Chicago commuters can rely on traffic being unreliable. That’s according to a new congestion study that looks at travel times and its impact on the local economy.

To measure travel reliability, you need to look at the amount of change there is in the traffic from day to day. Those variables could be a random accident, a presidential motorcade or a thunderstorm.

David Schrank co-authored the Urban Mobility Report from Texas A&M.

“One day it’s an hour. The next day it’s half an hour. The variability is much higher, so your planning time index goes up significantly because you the motorist don’t know what to expect when you get in your vehicle,” he said.

Shrank says the planning time index (PTI) is 3.95 for Chicago freeways.

That means you’d multiply that number by how long your commute would take without traffic. If it takes you 20 minutes to drive to work without traffic, then plan on leaving 79 minutes early to better ensure a timely arrival.

Shrank says it won’t always take that long to get to a destination. Some days, you might be very early.

The Washington D.C. area has the highest PTI value at 5.72. It also ranks at number one for worst congestion in the country.

Shrank says in 2011, overall congestion cost Chicago more than $6 billion, or just over $11,000 per commuter. That includes fuel costs and truck congestion costs.

Shrank applauds the city on some strategies, like the use of ramp metering. Those are the traffic lights at the entrance of the freeway.

“Those can keep traffic from backing up behind an incident as badly and keep some movement happening on the roadway instead of becoming a parking lot because there’s a crash or a stalled vehicle,” he said.

Randy Blankenhorn with the Chicago Metropolitan Agency for Planning says more could and will be done to get at the problem.

He says the agency is proposing congestion pricing on five new expressway projects under the GO TO 2040 plan. Congestion pricing is a way of charging users, like tollways.

“I think that the plan tries to look at how this region is going to grow and what that means for congestion. We’ve got to do a better job of managing the way people move in and out and around metropolitan Chicago,” he said.

Blankenhorn says the proposal also includes adding lanes to existing expressways.

“If we did this on the Stevenson Expressway where we added a lane and made it an express toll lane, you could really reduce your travel time from over 45 minutes down to 25 minutes at a cost of about $2.25,” he said.

Blankenhorn also says the report is evidence that public transportation needs to be improved around the region.

WBEZ spoke to Chicago commuters like Tony Brouzas about the report. 

“You can never tell which roads are going to be under construction downtown. It always seems like there’s some major thoroughfare that is closed off,” he said. “[Traffic] makes you not want to come downtown or to try to take the Metra—but the Metra raised the prices so much it almost makes more sense to drive than to take the train.”

Scott Kanowsky contributed to this report.