Chapter%208

Chapter%208 - 1 2 3 4 Trip assignment can be thought of as...

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
1
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2
Background image of page 2
3
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
4
Background image of page 4
Trip assignment can be thought of as the “choice” of a route by an individual. But, the choice of the route is dependent on level of service measures on that route, which are in turn dependent on the route choices made by other travelers. If a lot of the other travelers choose the same rate as a given traveler, then the route may get congested and may affect your choice of the route in the first place. Therefore, there is a clear feedback between the route choices of other customers and your own choice. Modeling trip assignment as an individual decision is possible but is fairly involved. Therefore, trip assignment is typically an aggregate process. Time periods are defined for the model, the daily trip table is apportioned into say, AM, mid-day, PM, Night and so on. These time period trip tables are then assigned to the highway network with the appropriate levels of service. 5
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
There are two key inputs to the highway assignment process. First, we need vehicle trip tables for each time period by class. In other words, we will need vehicle trip tables for SOV, HOV, trucks, toll road users and so on. Second, we need the highway network to which we can assign these trip tables. In many locations, the highway network is different for SOV and HOV users because HOV users can use the HOV lanes, while the SOV users cannot. Similarly, toll road users will need to be assigned to the network with the toll links included, but the non-toll users will be assigned to a network that omits toll links. The outputs of the highway assignment process are twofold: link traffic volumes on each link in the model highway network for each time period, and speeds on each link by each time period. 6
Background image of page 6
Most assignments are static equilibrium assignments. So how does static user equilibrium assignment work? There is an inherent circularity between traffic volume and travel time. Travel time on a link is a function of free-flow times, volume on that link and the capacity. But the volume on the link itself is a function of travel time! This circularity necessitates an iterative way of determining link volumes. We first start with the free-flow levels of service and obtain an initial set of volumes. Then, we recalculate the new travel times with these volumes, and once again repeat the entire process. In an equilibrium assignment we iterate until we reach an equilibrium. An equilibrium is reached when no traveler can improve her travel time by changing paths. 7
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
The travel modeling software treats equilibrium assignment as a series of all-or-nothing assignments. That is, for a zone pair connected by multiple paths, the software assigns all the trips between the zone pair to the path that has the lowest travel time (or cost). The software now calculates the new travel times on each path based on these
Background image of page 8
Image of page 9
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 31

Chapter%208 - 1 2 3 4 Trip assignment can be thought of as...

This preview shows document pages 1 - 9. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online