Synchronizing traffic signals is complicated.
The only signals in the City that are interconnected are on Avenue D, at First Street and Second Street. This upgrade was just completed in summer 2017. These signals are also connected to the Snohomish County traffic signal at Airport Road/Lowell-Snohomish Road on the south side of the Snohomish River. These traffic signals can be adjusted remotely if needed by Snohomish County.
Other City traffic signals are controlled by clocks and sensors (traffic loops and video detection). Signal cycle lengths are determined based on typical traffic flows through the day and are programmed into a clock to synchronize the traffic signals. However, the clocks at the various intersections run independently and when the individual clocks run a bit too fast or too slow, then the synchronization of traffic signals can be thrown off.
At a few of the traffic signals along Second Street, GPS clocks have been installed that will reset the clocks to resynchronize the traffic signals. If the sensors at an intersection detect a high number of vehicles, additional green time may be given at that intersection - which may throw off the synchronization.
Traffic signals also allow pedestrians to cross streets safely – this may require extra time when the pedestrian button is triggered. Anytime the synchronization is thrown off, it will take a few cycles to resynch.