How long does it take to cross the street?
Determining how much time a pedestrian needs to safely cross the road at a crosswalk is surprisingly complicated.
Traffic engineers calculate that time using a set of formulas that consider the width of the street, an average walking pace and intersection design, among other things.
Those calculations had previously assumed that walkers and rollers could travel 3.5 feet per second from the top of a curb ramp to the other side, before the light changes.
Now, through conversations with a pedestrian-focused advisory group, the Seattle Department of Transportation has updated its guidelines to give people a little more time to cross — a move celebrated by advocates for seniors and people with disabilities.
The change is an example of seemingly small tweaks that pedestrian advocates say can make a big difference in street safety, without requiring major construction or costs.
“It’s a really good step in the right direction,” said Anna Zivarts, the Disability Mobility Initiative director at Disability Rights Washington, who was part of the group advising SDOT. “I think it will make crossing the street much safer and much more comfortable for a lot of folks.”
Lars Sveum-Hanson, 74, welcomes the change. He lives at the Cedar Park senior apartments in Lake City and said the walking conditions in the neighborhood “put me in some unsafe situations.”
Before heading to the bank or to the Lake City Presbyterian Church for music and film classes, Sveum-Hanson dons a rainbow-colored hat and a bright-orange vest he kept from his days as a Minnesota bus driver.
But even with vibrant clothing and accessories, he still fears being struck while crossing the street.
“I’m pretty agile, and still the meter almost always runs out before I get to the other side,” he said.
Akira Ohiso, a social worker with Sound Generations, an organization that provides activities for seniors, said he regularly sees older people struggling to cross streets before the light changes.
“Crosswalks in busy urban streets do not allow enough time for older adults to cross safely, especially four-lane streets,” he said.
Under SDOT’s new guidelines, signals will be retimed with the updated formulas when existing intersections are modified, when community members request a change and when new signals are installed.
The city uses two different formulas for determining how long pedestrians get to cross the street, and it can get a little complicated.
To see how this works, consider the new signalized intersection at Northeast 125th Street and 28th Avenue Northeast in Lake City.
At Northeast 125th Street, walkers under the new guidelines have a total of 22 seconds to cross — eight seconds for the steady “walk” signal and 14 seconds for the flashing “don’t walk” signal.
The minimum amount of time for the “walk” signal is seven seconds, but SDOT can add more time depending on the width of the street and other conditions. At Northeast 125th, the time is eight seconds, said Dusty Rasmussen, a traffic-signal engineer at SDOT.
The time for the flashing “don’t walk” signal is based on the width of the street and the new assumption that most walkers can travel 3 feet per second — a slightly slower rate than under the old guidelines.
The distance between the tops of curb ramps on either side of Northeast 125th is 42 feet. So dividing 42 by 3 yields the 14-second flashing “don’t walk” signal. The idea is to give pedestrians time to clear the intersection if they enter the crosswalk as the flashing signal begins.
Engineers then use a different set of calculations to check whether the total crossing time — that is, the “walk” signal plus the flashing “don’t walk” signal or red flashing hand — is enough.
First, they measure the distance from the push-to-walk button on one side of the street to the top of curb ramp on the other side. That’s 48 feet on Northeast 125th. They use that measurement because a visually- or hearing-impaired person might need to stand further back from the curb to feel the vibrations from the push-to-walk button that signal when it’s safe to walk.
Engineers then take that distance and divide it by 2.5 — using the assumption that most people can walk or roll 2.5 feet per second. For Northeast 125th, 48 feet divided by 2.5 seconds per foot yields 19.2 seconds of total walking time.
That confirms that the 22-second crossing time is within the guidelines, Rasmussen said. If the calculation produced a number less than 19.2 seconds, engineers would add more time to the walk signal.
After 22 seconds, the flashing signal becomes a steady “don’t walk” signal. Cross traffic gets the green light a few seconds later.
SDOT’s crosswalk changes will mostly balance out for drivers, meaning that slightly longer red lights usually also lead to longer green lights, said spokesperson Ethan Bergerson.
The longer crossing times build on an earlier initiative that gives pedestrians a few seconds head start to enter the intersection before cars and trucks headed the same way are given a green light. The head start is intended to make pedestrians more visible in the crosswalk.
The recent changes also give more guidance to whether pedestrian signals should be automatic, as part of the traffic cycle, or rely on push-to-walk buttons.
An automatic walk signal will be used in busy parts of the city and, in some cases, during busy times of the day, under the new policy.
A push-to-walk button may be more appropriate in an area with fewer pedestrians.