Transportation leadership you can trust.

presented to

Florida Transit Modeling Workshop

presented by

Thomas RossiCambridge Systematics, Inc.

April 8, 2004

Transit Path Building

Objectives in Transit Path Building

Development of skims for mode choice (and maybe trip distribution)

Transit assignment

Need for consistency in path building throughout the modeling process

Path Building Options in TransCAD

Shortest path (all-or-nothing)

Optimal strategies

Pathfinder

User equilibrium (assignment)

Stochastic user equilibrium (assignment)

Impedance in Path Choice

In-vehicle time

Initial wait time

Transfer wait time

Access time

Egress time

Dwell time

Number of transfers

Costs

To ensure consistency with mode choice, variables used and their weights should be consistent with mode choice utilities

Non-Transit Links

For walk access/egress paths, include street layer—NOT necessarily the same as highway network—and centroid connectors

For auto access paths, include highway network and centroid connectors

Time of Day

To ensure consistency, mode choice and transit assignment must be performed for the same time periods, for example:

• Between trip distribution and mode choice

• Between trip generation and distribution

BUT NOT

• Between mode choice and assignment

Path Choice Example

Rail – 30 min headway, 30 min travel time

Bus – 20 min headway, 45 min travel time

OriginDestination

Ensuring that Paths Use the Chosen Mode

Affected by how alternatives are defined:

• Rail only

• Bus only

• Bus and rail

Access modes

Use of selection sets

Use of weights on links by mode

• Weights may be inconsistent with mode choice utilities

• However, may need weights to ensure consistency

Use of Weights

Increasing the probability of using the chosen mode

• Example: For a rail path, multiply rail time by 0.75 (keep bus time weight at 1.00)

Minimize the possibility of having paths with mostly (or all) non-transit links

• Example: Increase non-transit link weight to > 1.0

Use the chosen access mode

• For transit-auto access paths, increase weight for walk access links

Other Considerations

No set limits on variables

• Wait times

• Walk access times

• Walk egress times

• Auto access times

• Transfer times/distances

• Number of transfers

Alternatives

• Non-linear functions

• Piecewise linear functions