The development of a trip-assignment model designed to systematically compute emissions-optimized traffic flows for carbon monoxide is explored. Then the exploratory theoretical emissions-optimized tripassignment model is used to estimate the maximum carbon monoxide emissions reductions under varying congestion levels on a hypothetical network. The experimental results indicate moderate reductions in system-level vehicle emissions under emissions-optimized trip assignment compared with the conventional time-dependent user-equilibrium and system-optimized models. They suggest a possible method for approximating an upper boundary of expected emissions reductions using the standard travel-demand model with a modified trip-assignment objective function. The approximate boundary is useful as an estimate of emissions reductions that might be possible under optimal conditions (e.g., drivers choose emissions-optimized routes, fully implemented intelligent transportation systems, etc.). In other words, the upper boundary suggests that current traffic optimization strategies may be insufficient to fully remedy air quality problems created by mobile-source emissions.
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