How the WSDOT determines average travel times
Nov 6, 2014, 5:29 AM | Updated: 11:54 am
How accurate are those average travel times you hear me read on KIRO Radio’s Morning News traffic reports?
Sue Winter wrote in wondering just that question.
So how are those times derived?
Maan Sidhu, traffic engineer with the state’s traffic management center, says the normal or average time is based on historical data for that day and time over the past three months.
“Every day of the week, for every five minute period, we’re calculating travel times and averaging them,” Sidhu said.
So when you hear me on the radio say it should take you X amount of time to go from Everett to Seattle, it’s based on what it actually took to make that drive at that time on that day for the last three months.
It’s a large enough sample size to factor out a major accident on one day.
When I tell you how much time it’s really taking you to make that drive, it’s based on actual data being collected in real-time.
The state has embedded metal coils, called loop detectors, in the pavement on the freeways around the region that measure those times.
“We can add up the speed and distance among all these loop detectors we have, let’s say from Everett to Seattle, to calculate a time,” Sidhu said.
Those detectors send signals to nearby junction boxes, which upload the data to the state’s website. The detectors can tell when cars pass over them or if a car is sitting on top of them for an extended time.
They’re accurate about 95 percent of the time, according to Sidhu.
“We find that it’s within ten or fifteen minutes, usually,” he said. “It’s fairly accurate in that drivers can at least estimate how long it will take them to get to their destinations, so they can make that phone call to their office and say ‘hey I think I’m going to be late.'”
Normal will change over time, for better or for worse. It’s a three month average. Right now, what is normal factors in the entire month of August, which traditionally has less traffic.
Hear Chris Sullivan’s traffic reports every morning on KIRO Radio 97.3 FM
