Planners routinely overestimate how much traffic will grow in the future in order to justify new highways. Usage of the Intercounty Connector is still growing but it looks like the ICC, too, will get less use than planners thought.

ICC traffic levels, in vehicle miles traveled per year. Image by Claire Jaffe.

At first glance, traffic on the ICC seems sparse, and as many journalists report, drivers are taking far fewer trips on the road than predicted. But the trips are longer — about 9½ miles on average, compared to the 6½ miles forecasters expected. Also, the road’s initial “ramping up” phase, which is when traffic grows rapidly as drivers learn about a new highway, has not yet ended.

Shifting forecasts

The total miles traveled on a road in a year, which engineers call vehicle miles traveled (VMT), takes into account both the number and length of trips; VMT gives a more accurate measure of traffic density than looking at the number of trips alone. ICC forecasters, like the journalists, focused on trips rather than VMT, but we can tease some VMT approximations out of their reports:

  • In 2004, when the state decided to build the highway, planners foresaw VMT equal to 433 million miles in 2030.
  • A more detailed study in 2006 predicted 325 million VMT in 2020 and 390 million in 2030.
  • In 2009, after construction began, estimates went down further to 278 million in 2020 and 319 million in 2030.

From numbers in the state toll authority’s financial statement for the twelve months ending last June, I calculate that VMT in that year was about 195 million.  If traffic increases at the rate the forecasters expected (and the just-opened connection to US 1 adds 7% more VMT), it will reach 281 million in 2020 (the upper green line on the graphic above).  While that would fall short of pre-construction forecasts, it would at least match the estimate from 2009.

No matter the forecast, the ICC is seeing less usage than planners expected

But it doesn’t look like things are headed that way. When they made their models, planners assumed that the overall use of motor vehicles would grow. Data shows that this trend ended about 10 years ago, both in Maryland and elsewhere, and there’s not much to say it will start back up. Without long-term increases as part of the projection, the future level of ICC traffic drops to a steady 241 million miles a year (lower green line). That’s substantially below all the forecasts.

The range of possible outcomes is even wider than those two numbers suggest. If the initial ramp-up, whose duration is very hard to know beforehand, continues longer than expected and automobile use resumes its historic growth, future traffic might meet the early forecasts.

That, however, is not what’s been happening lately. If anything, the ramp-up is coming to an end more quickly than the forecasters predicted. Moreover, if affluent homeowners leave the outer suburbs and new residents who can’t afford tolls move in, traffic could peak short of 241 million VMT and then decline.

Even at best, the ICC’s tolls will repay only a third of its construction costs.  If, as seems probable, traffic on the ICC falls short of the 2009 forecasts, it will need even bigger subsidies. Before Maryland spends more money on toll highways, it should give its projections a second look.