TEMPORARY LOSSES OF HIGHWAY CAPACITY AND IMPACTS ON PERFORMANCE: PHASE 2

12. Next Steps

The TLC and TLC2 studies represent an initial attempt to estimate the loss of capacity and delay due to short-term events. Due to the scope of these studies, time and effort constraints, and limitations in available data, there are areas in which TLC delay estimates could be significantly improved or expanded. This chapter discusses further activities that would improve the initial estimates.

12.1 Estimate Impacts from Simultanteous Capacity-Reducing Events

TLC2 could be extended to estimate capacity reduction and delay resulting from simultaneous capacity-reducing events, such as a breakdown or crash in a work zone or during a snowstorm. These types of events might cause capacity losses and delays that are much longer than those that result when no interaction is considered.

12.2 Delphi Survey

A number of assumptions were made in the TLC study, some of which affected the methodologies and some of which affected equation parameters. Because of uncertainties about many of the key parameters of this analysis, it would be useful to conduct a Delphi survey of traffic engineers and other experts regarding the values they believe are correct. In addition to improving assumptions for key variables in the current methodology, the results of the Delphi survey could be used to determine distributions of values for sensitivity analysis.

12.3 Methodology for Analyzing Detailed Delay Estimates

The capacity loss and delay are estimated for detailed breakdowns by area type and size, highway type, and peak period/congestion level. It is often difficult to understand the reasons for the results A methodology could be developed to help answer why the models in TLC2 produced a given result for a specific category. This would improve our understanding of both the TLC2 models and the factors that affect specific delay estimates.

12.4 Sensitivity Analyses

The national-level scope of the TLC study and its novelty made it necessary to make a large number of assumptions regarding key parameters. Confidence in the resulting estimates could be greatly increased by performing a sensitivity analysis to determine which variables most affect capacity loss and delay estimates.

12.5 Methods for Estimating the 4 Rs

As stated often in this report, the TLC study did not attempt to assess impacts of re-routing, re-scheduling, reduced mobility, and reduced reliability on delay. Analytical methods for estimating re-routing, rescheduling, reduced mobility, and reduced reliability could be developed to give a more complete and accurate analysis of TLC impacts.

12.6 Assess Policy and Technology Impacts

At present, the TLC study attempts to estimate only the quantity or magnitude of delay from temporary losses of capacity. Of greater significance for policy decision-making would be estimates of the potential benefits of various policy and technology alternatives, such as work zone management strategies, electronic toll technologies, and other alternatives, that may reduce capacity losses and their impacts.

12.7 Methodology Improvements

During the TLC study, several areas in which the initial methodologies could be improved were identified:

Using probability distributions to assign incident duration instead of using mean duration
Obtaining more information about crashes on arterials and adjust the methodology to compute delays accordingly
For crashes at intersections, estimating the impacts on transversal roadway segments
Including the effects of crossover work zones, where the capacity of lanes in the other direction are affected by the construction
Improving the delay estimation methodology by considering demand reductions downstream of a capacity-reducing event
Using stochastic network simulation models to validate the deterministic delay calculation

12.8 Data Improvements

The completeness and reliability of TLC estimates could be improved by using, compiling, and/or collecting additional data.

Refine data for demand distribution multipliers for freeways and generate demand distribution multipliers for arterials instead of using freeway distributions as a surrogate.
Several other data sources for crashes and breakdowns on freeways were used as a surrogate for arterials. These data need to be validated for arterials or new data sources need be identified.
Collect more data on delay and capacity loss estimation due to vehicle breakdowns. This could also lead to methodology improvements.
Further study of the Monte Carlo simulations used with GES data to estimate delay and capacity loss due to non-fatal crashes should be performed.

12.9 Consider Publishing TLC Study as an Annual Report

The TLC study methodology could be used to produce an annual report on delays due to temporary capacity losses. A historical time series of nation-wide delay due to TLC could also be estimated.

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