Ada County Highway District

ACHD's Road Wizard Sunday, September 1, 2019 ACHD's Road Wizard

Green left turn traffic signal arrow for 13th Street, flashing yellow arrow traffic signal for Chinden Blvd. and Star Road, and a message from the Road Wizard

The Road Wizard Replies

Dear Road Wizard: Given road construction, at the evening commute, I have been attempting to go home by turning left from 13th Street onto State Street. However, it is both frustrating and dangerous because there is no green left turn arrow, only a flashing yellow. Even though left turners going north are first to the light, we have to yield to south bound cars that arrive intermittently and often. Thus, because we can't turn left, the northbound left lane stacks up and some end up in the intersection turning left after the light turns red. My question: why is there no green turn light at rush hour? Thanks.


Road Wizard:

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.

Dear Road Wizard: Could you please look into getting a flashing yellow left turn signal for the Chinden Blvd/Star Road intersection? Most roads I've traveled on allow for a left turn on a flashing yellow light, the intersection at Chinden Blvd (Highway 20) and Star Road does not allow this and it sure would be nice not to have to wait for the signals to cycle through just to turn left onto Star Rd. Thanks.


Road Wizard:

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.

Dear Road Wizard readers:  Though money is tight,

My wisdom is long,

And this is not the Road Wizard's swan song!

In local newspapers,

You will no longer see,

Any charming quips from me.

But fear not my friends,

For I am still around!

The ACHD website now houses this round.

If it's answers you seek,

About roads, signs and lights,

Send me your queries,

I'll give you my insights.

Road Wizard

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