Short of a time machine, travel demand models might just be the next best thing for taking a glimpse at the future of our transportation system.

So, what is a travel demand model?

Technically speaking, travel demand models are planning analysis tools that use statistical, econometric and mathematical algorithms to represent transportation system conditions and predict travel behavior response to those conditions.

Less technically speaking, a travel demand model consists of a series of interlinking computer programs that use statistics and demographic information to forecast current and future transportation system conditions.

That means ADOT can plug in data -- info like how many people live and work in various communities -- and the model will forecast the trips taken by drivers and transit passengers, where they’re going and what roads and transit services are being used. Basically, it gives planners a good idea of what future traffic patterns might look like and traffic’s impact upon air quality. (You might remember we talked before about modeling and predictions related to air quality.)

“It’s a planning tool that we use to anticipate traffic changes,” said Keith Killough, ADOT’s assistant director for travel demand modeling and analysis, adding that the model gives the decision-makers the information they need to make informed choices on transportation policies, plans, projects, and programs.

That’s because the model can actually run “what-if” scenarios by taking proposed changes (things like extra lanes, tolls, or new freeways) into account and give planners a realistic idea of what the alterations will do to traffic. This helps ADOT make decisions without costly trial and error implementations. In the past, planners had to rely solely on trend projections which generally do not consider capacity and growth constraints.

According to Killough, the key to better, accurate modeling results is to start with better, accurate data. He works closely with the Arizona State Demographer’s Office (in the Department of Administration) and utilizes data and assistance from throughout ADOT (particularly ITD, MVD, TSG, and ITG). Household statistics, employment data, travel surveys and more are taken into consideration.

The travel demand model is based on a lot of data and many different fields of science …

“It is complex, but still a very educated guess,” Killough said.

To make sure the model is producing the most accurate complete picture possible, the results are regularly checked and validated. Traffic counts help with this process.

For even more on how ADOT uses travel demand modeling, check out the Average Annual Daily Traffic (AADT) page.

A couple months ago we told you how ADOT works to minimize the noise impact a new freeway has on surrounding neighborhoods.

Well, similar efforts also are made when it comes to air quality …

ADOT is committed to improve air quality by making sure all projects comply with federal, state and local air quality laws and regulations.

That means ADOT’s Environmental Planning Group evaluates every ADOT transportation project to ensure air quality standards will be maintained.

ADOT Noise and Air Specialist Fred Garcia says smaller projects go through a qualitative review by his team to determine if the road work will have any impact on air quality. Larger projects (ones that increase traffic capacity or that have changes in the vertical and/or horizontal alignment) will go through a much more extensive quantitative analysis.

That quantitative analysis includes a sophisticated computer modeling process that helps predict localized air pollution concentrations in neighborhoods near the project.

According to Garcia, ambient background measurements must be taken first. ADOT gets these figures from either Maricopa County or from the Arizona Department of Environmental Quality.

That baseline number helps the modeling program generate an emissions report. From there, other conditions are entered into the program, including vehicle counts, atmospheric conditions and emission dispersion rates.

The program produces a report that can tell ADOT what the concentration of carbon monoxide will measure at different distances away from the future freeway.  The program is so sophisticated that it even takes the future design of cars into account. In coming years, vehicle manufacturers are required to make cleaner and more efficient engines … the computer program can factor that into its predictions.

Once the quantitative analysis is complete, ADOT can tell if air quality predictions will meet standards set by the U.S. Environmental Protection Agency. If standards are met, the project can move forward. If not, the project is modified to help meet those standards.

Transportation isn’t the only factor contributing to air pollution. Construction, manufacturing, and even wind, weather and Arizona’s terrain play a role. However, there are things everyone can do to help minimize their impact, including carpooling, using mass transit and driving less during rush hour.

For more on air quality and ADOT’s Environmental Planning Group, visit their webpage.