Large five-lane intersections dominate Bellevue. To eke out every little bit of roadway capacity, the city in 2015 finished installing adaptive signal technology at all 203 of its signalized intersections. The system adjusts the timing of the traffic signal cycle based on real-time traffic conditions. In theory, the less unused green left at the end of each light cycle, the better, resulting in more traffic moving through the intersection.
This system, known as the Sydney Coordinated Adaptive Traffic System (SCATS), has reduced afternoon delays at some intersections as much as 43%, but is also benefiting pedestrian and bus riders, the city says.
Traditional traffic signals work on a fixed cycle, which might allow for a couple of settings to be used over the course of the day. But the timing for a traffic signal cycle that works best for the morning commute isn’t always the most efficient for the evening rush or during non-commute hours. Instead, SCATS uses detectors embedded in the roadway to constantly monitor traffic volumes at intersections, adjusting cycle lengths based on current demand. The system tries to decrease delay by reducing the amount of unused green time during each cycle. Generally, the higher the traffic volume through an intersection, the longer the cycle length is to serve the demand.
Bellevue traffic engineers developed ‘personalities’ for each intersection, establishing a minimum and maximum length for each cycle. During non-peak times, intersections operate independently, but to handle commute traffic intersections are ‘married,’ and any cycle length timing adjustments are applied to the entire married group.
If traffic demand indicates more time is needed for a particular movement in the cycle, the adaptive system adjusts the timing for the next cycle. But if too much time is provided, the system can also trigger a red light early, reducing the amount of unused green.
“Generally, we’re getting cycle lengths that are a lot lower than what we had programmed before for the whole downtown,” said Chris Long, Bellevue’s Traffic Engineering Manager. “Because we would have had to program it for the worst conditions, now we can let it flex to that worst condition then come back down to a more normal state.”
The lower the cycle lengths gives pedestrians more opportunities to cross Bellevue’s massive streets, Long said.
“We have very big intersections. For a downtown environment, it’s unusual to have these five-lane roadways with protected, permissive left turns at all approaches,” Long said. “So it creates very long cycle lengths, so we want to get those as short as possible.”
The system also allows for safety features that give pedestrians an advance green light a few seconds before cars, protecting walkers from potential collisions with right-turning vehicles.
Adaptive technology backfired on pedestrians in Seattle last summer after the city installed it along Mercer Street from 3rd Ave W to Fairview Ave N. Shortened walk times had pedestrians rushing to cross intersections, while the real winners were the drivers whose commute along the corridor was cut in half during peak times, from 34 minutes to 17.
SDOT has since readjusted walk times, giving pedestrians significantly more time to cross the street. At the time, SDOT wrote in a blog post that though the amount of time given to pedestrians to cross Mercer St. may be shorter, the technology will actually provide pedestrians more opportunities to cross the street.
Bellevue has also programmed intersections to provide an early green to buses. Long said there is extra time, ‘slop time’, built into the cycle and if a bus is detected in the queue, the ‘slop time’ goes to the bus.
King County Metro Transit says combining SCATS with Transit Signal Priority (TSP) has improved travel times for buses along the B Line. A retiming of the signals along the B’s route last year resulted in travel time savings of up to 6 percent, according to the transit agency. But just using adaptive technology alone, without TSP, Metro said, may not improve transit travel times.
At some point during rush hour, especially where two large arterials come together, traffic engineers have to decide which street to prioritize. In those cases, the engineers weigh which approach has the capacity to store more cars without impacting other intersections.
The system can’t adapt to huge swings of traffic volumes which occur during special events such as the city’s Fourth of July celebration. Instead, a pre-planned special events strategy is deployed on the system.
“Prior to having the adaptive signals, it could take as much as two hours to clear out all the parking garages,” Long said, “and now it’s down to 45 minutes.”