Drive time: How we calculate travel times on Washington roads
Jun 19, 2018, 5:30 AM | Updated: 1:13 pm
When KIRO Radio 97.3 FM reports it’s an hour from Everett to Seattle, how does the station calculate that drive time and exactly where does that trip begin and end?
Chokepoints: Western Washington traffic news
I had a long email conversation with listener Tracy over the weekend that started out like this: “How do you figure out how long the ‘drive time’ is from city-to-city? Is it from taking pictures of license plates? Did someone have to actually drive it 20 years ago? Just been wondering.”
It’s a very common question. I base the travel times on the Washington Department of Transportation’s calculations. The state has embedded sensors in the roads that can judge when vehicles go over them and how long they stay on them.
“The travel times are calculated using loop detectors in the road,” state traffic engineer Maan Sidhu said. “They measure how many people are on the road, how long they’re staying in one spot, and their speed.”
All that data gets plugged into the computer algorithm before spitting out a travel time.
“This application is looking at all the loop detectors, from Everett to Seattle (for example), calculating speed and time, adding it up between all of these loop detectors to give you a travel time,” Sidhu said.
The data is updated every five minutes in real time, and Sidhu said it’s reasonably accurate.
“We find that it’s within 10 or 15 minutes,” he said.
Drive time records
In more than three years working with the system, I have found it to be even more accurate than 10-to-15 minutes on most days.
And if you’re wondering how I can compare the current travel time to what it should be, Sidhu said they keep three months worth of data for every destination combination for every time of day of every day of the week as a comparison. So when I say “you’re losing 10 minutes over what you should be” I am basing that on the travel times for that exact time of day, and day of the week for the previous three months.
As for what counts as Seattle or what counts as Everett, it’s pretty easy. I use the state calculations. Seattle is considered University Street. Everett is considered 41st Avenue.
