The Growth Factor Method is a technique used to estimate the trip distribution matrix for a given region or zone. This method assumes that the relationship between the number of trips generated in a zone and the number of trips attracted to another zone is proportional to a certain growth factor. The growth factor is calculated as the ratio of the total number of trips attracted to a zone to the total number of trips generated in that zone. The growth factor is then applied to the total number of trips generated in each zone to estimate the number of trips attracted to each other zone.
Uniform Growth Factor Model
The Uniform Growth Factor Model is a simpler version of the Growth Factor Method. In this method, a uniform growth factor is applied to all zones in the region. The growth factor is calculated as the ratio of the total number of trips attracted to all zones to the total number of trips generated in all zones. The total number of trips generated in each zone is then multiplied by the growth factor to estimate the number of trips attracted to each other zone.
A single growth factor for the entire area under study is calculated by dividing the future number of trip ends for the horizon year by trip ends in the base year. The future trips between zones I and j are then calculated by applying the uniform factor to the base year trips between zones i and j.
The origin and destination matrix can used to represent the given network.
ti = Trip generation in base year from zone i
Ti = Trip generation in horizon year from zone i = ti x Qi
Tj = trip attraction
Tcal = ti x Uniform growth factor (2.286)
| I | II | III | IV | |
| I | 0 | 25 | 50 | 25 |
| II | 25 | 0 | 150 | 75 |
| III | 50 | 150 | 0 | 200 |
| IV | 25 | 75 | 200 | 0 |
From the above origin-destination matrix, we can the following in matrix form.
| I | II | III | IV | ti | Qi | Ti = ti x Qi | Tcal | |
| I | 0 | 25 | 50 | 25 | 100 | 3 | 300 | 229 |
| II | 25 | 0 | 150 | 75 | 250 | 4 | 1000 | 571 |
| III | 50 | 150 | 0 | 200 | 400 | 2 | 800 | 914 |
| IV | 25 | 75 | 200 | 0 | 300 | 1 | 300 | 686 |
| Tj | 300 | 1000 | 800 | 300 | 1050 | 2400 | 2400 |
Uniform growth factor F(1) = 2400/1050 = 2.286
Tcal is tabulated using the calculated growth factor in the last column.
Uniform growth factor F(2) = 2400/1050 = 2.286. Since the growth factors remain equal, there is no need of further iteration.
There are some drawbacks of using this method. These are:
1. The assumption of a uniform growth rate for the entire study area is not correct as the growth factor varies across zones.
2. The land use pattern changes with time, but not uniformly as assumed. Hence growth factor changes with time.
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