Discussion-piece #3

Evaluation of Highway Capacity Congestion Relief Benefits

Federal Transit Administration

June 6, 2006

1.  [Motivations.]{.underline} FTA recognizes that the impacts of major
    transit projects include reductions in highway congestion and,
    consequently, greater mobility for highway users. Therefore, FTA
    defined the New Starts measure of mobility benefits --
    Transportation System User Benefits -- to capture mobility
    improvements accruing to both transit [and highway]{.underline}
    users. The measure captures impacts on both groups of users because
    it obtains information on mobility differences between alternatives
    from the denominator of the logit mode choice model -- that
    quantifies the mobility of all travelers, regardless of mode.
    Consequently, the user-benefits measure as it is currently defined
    (in FTA regulations) and implemented (in FTA-distributed software
    "Summit") is able to credit projects with both the direct
    improvements in transit service caused by the transit project and
    the indirect improvements in highway conditions caused by the
    diversion of some auto travelers to the transit system. Congress has
    expressed support for the inclusion of congestion relief among the
    measures of benefits generated by New Starts projects and, in the
    FY2005 transportation appropriation, directed FTA and FHWA to study
    how highway congestion relief benefits could be incorporated into
    the New Starts ratings process.

2.  [Difficulties and responses.]{.underline} Unfortunately, initial
    results for projects whose sponsors attempted to capture the
    benefits of highway congestion relief suggested that these benefits
    are not predicted reliably in current models -- in both their
    magnitude and their geographic location with respect to the transit
    project. Because of those problems, FTA has elected to consider only
    direct transit benefits in rating proposed projects and to sponsor
    research on the prediction of highway-congestion impacts in hopes of
    correcting the difficulties. The research effort was designed to (1)
    confirm the existence and magnitude of unreliability problems, (2)
    diagnose the source(s) of unreliability, (3) propose solutions to
    the problem(s), and (4) prepare recommendations on how project
    sponsors could proceed to develop reliable estimates of highway
    congestion-relief benefits.

3.  [Nature and sources of the problems.]{.underline} The apparent
    unreliability of congestion-relief estimates came to FTA's attention
    through Summit-generated reports and graphics that appeared to show
    a wide range of highway-side impacts that were largely unexplainable
    as consequences of the underlying transit project. Unexplainable
    results might have been caused by a wide range of potential causes
    including undetected problems in the transit portions of the model.
    To substantiate that the problem was caused by the highway
    congestion elements of the model, initial tests focused on estimates
    of highway congestion relief benefits generated by two models
    previously reviewed by FTA and found to be generally capable of
    forecasting transit ridership impacts. The forecasts in these two
    cases -- a relatively modest-sized project with limited region-wide
    impacts and a very large project with large regional impacts --
    produced vehicle trip tables that had appropriate reductions in auto
    trips that were strongly related to the underlying transit project
    in both magnitude and location. However, highway assignments of
    vehicle trips from the baseline and build projects (using the local
    model's calibrated equilibrium assignment approach) generated both
    positive and negative congestion impacts on trip-ends throughout the
    entire region. These patterns gave the appearance of "randomness"
    and suggested that corresponding estimate of user benefits rested
    entirely on the net difference between the sums of the (apparently
    "random") positive and negative outcomes. A similar pattern was
    evident in plots of the differences (build versus baseline) in
    assigned vehicle hours of travel by link, as illustrated in
    Figure 1. These findings led to conclusions that (1) estimates of
    highway congestion relief user benefits were not reliably estimated
    in the two test cases and (2) the underlying cause appeared to be
    related to instability in the highway assignment process that
    rendered meaningless any comparison of highway-side changes between
    two transit alternatives.

4.  [Test of solution #1: elimination of integer bucket
    rounding.]{.underline} Both models generated the vehicle trip tables
    for assignment using integer bucket rounding techniques. Since these
    techniques move fractions of trips from cell to cell in response to
    minute changes in mode shares, both models were converted to
    generate (1) un-rounded integer trips in 100ths where 0.01 trips was
    represented as 1 trip in the trip table, (2) single precision real
    numbers, and (3) double precision real numbers. The resulting
    highway assignments showed a slight reduction in random noise but
    left most of the problem unresolved.

5.  [Test of solution #2: fixed iteration weights.]{.underline} To
    eliminate possible instability caused by the different weights
    dynamically derived in equilibrium highway assignments for the
    baseline and build alternatives, the iteration weights for the build
    assignment were fixed to equal the weights derived for the baseline
    assignment. The test resulted in no observable improvement to the
    assignment stability for the two test-case models.

6.  [Test of solution #3: tighter closure criteria.]{.underline} The
    assignment closure criteria for both models were typical of common
    practice: Model 1 was set to cease iterations when the gap fell
    below 1 percent; and Model 2 was controlled with a closure gap of
    0.1 percent and a maximum of 15 iterations. Tighter closure criteria
    -- a closure gap of 0.001 percent and no maximum on the number of
    iterations -- produced substantially more stable estimates in both
    models of the change (build versus baseline) in link-level VHT. The
    tighter closure required thousands of iterations to achieve and
    increased execution times by many hours. However, the results
    represented the first apparently reasonable estimates of the impacts
    of changes in vehicle trips on underlying traffic congestion for the
    two projects.

7.  [Test 4: an incremental assignment approach.]{.underline} An
    incremental approach is an entirely different strategy for improving
    the reasonableness of the change in link-level VHT. The incremental
    test used a 1-iteration all-or-nothing method to assign the
    [change]{.underline} in vehicle trips (build minus baseline) onto
    the congested network from conventional equilibrium assignment of
    vehicle trips in the baseline alternative. (Since the software
    packages did not allow assignment of negative trips, the change in
    the vehicle trip table was separated into two tables of positive
    number -- one for increases in vehicle trips and the other for
    decreases. Each table was assigned separately and the sign of the
    incremental trips was addressed during the reporting process.) This
    approach also resulted in highway assignments results (build versus
    baseline VHT) that appeared reasonable and highly correlated with
    the underlying transit project. The execution time for this approach
    was not substantially different from the original assignments.

8.  [User benefits from Tests 3 and 4.]{.underline} Detailed review of
    the estimated user benefits from the incremental approach (#4)
    suggested that the strategy removed large numbers of trips from the
    shortest highway paths and did not allow for the backfill in traffic
    volumes that would surely occur. Therefore, the highway travel time
    savings were evidently too high for the primary routes most directly
    affected by the project and too low for the facilities carrying
    spillover traffic that would -- in part -- be diverted back to the
    primary routes by their now-lower traffic volumes. Further, the
    incremental approach would be difficult to apply in cases where the
    baseline and build highway networks are different from each another.
    For these reasons, further consideration of the incremental
    technique was abandoned. A review of the highway user benefits from
    the tighter closure criteria (#3) suggested that, while this
    strategy produces much more reasonable VHT results, it does not
    correct the largely random patterns of positive and negative highway
    user benefits.

9.  [Internal inconsistencies.]{.underline} Much additional research
    into the remaining problem (unstable highway user benefits even with
    reasonable underlying highway assignments) identified the root cause
    to be inconsistencies in the impedance weights used in different
    model components. Specifically, the highway path building procedures
    included weights on components of time and cost that were difference
    from the relative weights on those same characteristics in the mode
    choice model. (This inconsistency enables the pathfinder to find a
    "better" path in the build alternative that the mode choice model
    then evaluates as being worse, rather than better, because it has a
    different opinion on the relative importance of times and costs.)
    Test runs using impedance weights set identically in all model
    components substantially reduced (improved) the random appearance of
    the computed benefits accruing to highway users. As with many
    insights about travel forecasting, this finding was not surprising
    in retrospect: it is *identical* to the early (2002) finding on the
    need for consistency between weights in mode choice and
    [transit]{.underline} pathbuilding derived from initial reviews of
    Summit reporting of ridership changes and associated user benefits
    for New Starts projects.

10. [Possible implementation of findings.]{.underline} Taken together,
    these research results suggest that it may be possible to derive
    meaningful estimates of the congestion-relief benefits of New Starts
    projects. Useful forecasts might be possible from model sets
    that (1) hold trip tables in sufficiently precise formats --
    hundredths of trips or better; (2) iterate highway assignment
    procedures to sufficiently stable closure -- at least for "official"
    New Starts submissions; and (3) employ consistent impedance weights
    "across the board" in transit pathbuilding, highway pathbuilding,
    and mode choice. Implementation of the first condition requires
    relatively minor effort and most travel models are already migrating
    to software packages that hold table information in real numbers.
    Implementation of the second condition is dependent on the locally
    implemented software package but requires minor effort (though lots
    of computer time) to prepare comparable highway assignment results
    for pairs of baseline and build transit alternatives. Implementation
    of the third conditions is a potentially significant effort because
    it would require recalibration of highway pathbuilding/assignment
    procedures, the mode choice model, and/or both model components.

11. [National consistency.]{.underline} The approach sketched in item 10
    addresses only the conditions necessary to produce potentially
    comparable highway assignment results for a pair of transit
    alternatives. Pair-wise comparability of predicted congestion levels
    is necessary for local evaluation of the potential congestion-relief
    benefits of a major transit project. That condition does not,
    however, begin to address two other issues necessary to the
    usefulness of congestion-relief benefits in New Starts project
    ratings. The first issue is whether or not the estimated highway
    travel times are "reasonable" within individual metropolitan areas.
    In order to properly estimate highway user benefits, a detailed
    assessment of the reasonableness of free flow speeds, capacities,
    and volume-delay functions would be required before any outcome
    could be used for project evaluation. The second issue is whether or
    not sufficient consistency exists (or could exist) across
    metropolitan areas to support meaningful national comparisons of
    congestion-relief benefits. Since model structures, parameters, and
    other characteristics vary widely across the United States, the
    possibility exists that equally "reasonable" -- but different --
    approaches to highway assignment could generate significantly
    different estimates of highway congestion relief benefits. These
    changes could be sufficiently large to affect project ratings in
    ways that are not related to the relative performance of the many
    projects that are proposed nationally.

12. [Recommendation.]{.underline} The research accomplished in response
    to the Congressional instructions regarding congestion relief
    suggests that current practice in travel forecasting may have the
    fundamental tools necessary to produce useful insights. However, the
    research also suggests that major efforts would be needed -- by
    project sponsors and by FTA -- to refine the details of current
    practice sufficiently to make such an outcome possible. Necessary
    local efforts would likely include substantial recalibration of
    existing model sets, collection of data on highway speeds,
    refinement of coding procedures for highway networks, and
    adjustments (or wholesale replacement) of methods used to load
    traffic on those networks and estimate congested speeds. Even more
    consequential, perhaps, might be the impact of these changes on
    existing findings of conformity with national air-quality standards
    and any additional work to bring air-quality-related work into
    alignment with substantially revised travel models. Necessary FTA
    efforts would include quality control on both the reasonableness and
    the consistency of predicted congestion relief benefits (to ensure,
    for example, that transit projects with similar ridership impacts in
    settings with similar congestion levels would be evaluated with
    similar congestion-relief predictions). Because of the potentially
    large local burden and the likely requirement that FTA take a
    substantial role in efforts to improve forecasts of highway
    performance, FTA has recommended to Congress that evaluations of New
    Starts projects continue to be done with [transit]{.underline} user
    benefits (and that FTA continue to make general allowances in
    cost-effectiveness ratings for the absence of highway user
    benefits).

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Figure 1. Display of Predicted VHT Changes for a Rail Project in a
Medium-Sized Area