Topics: Traffic Forecasting, Transportation Forecasting

Websites and Citations:

Atlanta Regional Commission Travel Demand Model Documentation

Theme Music: Five Star Fall, Mercurial Girl, Magnatune.com

Episode 45 – Traffic Forecasting

Hello and welcome to another episode of Talking Traffic. My name is Bill Ruhsam and I host this podcast and its sister website, talkingtraffic.org. Today is March 3 and this is episode 46 of Talking Traffic. Today’s episode topic is Traffic Forecasting.

I know what you’re thinking…Boooooorring. And I won’t lie, this is not the most engaging topic ever. Like, you’ll probably never sit down around a dining room table with friends, open a bottle of wine and say, “Let’s talk about the traffic analysis zones near home to better engage in the discussion of future land use with our local government!” Well, unless you hang out with a bunch of transportation planners. If that’s the case, bring more wine.

Before we get into some of the details, let me give you some definitions.

Traffic Forecasting, or Transportation Forecasting, according to the Wikitionary, “…is the process of estimating the number of vehicles or people that will use a specific transportation facility in the future.” In the general context, “vehicles” means literally any vehicle: Bikes, Autos, trucks, trains, ships, airplanes, etc. So transportation forecasting means any transportation. I’m going to concentrate on the specific example of forecasting that deals with roads and highways.

What this means on a day to day basis depends on what it’s being used for. I’ll give you some concrete examples to illustrate the differences.

Ok, your local municipality may have a problem with an intersection that is obviously related to the number of people turning right. The turning traffic is backing up into the through lane and causing a congestion problem. A solution is to make a turn lane, or extend the existing turn lane. But how long should that turn lane be? Nobody wants to spend more money than necessary, so you want to extend the turn lane as long as it needs to be, but no longer. And that “need” should extend into the future to meet the future traffic that will be coming through the intersection. So a traffic forecast here would simply be to predict the future number of vehicles using the intersection and turning right.

Now let’s say you’re Walmart and you want to install a new SuperDuperWalmartCenter. Obviously, as soon as you open you’ll have a direct impact on the street right next to the Walmart, but what about downstream at the next several signals, or the hypothetical interstate interchange that’s down that road? Now we’re talking about a whole system of roadways being affected by a large installation. That requires a much more detailed and in-depth analysis to forecast the traffic that will occur.

For a third example, let’s say you’re a metropolitan region. You need to plan for the future based on your best available information. You need to know where the traffic congestion will be worst 10, 20, 30 years from now. You have literally thousands of different variables and assumptions to incorporate into a forecast model that will look through its crystal ball out into the future. Based on that model, you’ll evaluate your transportation network and (hopefully) develop a plan to meet your future needs.

These are examples of the different scales of traffic forecasting. But they all generally use the same process. The process is called the four-step transportation forecasting process. Step one is Trip Generation, Step two is Trip Distribution, Step three is Mode Choice, and Step four is Assignment.

A “trip” is a journey from an origin to a destination by a particular vehicle type. These types are referred to as “modes”. So if you walk from your doorstep to the CVS, you’ve just made a trip by the pedestrian mode. The walk back is another trip by the pedestrian mode. If you drove, it would be a trip by an automobile mod[...]