. One runs into problems, if one wants to talk about tours within tours, for example going to lunch and coming back to the workplace. Some parts of the literature talk about sub-tours then. Equally, other parts of the discussion need a word to talk about the movement from home to the main stop of the tour. You will find the word ‘commute’ to describe just this, even if it includes activities, such as dropping out the children at school, a quick coffee at Starbucks and time at the gym before arriving at work.

The daily schedule are all the tours undertaken between getting up and going to bed again.

It should be clear now, that discussion about mode choice should always refer to the element talked about. Walking (stages) will always be part a trip to reach the vehicle(s). Walking will therefore always have the highest mode share among the stages, but not of the miles travelled. At the higher levels the planners have to decide which mode they allocate to the trip or tour. Normally they choose the mode of the stage with the longest distance. In this process the other stages are forgotten and often their mileage allocated to the maid mode. The chances for confusion are endless, unless this is made clear.

Remember: Tours are sequences of trips, which are sequences of stages.

PREDICTED TRAFFIC AND ACTUAL TRAFFIC AFTER A ROAD WIDENING

by Wes Marshall

You already have a congested roadway, and the transportation planners predict even more traffic on that road in the near future. What do you do? For most of the last century, the answer was to increase capacity. In the short-term, this seemed to work. Time and time again, over the long-term, the actual amount of traffic after the capacity increase grew far more than expected. What seemed like an obvious solution to a congestion problem continued to disappoint. But why?

The reason for these failures lies with the principle of induced demand. Once capacity increases, not only do you get the originally predicted traffic growth, but you also facilitate some often unanticipated changes in travel behavior. First, existing road users might change the time of day when they travel; instead of leaving at 5 AM to beat traffic, the newly widened road entices them to leave for work with everyone else. Second, those traveling a different route might switch and drive along the newly widened option. Third, those previously using other modes such as transit, walking, bicycling, or even carpooling may now decide to drive or drive alone instead. Together, these unwanted behavior changes fall under what is termed the theory of triple convergence (also known as the ‘Iron Law of Congestion’). This latent demand induces more traffic than originally expected and saps the supposed improvement of the expected benefits.

The joke is that adding lanes to cure congestion is like loosening your belt to cure obesity. Empirical results over the last century – due to the principle of induced demand – have borne out that this issue is real and should always be accounted for when considering adding capacity as a solution to congestion.

References:

Downs, A. (2004). Still Stuck in Traffic. Washington DC, Brookings Institute.