Traffic Forecasting – Past Performance Does Not Guarantee Future Results
I'm working on a traffic study where the reviewing agencies asked us to prepare 20 year forecasts in addition to looking at the build out year. Typically, we look at data trendlines on nearby roads and throughout the county to determine a "typical growth rate" for traffic in the area. This has historically been an annual growth rate of 1 to 3%. This often leads us to factoring traffic up 50% or more over 20 years and then layering on the traffic from the proposed development. Factoring existing traffic volumes up approximately 50% is also how traffic forecasts are often prepared for road design projects.
I'm strongly reconsidering this approach. Consider Figure 1 below from the Federal Highway Administration's Office of Highway Policy Information website. From 1986 to 2006, traffic on all of our highways did fit the model of growing by about 2 to 3% a year (or 50% to 60% over the 20 years). But since 2005, we've had a significant drop in traffic and the trendline sure makes it look like traffic growth has plateaued.
I've also been reading through the Texas Transportation Institute's 2011 Urban Mobility Report. They talk about the terrible state of congestion. I'm not going to argue that congestion doesn't suck, but take a look at Exhibit 1 from their report. Compared to 1982, congestion is indeed terrible now compared to 30 years ago. But looking at the 2000 to 2010 data shows congestion peaked in 2005 and we're not much worse off than we were a decade ago.
Also consider the August 2011 to August 2010 data below from the Federal Highway Administration. From their website, "Travel on all roads and streets changed by -1.7% (-4.6 Billion vehicle miles) for August 2011 compared with August 2010."
But maybe Minnesota's different…… NOPE. Below is Figure 1 from Mn/DOT's Vehicle Miles of Travel Trends in Minnesota: 1992-2010 showing traffic in Minnesota has followed the national trend in plateauing.
Based on national and local trends, my conclusion is that it is very reasonable to think traffic growth has plateaued. The punchline for traffic impact studies: the "no-build" traffic forecasts should be the same as the existing traffic volumes. We don't need to do opening day forecasts and 20 year forecasts because they can reasonably be expected to be similar.
And given our huge budget shortfalls, this should also mean a policy of fixing the infrastructure we have. NOT expanding our transportation system to add capacity.