8.2.2. Generalized cost

One of the factors that influences trip distribution is the relative travel cost between two zones. This cost element may be considered in terms of distance, time or money units.

Travel costs play a significant role in determining if individuals will travel between two zones and how often. When we talk about travel costs, we can think about three main elements: distance, time, and money. Distance refers to how far two locations are, time refers to how long it will take to travel that distance, and money refers to the financial cost of the journey, such as fuel or fare fees. If traveling between zones is costly in any of these aspects, people might choose to travel less or to a different zone where these costs are lower.

Imagine you're deciding whether to visit a friend who lives 30 minutes away. If you have to pay $10 for gas or public transport fare, the time it takes to travel also must make sense compared to just chatting on the phone. If the trip seems too costly in terms of time and money, you might decide it's easier to stay home and call instead.

It is often convenient to use a measure combining all the main attributes related to the dis-utility of a journey and this is normally referred to as the generalized cost of travel.

The generalized cost of travel helps to combine various elements of travel disutility into a single measure, making it easier to analyze and compare trips. This concept acknowledges that different travelers value aspects of a trip differently. For example, some may prioritize time whereas others may focus on the monetary cost. By combining these elements, planners can better understand overall trip behavior.

Think of it this way: if you have to choose between two modes of transportation to work, say driving or biking, you might consider not just the fuel cost of driving, but also the time it would spend in traffic versus the health benefits and enjoyment of biking, even if it takes longer. The generalized cost factor evaluates all these aspects.

This can be represented as c = a₁tᵥ + a₂tₜ + a₃tₜ + a₄t + a₅F + a₆φ + δ.

The formula for the generalized cost includes multiple variables: tᵥ (in-vehicle travel time), tₜ (walking time to and from stops), tₜ (waiting time at stops), F (fare), φ (parking cost), and δ (a comfort parameter). Each element captures a specific cost associated with travel, which can be weighted (represented by a₁, a₂, etc.) based on how important each factor is to a particular traveler or type of trip. This allows for a more nuanced understanding of travel behaviors across different populations.

Imagine two people planning to attend the same event. One is in a rush to get there for a meeting; for them, waiting time (tₜ) and in-vehicle time (tᵥ) are significant weights in their generalized cost calculation. The other person might prioritize cost (F and φ) because of a strict budget. Depending on their priorities, the weights for components in the equation will shift, reflecting how they value different aspects of their journey.

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