Induced and Generated Traffic

In this note, we describe the methodology for determining the level of generated, induced and other special traffic.

The traffic forecasts will be developed for each highway section by vehicle type and for each component of traffic growth including:

• Normal growth of the existing traffic;
• Diverted traffic, to and from the toll highway, if applicable;
• Generated traffic to the extent that the improve travel conditions on the toll highway will reduce travel times and costs and thereby result in an increased frequency of existing trips or in trips being made that are not now made at all;
• Induced (development) traffic resulting from increased economic activity in the toll highway’s area of influence that may be due to and attributable to the improved access and service levels provided by the toll highway; and
• Any special traffic, unusual or non-recurring traffic that may occur.

Generated Traffic

As described above, generated traffic is a result of increased travel frequency of travelers because of a reduction in the travel times and costs.  In other words, under existing conditions because of high travel costs (in terms of travel time, congestion, out-of-pocket costs) people do not make all the trips that they want to make.  However, after the improvements in the transport infrastructure the generalized cost of travel is expected to reduce thereby making it possible for travelers to make the trips that they were not making under existing conditions.

To determine the level of generated traffic the study team should develop an elasticity based model through the following steps:

1. Determine the generalized cost of travel under existing conditions. The generalized cost of travel is defined as: Generalized Cost = Travel Time + ( (Tolls + Length * VOC) / Value of Time) where VOC: vehicle operating cost.
2. Determine the generalized cost of travel after improvements: After the concession is in place the highways are expected to have a different generalized cost of travel.  Most likely, the travel times and the vehicle operating costs are expected to reduce while the value of tolls will be higher than the existing conditions.  Using this information the study team should determine the new generalized cost of travel.
3. Determine the difference in generalized cost before and after the concession.
4. Determine the elasticity of generalized cost: The elasticity of generalized cost refers to the percentage change in number of trips produced by an equivalent percentage change in the generalized cost of travel.  This elasticity value should be computed through gravity models.
5. Determine the generated traffic: The generated traffic should be computed using an elasticity based model with the following structure:

Generated Trips = ∫(Existing Trips, GC^base, GC^future, ε_GC)

where:

GC^Base = Generalized cost before improvements

GC^Future = Generalized cost after improvements

ε_GC = Elasticity of generalized cost of travel

Note that the generated trips will be computed for each zone pair.  By including the existing trips in the model, the generated trips become a function of existing trips making the model conservative and reliable.

Induced Traffic

Induced traffic is defined as the traffic resulting from economic development due to improved accessibility that a new or rehabilitated road provides.  Often certain regions are not developed to their maximum potential because of poor transport infrastructure.  After the concession of a road, it is likely that it will be easier to reach such regions making it attractive for economic development.  New industries can locate to such regions to take advantage of being closer to the sources of raw materials or markets or low labor costs.

The determination of induced traffic is quite complicated.  The study team has to take into account the reduction in generalized cost of travel (see section on generated traffic) but also has to take into account the macro economic conditions of the area of influence of the highway.  The study team should adopt a conservative methodology for determination of induced traffic as described in the following steps:

1. Identify principal economic activities: The analysis team should study the macro-economic conditions of the area of influence of the highway.  This analysis should result in a description of the principal economic activities of the region (such as agriculture, mining, services, etc.)
2. Evaluate growth possibilities: The study team should evaluate the possibilities of growth in the existing and new economic activities in the area of influence.  For example, in an existing agricultural region it is possible that an improved transport infrastructure could induce food processing industries to develop.  The study team should evaluate not only the type of growth possibilities but will also evaluate the level of such growth and the time horizon in which such growth will occur.
3. Determine the trips produced/attracted by new activities: Once the potential growth areas are identified, the consultant will determine the number of trips produced/attracted by such new activities.  The study team should use simple trip generation rates to determine the induced trips.  Appropriate sources (such as the ITE Trip Generation Manual) could be used for this task.
4. Perform reasonableness checks on estimated induced trips: As a final step, the study team should perform reasonableness checks to ensure that the estimated induced trips are not exaggerated and are within reasonable limits.