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Traffic's path of least resistance
While driving the streets of Federal Way, Raid Tirhi's time is not spent admiring the landscape.
Instead, Tirhi observes the driving patterns of those around him and takes note of needed traffic signal improvements.
Tirhi, a senior traffic engineer, has been in charge of ensuring Federal Way streets are safe to travel since 2002. As part of his job, Tirhi adjusts the timing cycles on street lights so that motorists can travel from "point A to point B" without having to stop at every light along the way.
"Signal coordination is always a moving target," Tirhi said. "Traffic is always moving, changing and increasing."
In Federal Way, street light coordination is most obvious along four high-traffic corridors: South 348th Street, South 320th Street, Pacific Highway South and 21st Avenue Southwest.
Tirhi's goal along these roadways is for motorists to travel the length of the streets without stopping frequently.
Determining how best to implement signal coordination can be tricky. One of the biggest factors in whether drivers will benefit from signal coordination is the number of vehicles traveling the road, Tirhi said.
If there are too few or too many vehicles on the road, signal coordination will be useless, Tihri said. With too few vehicles, motorists will wait at lights that would otherwise be signaled by their approach. With too many vehicles, it is impossible to avoid clogged streets with or without coordinated signals, he said.
Generally, motorists travel busy streets in packs, or cues. Tirhi's goal in his signal coordination is to keep these cues moving and keep them from becoming one long stretched out congested mess.
To achieve a steady flow in traffic, signals must utilize a signal cycle length.
All lights within a coordinated system are set according to the cycle. The cycle details how many seconds are allowed for each movement (such as a green arrow for left turns) within a given intersection to be completed.
For example, Enchanted Parkway and State Route 18/South 348th Street is a well-traveled intersection where signal coordination is in place. Here, eight possible traffic movements that require traffic signal indications may take place. The lights at this intersection will rotate through the eight movements in a 150-second cycle length, Tirhi said.
The lights at Pacific Highway and South 320th Street are coordinated for the South 320th Street corridor. Their signal cycle length is 140 seconds, meaning that any driver who approaches the intersection and is forced to stop at a red light will not receive a green light in less than two minutes and 20 seconds.
Drivers may only be stopped for a few minutes, but for some, a minute here and there adds up. This is why Tirhi advises that drivers travel the coordinated systems at the speed limit and take full advantage of all the lanes.
In many cases, drivers speed from one light to another. This approach leaves the driver waiting at a red light for the coordination to catch up. In other cases, drivers get stuck in traffic because several cars line up in one lane, leaving another lane open, Tirhi said.
Traffic must be equally distributed for signal coordination to work at its best, he said.
Appealing to the majority
Tirhi must take into account where the majority of traffic is going or coming from when deciding how to coordinate traffic signals.
Because he is unable to provide a green light for everyone at a given intersection, he chooses to provide that green light where it will be most useful.
"I have to sacrifice the minority of traffic for the majority of traffic," he said.
For example, in the morning hours of a weekday, most people traveling through the intersection at Enchanted Parkway and South 348th Street are driving east on South 348th Street or north on Enchanted Parkway toward Interstate 5, Tirhi said.
Throughout the day, each coordinated stretch of lights goes through a series of four to six cycle changes. While the total signal cycle length remains the same, Tirhi programmed the signal controller box to allocate more or less time to complete individual movements.
Movements with higher traffic volumes get more green light time. For example, between the hours of 5 and 9 a.m. on weekdays, drivers traveling east on South 348th Street toward Interstate 5 through the intersection at Enchanted Parkway will receive a green light that lasts for 69 seconds, Tirhi said.
However, from 2 to 7 p.m. drivers traveling east through the same intersection will witness a green light lasting 57 seconds, he said.
Allocating green light time and determining where traffic will benefit from signal coordination is a process that requires computer data as well as firsthand observations.
Tirhi is continuously altering the signal cycles to better serve Federal Way drivers. Although he cannot fix all the traffic signal problems, he encourages people to let him know where improvements can be made.
"If I knew of a problem, I would like to try to fix it," he said.
Contact Jacinda Howard at firstname.lastname@example.org or (253) 925-5565.
South 320th Street corridor:
Between the hours of 4 p.m. and 6 p.m. on weekdays, more than 5,000 vehicles travel through the intersection at South 320th Street and Pacific Highway South, Tirhi said. Because of the large number of drivers traveling on South 320th Street it has 12 coordinated traffic signals beginning at 1st Avenue South and continuing East to South 321st Street.
Pacific Highway South corridor:
Pacific Highway South is a key access road to and from Federal Way. It delivers drivers from neighboring cities, such as Tacoma and Kent, into downtown Federal Way. Pacific Highway South also has 12 coordinated traffic lights in place. The coordination begins at South 288th Street and continues south to South 340th Street.
South 348th Street Corridor:
State Route 18/South 348th Street/Southwest Campus Drive is another stretch of road that has coordinated traffic signals. Here, between the hours of 4 p.m and 6 p.m. more than 6,000 vehicles pass through the intersection at 348th Street and Enchanted Parkway, Tirhi said.
Three lights are timed to provide flowing traffic on State Route 18/South 348th Street between the crossroads of Enchanted Parkway and 9th Avenue South. The light at 1st Avenue South is part of the coordinated system on weekdays from 2 p.m. to 7 p.m. Plans to include this light at other times of the day are forthcoming, Tirhi said. Coordination also occurs farther west on Southwest Campus Drive between the lights at 21st Avenue Southwest and Hoyt Road Southwest.
21st Avenue Southwest corridor:
Six traffic signals make up this coordinated system, which runs from Southwest Dash Point Road at the Southwest 312th Street intersection to Southwest 336th Street.