Guide for Highway Capacity and Operations Analysis of Active Transportation and Demand Management Strategies
4 Overview of ATDM Analysis Methodology
The methodology for capacity and operations analysis of ATDM presented in this Guide is designed to provide estimates of the effects of ATDM strategies on person throughput, mean facility or system travel time (and therefore delay), and facility or system travel time reliability for two conditions:
- Before implementation of ATDM Strategy; and
- After implementation of the ATDM strategy; both “opening day” conditions as well as equilibrium conditions (3 to 6 months after implementation) can be analyzed.
The before conditions are used to calibrate and error-check the selected traffic operations models to be used to estimate maximum person throughput, mean travel time and travel time reliability. Depending on the availability of “before” information on counts, incidents, weather, mean travel time, and reliability, the calibration can be relatively simple or quite sophisticated.
After conditions predict how facility throughput, mean travel times and travel time reliability will change after implementation of the ATDM strategy. Two levels of “after” analysis are provided: 1) ”opening day” conditions, which represent what happens immediately after implementation and 2) equilibrium conditions which represent conditions travel after travelers have had a chance to become familiar with the facility’s new performance levels. That is, travelers have had a chance to shift trip starting times, shift routes, shift destinations, and shift modes in response to the new performance levels. Equilibrium conditions would typically be experienced 3 to 6 months after activation of the ATDM strategy.
The ATDM Analysis Methodology is designed to be applied to a system of highways or a single highway facility. Its capabilities are determined by the tool selected by the analyst for computing the travel time effects of ATDM. For example, ATDM evaluations using the Highway Capacity Manual (HCM) will be limited to single freeway or single urban arterial facilities until such time as the HCM provides systems analysis capabilities. An ATDM analysis using the recommended methodology in combination with a microsimulation model will be able to perform systems evaluations.
A flowchart of the analysis within the ATDM Analysis methodology is given in Figure 6. The stages of the analysis and the major steps of the analysis are summarized below.
- Preparatory Steps: This first task consists of setting the scope and purpose of the analysis, defining the target study area, and collecting data.
- Generate Scenarios: Using historic data on demand, incidents, and weather, a set of demand and capacity scenarios are generated against which to evaluate current conditions and to test the new ATDM strategy. This is the “before” condition.
- Apply Selected Operations Analysis Tool to Scenarios: The appropriate operations analysis tool (HCM, simulation, etc.) is used to evaluate facility operations for each scenario. Each scenario consists of a given total demand level for the overall peak period, and a given incident and weather condition. This same analysis tool is applied for the Before, Opening Day, and Long-Term Demand Analysis conditions. The demands input to the operations analysis tool will vary according to the condition being evaluated.
- Compute MOEs (Performance Measures): The results output by the operations analysis tool are combined to yield the desired throughput, delay, and travel time reliability strategies of effectiveness for the “before” condition.
Source: Cambridge Systematics, Inc.
- Design ATDM Strategy: Based on an assessment of the “before” conditions and an identification of the relative contribution of weather, demand, incidents, and work zones to the undesirable performance of the facility or system, the analyst selects and designs the ATDM strategy that he or she wishes to test. If ATDM is already in place for the before condition, then the analyst identifies the changes in the existing ATDM strategy to be tested.
- Convert ATDM Strategy into Operations Analysis Tool Inputs: The ATDM strategy to be evaluated must be converted into the appropriate demands, capacities, and control inputs required by the operations analysis tool for each specific scenario. This is a key part of the process – the user must be able to translate the effect of an ATDM strategy into the inputs used by the analytical engine.
- Apply Selected Operations Analysis Tool to Scenarios (Opening Day): The same operations analysis tool as was used in the “before” analysis is used to evaluate the “after” performance. Opening day demands are held essentially constant at this stage with the exception that drivers are assumed to cooperate with the new controls in effect (wait for ramp meters, obey new speed limits, etc.) and take advantage of any new capacity provided (simple lane shifts, but no route, time-of-day, or mode shifts). These are the demand changes estimated to occur on “Opening Day” prior to travelers experiencing or recognizing that the travel time has changed on the facility.
- Compute MOEs (Opening Day): The results output by the operations analysis tool for Opening Day are combined to yield the desired MOEs. The “after” results are assessed by the analyst to determine if the ATDM strategy should be fine-tuned and reevaluated.
The equilibrium effects of ATDM come into play as travelers on other facilities in the area recognize the time and reliability savings of the ATDM improvements on the subject facility and shift their route choice, time-of-day choice, and their mode choice to take advantage of the improved operations on the subject facility. For the purposes of estimating the benefits of ATDM investments it is not strictly necessary to account for the equilibrium effects of ATDM, because travelers drawn to the facility from other facilities (or modes or times of day) do so because they also experience a net benefit from the ATDM improvements to the subject facility. In addition, their leaving the other facilities also improves the operation of the other facilities for those drivers remaining on the other facilities.
Accounting for the equilibrium effects of ATDM is important when one wishes to obtain a more accurate estimate of facility performance after drivers in the area have adapted to the improved conditions. The procedure for equilibrating the estimated facility performance with ATDM is described in Appendix M: The Equilibrium Effects of ATDM.previous | next