Calibration Report for the TPB Travel Forecasting Model, Version ...

Calibration Report for the TPB Travel Forecasting Model, Version 2.3
1.5 Special modeling applications
A single standard execution of the regional travel is sometimes insufficient for representing a special
policy condition that is relevant to a particular forecasted scenario. In recent years, TPB staff has
established two model execution “variations,” which address the treatment of special transportation
policies or operations.
The first such variation is known as the “transit constraint” option. Since the year 2000, TPB forecasts
have been subject to a Metrorail‐related capacity limitation in the core area. It is currently assumed
that, beyond the year 2020, all Metrorail trips moving to and through the regional core area will not
exceed 2020 levels. Consequently, procedures have been added to all post‐2020 model executions to
compare the modeled Metrorail trip table with a pre‐existing 2020 Metrorail trip table. Under current
procedures, post‐2020 Metrorail trips exceeding 2020 ridership levels to and through the regional core
are converted into auto trips.
The second variation, known as the “HOV‐3+ skims substitution” option, is undertaken to the model of
planned HOT (High‐Occupancy/Toll) lanes in Northern Virginia. Modeling this type of facility has proven
to be especially challenging given:
1. The HOT facility will operate using dynamic tolling, i.e., the tolls will change in real time to
maintain a service speed at or near free‐flow levels;
2. The legislative stipulation that SOV’s and 2‐occupant HOVs will pay for the HOT lane access
while 3+ occupant HOVs will be allowed access to the HOT facility at no charge; and
3. the legislative stipulation that the HOV travel market will suffer no service degradation from the
HOT lane operation.
Obviously, a central modeling objective in representing HOT lanes is to specify detailed toll rates that
will result in demand levels that do not degrade the prevailing speed on the HOT facility. Another
modeling objective is to ensure that HOV 3+ service levels will remain unaffected by the HOT operation.
To achieve these two objectives, the following four steps are currently undertaken by TPB staff on a
year‐by‐year basis to simulate HOT lanes in Virginia.
1. The travel model is fully executed whereby all Virginia HOT lanes in the highway network are
coded as HOV 3+‐priority lanes. The resulting HOV 3+ LOS skim files corresponding to each
iteration (pump‐prime, iteration 1,…, iteration 6) are preserved for later use. This step is known
as the “base” execution.
2. The travel model is fully executed again. This time, the Virgina HOT lanes are coded as mixed
use lanes (i.e., allowing access by both SOVs and HOVs). The HOT lanes are assigned a “straw”
toll level of 20 cents per mile during the peak periods and 15 cents per mile during the off‐peak.
The toll is included into the overall highway impedance during path building in the traffic
assignment step. Therefore, the toll level impacts the loading in the HOT lane facility.
3. The final (iteration 6) highway assignment process (including three traffic assignments for each
time period) resulting from Step 2 is run iteratively, on a trial‐and‐error basis, to identify HOT
lane toll rates (cents/mile) which yield optimum speeds on the facility. The toll rates are
developed on a freeway segment by segment basis. The queuing delay function is disabled on
the HOT lane facility during this particular step as such delay will presumably be minimal given
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