Carl from Denver writes, “What's driving you crazy? Why is there no green arrow to turn east onto Morrison Road from southbound Sheridan Blvd? As we all know, Sheridan is one of the busiest streets in Denver. It only allows a single car to turn left on a yellow or red light. If you actually obeyed the law, there’d be many cycles of the traffic lights before you could cross northbound traffic. Feelin' lucky?”
In general Carl, the presence or absence of a left turn signal is ultimately the result of two considerations: What is the risk of a crash and how much demand there is for the left turn versus the time that should be allocated for stopping traffic?
Traffic engineers tell me they weigh that left turn demand against this question: Does the benefit in reducing net delay for the left turning drivers outweigh the net delay cost to higher-volume through drivers? Basically, who do you delay more, the majority or the few?
This is the basic reason why traffic engineers don’t provide left turn lights at every traffic signal.
This traffic signal is operated and maintained by the City and County of Denver even though Sheridan Boulevard is a Colorado Department of Transportation-maintained roadway, as it is Colorado State Highway 95. Vanessa Lacayo, marketing and communication specialist with Denver’s Department of Transportation and Infrastructure, tells me they periodically review traffic signals throughout the city to determine where left turn signals are needed.
“DOTI reviewed the intersection at Sheridan and Morrison/Mississippi in November 2015. At that time, we decided not to add a turn signal based on the data we collected. Given additional housing in Westwood as well as new development along Morrison Road since 2015, we plan to update our review.”
Lacayo tells me if DOTI decides to add a left turn arrow, the process usually takes several months depending on the design approval process and scheduling.
When I talked with Bob Kiene, CDOT Region 1 traffic operations engineer, he told me one factor in the decision to add or refrain from adding a left turn arrow is partially determined by the change to the total cost to intersection delay. “Each additional phase at a traffic signal takes cycle time away from signal phases serving movements in conflict with it and introduces additional 'lost time' that reduces the intersection's overall capacity.”
MORE: Read more traffic issues driving people crazy
The lost time at a traffic light Kiene is referring to is the brief pause that no vehicles are in the intersection. For example, at a simple two-way intersection when the signal turns red, the cross traffic signal doesn’t usually immediately turn green. The delay is short, but not immediate since it is a simple intersection. That delay allows for a brief few moments that no vehicles are in the intersection to make sure the intersection is clear before another direction turns green.
At busier intersections, like at 6th Avenue and Lincoln Street in downtown Denver, that brief hold time is several seconds since there are more vehicles rolling through and there is a problem with red light runners.
“While this may not seem like much time, when an intersection is at or near capacity, large backups can develop over the course of the peak period caused by one or two more vehicles being stopped by the signal than would have been otherwise,” Kiene said. “As a very simplified example, if the signal has a 60-second cycle and serves eight vehicles in a cycle in a lane on one approach when the demand averages 10 vehicles each cycle, the queue will build by the excess each cycle. And after one hour, the queue would build to 120 vehicles, or around a half mile, as a result of this small change in timing. When left turn volume is significant, the amount of delay at the intersection begins to exceed the savings from not having a left turn phase.”
Traffic engineers say at these lower turn volume intersections, because vehicles may exit — make the left turn — but not enter — roll past the red light into the intersection to make the turn — an intersection during a red light, very low-volume left turn movements can be adequately served without a left turn light. They say this left turn works best when drivers safely stop within the intersection, pulling into the intersection making sure not to impede other traffic, and wait for a gap in oncoming traffic or proceeding with the left once the oncoming traffic has stopped due to the red light.
Whether you feel comfortable with pulling halfway out or not when making a left is a separate question that I tackled in a previous Driving You Crazy story you can read here.
Both CDOT and DOTI tell me they will reevaluate the placement of a left turn arrow at a lower turn volume intersection if there is a sight distance issue, identified crash pattern — especially ones that may have been prevented with a protected turn signal — or high opposing traffic speed causing difficulty in creating a long enough gap in traffic flow to make the left turn safer.
The simplest signal type, known as a "two-phase" operation that has one phase for the major street and one phase for the minor street, introduces the lowest possible amount of lost time. An equivalent signal with left turn phases added in all directions experiences twice as much lost time.
In the short term, you will still have to use extra caution when making that turn. But for the long term, the final determination to add a turn light or not will ultimately be determined by that new traffic signal review DOTI will conduct.
Denver7 traffic anchor Jayson Luber says he has been covering Denver-metro traffic since Ben-Hur was driving a chariot. (We believe the actual number is over 25 years.) He's obsessed with letting viewers know what's happening on their drive and the best way to avoid the problems that spring up. Follow him on Facebook,Twitter or Instagram or listen to his Driving You Crazy podcast on any podcast app including iTunes , Stitcher , Spotify and Podbean.