10.6.1. Incremental assignment

Incremental assignment is a process in which fractions of traffic volumes are assigned in steps.

Incremental assignment refers to a method of traffic volume management where a portion of total traffic is assigned to a transportation network gradually rather than all at once. This is done in multiple phases or steps, which allows for a more controlled approach to understanding how traffic flows through the system.

Think of it like pouring a gallon of water into a glass a little bit at a time instead of dumping the whole gallon in at once. By pouring slowly, you can see how the glass fills and make adjustments if needed.

In each step, a fixed proportion of total demand is assigned, based on all-or-nothing assignment.

During each incremental step, a predetermined fraction of the total expected traffic is allocated using the all-or-nothing assignment method. This means that all vehicles from a specific origin-destination pair are assigned to the best available route based on the current traffic conditions, without spreading them across multiple routes.

Imagine planning a school trip to a theme park. You check the weather forecast and decide to book tickets for half the class on the first bus. If it rains, you reassess for future trips, but for now, all the first bus tickets go to the first half of the class, reflecting an all-or-nothing approach.

After each step, link travel times are recalculated based on link volumes.

Once traffic volumes are assigned in a step, the travel times for each road link are adjusted according to the current traffic conditions. As more vehicles are added to a road, its speed may reduce, leading to longer travel times. This recalibration is necessary to provide updated insights for the next round of volume assignments.

It’s like a busy restaurant where initially only a few tables are filled; the service is fast. As more patrons arrive, the wait times increase, so the restaurant may adjust their seating assignments to manage flow better, ensuring that not too many people are waiting at once.

When there are many increments used, the flows may resemble an equilibrium assignment; however, this method does not yield an equilibrium solution.

Using many increments in the incremental assignment process can lead to traffic flows that start to look like those in an equilibrium state, where all routes are balanced. However, the key point here is that this method will still not reach a true equilibrium because it does not consider all factors that influence travel times and route choices comprehensively like other methods might.

Think of a game of musical chairs. Everyone starts around the chairs. As the music plays (the increments occur), people take their places based on what's available but may not find a spot that represents the best scenario until the game finishes with the last round (the final solution).

Consequently, there will be inconsistencies between link volumes and travel times that can lead to errors in evaluation measures.

Due to the methodology of incremental assignment, discrepancies can occur between the expected traffic volumes on roads and the actual travel times experienced. These inconsistencies can yield inaccuracies in evaluating how well the transportation system is performing, which may affect future decisions.

This is similar to trying to guess how much candy is in a jar by only observing a few handfuls. If too few peeks are taken at some parts of the jar, you might underestimate or overestimate the total amount without a comprehensive view of the entire jar.

Also, incremental assignment is influenced by the order in which volumes for O-D pairs are assigned, raising the possibility of additional bias in results.

The order in which different origin-destination (O-D) pairs are assigned traffic volumes can significantly impact the results of the incremental assignment. If the most congested routes are accounted for first, they may lead to higher delays for later assignments, causing unequal distribution of traffic across the network.

Imagine a group of friends trying to fill a car with groceries. If the heavier bags are loaded first at the bottom, the later bags might not fit well, affecting how accessible the groceries are once you reach home. The order of loading impacts the overall outcome.

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