Emissions Analysis Techniques for TCMs
Overview - Traffic simulation models are used to evaluate the impacts of changes in traffic volumes and transportation network characteristics (capacity, signal timing, etc.) on traffic flow patterns (vehicle speeds, acceleration, and delay). A number of the models are capable of calculating changes in emissions resulting from changes in traffic characteristics.
Strategies Addressed - HOV lanes; traffic flow improvements.
Methodology - Traffic simulation models can be divided into two general classes: macroscopic and microscopic. Macroscopic models (TRANSYT-7F, TRAF, FREQ) are based on deterministic relationships between roadway and intersection characteristics and traffic flow. Microscopic models (TSIS, INTEGRATION, Paramics, SIDRA, Synchro/SimTraffic) simulate the movement of individual vehicles through the network being modeled. In either type of model, emissions are estimated based on vehicle speed and acceleration characteristics.
Some simulation models are designed for analysis of individual intersections or specific types of facilities, while others are designed for network-level analysis. Models capable of network-level analysis include TRANSYT-7F, Synchro/SimTraffic, INTEGRATION, and Paramics. The TRAF and TSIS sets of models include NETFLO and NETSIM (respectively) for network analysis, FREFLO and FRESIM for freeway analysis, and CORFLO and CORSIM, which are integrated packages of the network and freeway models. SIDRA is designed for intersection analysis. FREQ simulates corridor traffic operations including one freeway and one parallel arterial.
Data Requirements - Requirements for traffic data include volumes by link and intersection turning movement, link travel times, and percent heavy vehicles and buses. Requirements for network data include characteristics of each link (e.g., lanes, turning lanes, speed limits, lane widths) and intersection (e.g., signal phasing).
Outputs - Changes in average speeds, travel time, delay, and emissions.
Level of Effort - The individual models vary in their ease of use. A significant amount of effort generally is required to learn to use traffic simulation models, including setting up the appropriate inputs and parameters. (Most regional and local transportation agencies will have staff that are familiar with the use of one or more of these software packages.) A significant amount of effort may also be required to obtain traffic and network data to conduct the analysis and to calibrate the model to local conditions. Data requirements are proportional to the extent of the network being modeled.
Advantages - Traffic simulation models are the best means of estimating changes in emissions resulting from strategies that affect traffic flow, and can provide a relatively accurate assessment of impacts. Microsimulation models can account for the effects of the variance of driver behavior on emissions.
Limitations - The models generally require a non-trivial analysis effort as described above. Also, the quality of the emissions data and the vehicle trajectories (speed/acceleration profiles) produced by the models varies; for example, emissions factors may not be available or are often based on old data and may not closely represent the current vehicle fleet.