Signaling the future

Feb. 19, 2016

Success with adaptive signal system in Bellevue, Wash.

The city of Bellevue, Wash., is booming, with rapid growth in both residency and jobs. 

Located 10 miles east of Seattle, the city’s population now stands at 135,000, with more than 140,000 jobs. By 2035, Bellevue is expected to grow by about 30,200 people, 52,000 jobs and 15,800 housing units. With all of the recent and predicted growth, Bellevue traffic engineers have faced a big challenge: How to keep all modes of transportation moving in an efficient, yet safe manner. To complicate matters, property is in short supply and extremely expensive, so it wasn’t just a matter of adding more lanes to build toward traffic Nirvana.

For those reasons, the focus was on using existing infrastructure more efficiently, and the city has invested heavily in smarter traffic lights that utilize cutting-edge technologies. By the end of 2015, Bellevue was operating all of its 200 traffic signals via an adaptive traffic-signal system called Sydney Coordinated Adaptive Traffic System (SCATS). SCATS has taken five phases across five years to implement citywide with a price tag of $5.5 million. 

City of Bellevue engineers’ objectives in adopting and implementing SCATS were to:

• Minimize user delay for all modes of transportation including pedestrians, bicyclists and drivers;

• Minimize the number of stops and rear-end collisions;

• Manage traffic queues to reduce left-turn areas blocking a through lane or an upstream intersection; and

• Maximize the green bandwidth yet also utilize the optimum cycle length.

The system

Traffic engineers understand that reducing delay at signalized intersections requires increasing the cycle up to the optimum cycle length. Additional increases in the cycle generate traffic inefficiencies that yield delay, frustration and possible collisions. Bellevue’s adaptive signal system doesn’t just save time for travelers and reduce air pollution from idling vehicles, it’s user-friendly for engineers.  

The system: 

• Shows second-by-second information such as current cycle and splits;

• Provides charts for historical volume data;

• Logs an emergency vehicle pre-emption and gets back in sync within two cycles; and

• Alerts the engineer if a vehicle detector or a pedestrian push-button fails. 

Major disadvantages to the system are that SCATS is limited to 24 detectors and seven stages or phase groups. Therefore, lead/lag left-turn phasing could be limited to some directions. Also, only four of the stages get to vote for more time, while all other stages get a predetermined percentage of the cycle.

After communication, the most critical part for any adaptive signal system to properly work is detection. SCATS uses the gaps between vehicles to calculate a degree of saturation (DS). The max DS in the lane group is used for each cycle. Green times are then assigned based on average DS in the last three cycles of all voting stages. When the intersection DS goes up, the cycle length increases up to a maximum set by the engineer. 

The optimum cycle length minimizes both delay and cycle time.

Changes to traffic conditions such as school schedules, holidays, collisions, snow days or even the normal variations in daily operations yield dynamic traffic conditions requiring an adaptive signal system. SCATS can handle daily events automatically. On the other hand, if goals and objectives change, then the engineer needs to intervene with special changes. 

One example of an annual city-sponsored event in Bellevue is the arts and crafts fair, which draws more than 300,000 people to the downtown area over a three-day period, during which time engineers program permanent pedestrian demands for specific intersections. Another example is Bellevue’s July 4 celebration, which accommodates more than 50,000 people trying to leave downtown at the same time immediately following the fireworks, for which special changes such as increasing the maximum cycle to 180 seconds and increasing walk times from seven to 30 seconds are implemented. During the July 4 event, engineering objectives change from moving pedestrians only, to serving both pedestrians and vehicles, and finally to flushing out traffic. City of Bellevue engineers invested their time in understanding special-event traffic patterns, then continued to fine-tune the system, yielding demonstrable success in clearing the majority of traffic and saving drivers up to two hours of time wasted in gridlock.

Seeing results

Bellevue converted 110 left-turning movements from protected-only to protected/permitted phasing utilizing a flashing yellow arrow (FYA) configuration. Using a $15/hour driver’s value of time, the conversion reduces driver delay and yields $3.2 million in annual public savings. Bellevue is the first agency to create and utilize an adaptive left-turn FYA in the SCATS system. When traffic volumes reach specific levels, engineers skip the FYA and run a protected left-turn phase. This is done for driver safety due to sight distance resulting from long traffic queues or gap-acceptance issues with heavy volumes. Engineers also programmed a FYA with a “Ped-Minus” phasing. When a pedestrian push button is activated, the FYA-permitted phase is skipped for pedestrian safety. 

Due to slow freeway ramp-metering rates, traffic backs up onto city surface streets and large platoons of left turns have no space to go to. SCATS allowed engineers to double-service a left-turn phase using two smaller splits within the same cycle. It also helped shorten three blocks of traffic queues and reduce the evening peak gridlock from three to two hours. Figure 1 shows before-and-after results of SCATS implementation.


For each SCATS phase of implementation, engineers conduct before-and-after travel-time studies, documenting where drivers stop, for how long, and total travel times within a corridor. For example, 148th Avenue carries an average of 40,000 vehicles per day and has 19 traffic signals within a 4.5-mile stretch. Before and after SCATS studies yielded up to 38% reduction in the number of through stops. Implementing SCATS on other principal arterials such as NE 8th Street in the downtown area yielded up to 43% improvement in total travel time, as shown in Table 1.

Moving traffic efficiently has yielded safety benefits in collision reductions. A review of the collision database and a comparison of before-and-after SCATS implementation revealed a 22% reduction in red-light running collisions. 

The overall improvement of signal timing and synchronization plans, vehicle arrival type, reducing stops and delay, and level of service for all modes of transportation have yielded additional safety, societal and environmental benefits.         TM&E

About The Author: Tirhi is Senior ITS Engineer for the city of Bellevue, Wash.