Did you know that TMODEL2 can now be used to compute emissions on a 24-hour basis? With the last update in November 1994 a new module (section 1.7.2) was released that makes emissions computations easier than ever.

One of the problems of computing emissions from transportation planning models has been that modelers sometimes adjust design or free flow speeds to make the model calibrate. A slight change in these speeds may be necessary to reflect true conditions. Large changes, however, may not be as representative of true operation. Emissions can be very sensitive to operating speed. It is best to use a design speed as close to reality as possible so that the link and node delay equations can be used to compute representative results.

In fact, this has been our philosophy here at TModel Corporation from the beginning. We believe in modeling streets (links) and intersections (nodes) with realistic (spelled ENGINEERING) speeds and capacities. Using "planning" speeds and capacities, which need to be used in link- based models, may make the model unsuitable to compute emissions. If you are using your model for emission computation purposes, it is recommended that you compare model operating speeds with observed speed/delay data.

However, if you have adjusted speeds for assignment calibration purposes you can still compute the emissions using realistic speeds. The assignment volumes can be inserted into a link file with realistic speeds by using the compare/combine feature in the link line editor (section 1.2.3). Also, if you want to make adjustments to the volume assignments (based upon known weaknesses in the calibration), these adjustments can be made in this same compare/combine section.

Emissions are computed for each of the 24 hours by coding in a traffic volume variation profile. Of course, you should use profiles specific to your area. Up to ten profiles are allowed. Emission computations are made using the revised or adjusted assignment volumes, the speeds present in the link file to be analyzed, as well as the link capacities, node capacities, and link and node delay coefficients. The module then computes adjusted volumes and uses these volumes to compute operating speeds and delays, VMT, and VHT for each hour. This way, the speeds and volumes at peak hours are different than those at off-peak hours. This should result in greater accuracy than using average speed and volume.

The missing data that you must supply are the emissions factors for each emission that you want to analyze by speed. These vary from area to area based upon the age/use mix of vehicles. Your State DOT should be able to help.

This procedure is being used, but we want to keep improving it. We are currently planning on adding options to compute the emissions on a disaggregate basis for links and nodes as well as to compute emissions into the link and node files for graphical display and further analysis. After trying it out on your projects, please give us a call with your comments. Thanks!