Tour-Based Modeling: Why is it Important?
One thing that is constantly bounced around is why tour-based modeling is better than trip based modeling. We’ve been using trip based modeling for 50 years, isn’t it timeless?
No.
Fifty years ago, when the trip based modeling methodologies were developed, the primary reason was to evaluate highway improvements. While tolling was in use, the bonding requirements were likely different. Transit, while extremely important, was not in the public realm (the streetcars were normally privately owned by the area’s electric company).
Now, there are a lot of demands on travel models:
- Tolling/Toll Road analysis at a better level
- Different tolling schemes (area tolling, cordon tolling)
- Travel Demand Management (telecommuting, flex hours, flex time, alternative schedules)
- Better freight modeling (which now is becoming commodity flow and commercial vehicle modeling)
- Varying levels of transit (local bus, express bus, intercity bus, BRT, light rail, and commuter rail
While many of these can be done with trip based models, most of them cannot be done well with trip based models. There are a number of reasons, but the few that come to mind are aggregation bias, modal inconsistency, and household interrelationships.
Aggregation Bias
Aggregation bias occurs when averages are used to determine an outcome. For example, using a zonal average vehicles per household, you miss the components that form the average, such as:
20 households, average VPHH = 2.2
2 HH VPHH = 0
5 HH VPHH = 1
4 HH VPHH = 2
6 HH VPHH = 3
3 HH VPHH = 4+
The trip generation and modal choices (car, bus, bike, walk, etc.) among these households are all different, and are even more more different if you look at the number of workers per household.
Modal Inconsistency
In trip based modeling, “people” are not tracked throughout their day. So, if someone rides the bus to work, there is nothing in the model to ensure that they don’t drive from work to get lunch. While we don’t want to force people to use the same mode, since many people will use the bus to get to work and then walk to lunch or to go shopping during lunch, we want to make sure that there is some compatibility of modes.
Household Interrelationships
One of the features of of tour based models is determining each person’s daily activity pattern. During this process, certain person types can determine what another person is doing. For example, if a preschool age child is staying home, an adult (whether they are a worker or not) HAS to stay home. Another example is if a school-non-driving-age child is going on a non-mandatory trip, an adult must accompany them. Trip based models don’t know about the household makeup and the household interaction.
The above are only three of the many reasons why tour-based modeling is important. There are many more, but I feel these are some of the most important and some of the easiest to understand.
Tags: tour-based-modeling, Transportation, trip generation, trip-based-modeling
