travel demand forecasting report - Sonoma County Transportation ...

Appendix I: Travel Demand Forecasting Report
In the AM peak period best paths, walk access to transit competes with auto access to transit to produce
the best paths. AM peak walk-only paths and midday walk-only paths produces walk access paths to
transit.
2.6 Modifications to the MTC/VTA Model
This section presents modifications made to the MTC/VTA travel demand model for the SMART EIR.
Specific changes were made to the mode choice portion of the model to improve its ability to estimate
rail travel. Each modification was made with the intent of improving the model’s ability to replicate data
observed in the region of interest (the San Francisco Bay Area). Observed data/behavior was obtained
from the Caltrain On-Board Survey used to calibrate the model (Caltrain, 2003).The approach first
introduced a new variable to the mode choice model, and the coefficient acting on the new variable was
then calibrated using observed data. The changes are discussed in two sections below: for the homebased
work trip purpose and the other trip purposes.
2.6.1 Home-Based Work Modifications
The home-based work mode choice model includes walk-to-transit modes segmented by service
type (i.e. walk to passenger rail) and generic auto access modes (i.e. drive-to-transit; where
transit could be passenger rail, light rail, local bus, and so on). The modifications to the model
are discussed separately for each of these access types below.
Walk-access to Passenger Rail: The segmentation of transit modes in the walk access nest allowed
for the introduction of passenger-rail specific coefficients. One shortcoming of the mode choice model is
the lack of a penalty applied to bus-to-rail or rail-to-bus transfers. In the calibration year, this shortcoming
caused an overestimation of transfer trips (a Caltrain on-board survey observed a transfer rate near 50%
versus an estimated rate near 86%). To alleviate this problem, a variable to capture the number of
transfers was introduced to the model and a coefficient was calibrated to allow the estimated transfer
rate to more closely match the observed transfer rate. The final calibrated value for this coefficient was -
3.400, which corresponds to 70 equivalent minutes of in-vehicle travel time.
Drive-access to Transit: Transit modes in the drive access nest (park and ride and kiss and ride) are
treated as generic. As such, variables specific to the passenger rail choice are introduced conditionally
only if a passenger rail in-vehicle time is present in the input skim matrices (new drive access to
passenger rail skims were produced for the SMART project and the mode choice source code was
modified to accept the new skims). Three variables specific to the drive-to-passenger rail mode were
introduced, namely: transfers (same as the walk access), a ratio of the drive access to passenger rail invehicle
time, and an alternative specific constant.
The transfer variable behaves similar to the walk-access mode, discussed previously. Again,
this variable was calibrated to match the observed transfer rate. The final calibrated coefficient
is -4.200, which translates to 86 equivalent in-vehicle minutes.
The travel time ratio variable computes the ratio of the drive access time to the passenger rail invehicle
time. Such a variable has been shown to be an important determinant of passenger rail
choice in places such as San Diego. The coefficient on this variable was calibrated to
approximately match the observed ratio in the San Diego Region. The final coefficient is -1.000
(20 minutes of in-vehicle time).
The introduction of negative utilities based on transfers and the travel time ratio causes the
passenger rail portion of the drive-access to transit mode to lose market share. To regain this
share, an alternative specific constant is introduced specific to the drive to passenger rail mode
(again, the input skim is used to determine when this constant should be applied). The final
value of this coefficient is 0.9732, which translates to 20 equivalent in-vehicle minutes.
Sonoma-Marin Area Rail Transit 11 Travel Demand Forecasting Report
September 2005