Tim Harlow, of the StarTribune’s The Drive column asks if “our roads are really more congested?” and interviewed me:
Of course, almost every time a report on traffic comes out, it makes the headlines. But how do we make heads or tails out of them when they seem to contradict one another?
“They are not necessarily contradictory,” said David Levinson, a professor of civil engineering at the University of Minnesota. “They all have different data, but they are measuring roughly the same thing.”
The differences can start with how the report makers define congestion. MnDOT defines congestion as traffic flowing at speeds less than or equal to 45 miles per hour. TomTom defines it as increased travel time when compared to free-flow conditions.
The data used to compile reports comes from different sources, too. MnDOT uses loop detectors embedded in the pavement while INRIX uses GPS data. Another difference is that the INRIX report looked at traffic volume and delays while MnDOT’s congestion report details the location and percentage of freeways experiencing daily congestion.
New business or housing developments can alter traffic flow in areas, making roads that were adequate suddenly become packed, creating the perception that congestion is getting worse.
“You might see more this year because the economy has picked up, but generally it has been flat and has been for a while,” Levinson said. “If it gets too bad, people will change their behavior. … There are limits on how bad the congestion can get.”