Cross-classification model

Cross-classification model, sometimes referred to as category analysis model, is based on the assumption that the number of trips generated by similar households or households belonging to the same category is the same. According to this model if in Zone $i$ there are $n_k^i$ households in category $k$ and if $g_k$ is the average rate of trip generation per household in category $k$ then the trips generated (or produced) by Zone $i$, $T_i$ is given by

$$T_i = \sum_{\forall k} n_k^i g_k$$ (1)

The model predicts the trips produced by a zone by simply aggregating the total trips produced by the all the households in that zone. However, two basic questions need to be answered here: (i) how does one define similar households, or alternatively how does one define categories of households, and (ii) how does one determine the rate of trip generation for a given category of household. The answer to both these questions is: through empirical observations and analysis. What is done is that, first, data on demographic characteristics and trip making behaviour of large number of households are collected. This data is then analyzed to see what characteristics of the households are important in defining a homogeneous group -- households which produce approximately the same number of trips.

Based on the above analysis tables are made which define each category of households by listing its properties in terms of different demographic variables. For example, a particular category of households may be defined as households with 3 to 4 members in the age group 6 to 60, with income in the range of Rs. 30,000 to Rs. 40,000 per month, and one automobile. Finally, for each category of household the average number of trips generated are listed. The listing of the definition of categories and the associated trip generation rates are generally referred to as trip tables.

This method of analysis although simple in its structure have few difficulties. The foremost is the problem with defining categories correctly -- at best it is very difficult. There are other problems like handling additional data on trip generation behaviour -- the trip tables are not amenable to simple updating but generally have to be completely revamped every time new data is available.