ACHD's Road Wizard Sunday, September 1, 2019

The downtown environment is an especially challenging area to install left turn signals for two main reasons. First, signal spacing is much closer compared to areas where the street grid is less robust. Second, maintaining a consistent cycle length, or the amount of time it takes to serve all movements at a given intersection, is key to avoiding gridlock.

In order to maintain a consistent cycle length, even this Wizard can't simply add time for a left turn phase without taking away green time from another movement. At the intersection of 13th Street and State Street, recent counts in the morning and evening peak hours show about 50 vehicles making the southbound left turn from 13th on to State. For comparison, there are about 900 vehicles westbound on State during the evening peak hour (about the same number eastbound in the morning peak). Any time given to an exclusive left turn phase for 13th would by necessity be taken away from State, which would have a substantial negative impact on the dominant flow of traffic.

Unfortunately, this creates a challenge for the safe implementation of a flashing yellow arrow signal on Chinden. First, the posted speed limit is 55 mph on Chinden, so east and westbound vehicles are traveling pretty quickly through the intersection. Second, with the widening only at the intersection, drivers have the ability to use the non-continuous lanes to pass slower moving vehicles. This, in turn, can create sight distance challenges for left turning vehicles. Those two factors, in combination, make this a poor candidate for a flashing yellow arrow.

ITD has plans to widen Chinden from two to five lanes west of Eagle Rd. starting this year, though this section of Chinden is slated for 2024 construction. Flashing yellow arrows will be a part of design considerations with these improvements.