As the volume of vehicles changes throughout the day on Carson City's roads, its network of traffic signals is engineered to adjust for optimal flow, said Carson City Public Works Operations Manager for Control Systems James Jacklett.
"It's all about optimizing efficiency and reducing overall demand to get to that level of service," he said.
Some areas of Carson City are more heavily congested than others, Jacklett said, so signal changes take longer for vehicles seeking to cross them at controlled intersections. This is by design.
"The delays out there, in order to have progression and coordination, requires long cycle lengths," he said. "The delays on the side streets can be significant due to the volume and all of the cars that have to travel through there."
Cycle length is the total amount of time a signal is programmed to serve all of the approaches, Jacklett said.
Major traffic corridors and arterials can feature disproportionate cycle lengths between signal changes for cross streets, because of the higher volume of vehicles traveling along these routes.
"If you are on the side street, you are waiting and it's not enjoyable," Jacklett acknowledged. "But if you are on the main street, you are thankful because you have progression and you're not spending many minutes at a light."
Jacklett said the South Carson Street corridor from Koontz Lane south to Mica Drive in Northern Douglas County attracts the highest volume of vehicles in the Carson City area, currently operating at or beyond its capacity for the amount of traffic it was designed for.
Without longer cycle lengths allowing for the progression of north and southbound traffic, vehicle congestion would only be worse there than it is now.
Signals times along the South Carson Street corridor are also coordinated to change together during peak traffic periods, Jacklett said, which helps further keep the flow of vehicles moving and reduces congestion caused by back up.
In fact, 31 of the 50 traffic signals throughout Carson City are coordinated to change together during peak traffic, so the series of lights in South Carson City will change in coordination with one another when vehicle use is heaviest.
"They are in coordination during certain times of day when the volumes warrant it," Jacklett said.
Signal coordination is warranted based on traffic density as well as how far apart lights can be from one another. The closer together they are, such as the signals in downtown Carson City, the more timing coordination is warranted when volume is high.
That doesn't mean traffic signals are always timed to coordinate with one another. In other parts of town, coordination can actually be a hindrance when signals are farther apart.
Jacklett used the distance between College and Medical parkways along North Carson Street as an example. Due to their distance from one another, coordination would only mean cross traffic at those busy intersections would have to wait longer for the lights to change.
"Those two signals on Carson Street are so far apart and the volumes are low that they don't warrant coordination," Jacklett said. "Coordination actually causes more delay, because it provides preference to the main street for progression."
Jacklett said all of Carson City's signals are fully actuated, meaning they are equipped with vehicle detectors on all approaches at an intersection. During off periods when traffic volume is low, the wait time at a light will be much shorter than it is during hours of peak vehicle flow.
When a signal is operating off of its detector inputs, that's known as free mode of operation, which responds entirely to demand.
"That's why, within a few seconds, if there is nobody on the other approaches, you'll see those approaches clear and it will serve you," Jacklett said. "That mode of operation is preferred when signals are further apart and volumes are low."
Each signal in Carson City is also programmed with multiple timing plans that can adjust to changes in traffic volume and flow throughout the day, Jacklett said.
"There can be multiple plans in a single day," he said. "It's not uncommon to see an a.m., a mid-day and a p.m. peak plan depending on the characteristics of that corridor and its needs."
Actuated signals are programmed to change their plan from free to timing mode depending on the traffic detected.
"It's typical for these signals to operate in the free or fully actuated mode during the off period or in the middle of the night," Jacklett said. "Then at some point in the morning, prior to when traffic volumes really pick up and the commuters come through, the signals move and activate a plan and start to work together to optimize progression in a corridor."
Engineering is the key to proper signal operation and planning, he said, but there are always variables to traffic that drivers need to take into consideration when sitting and waiting at a light.
"Even at the best signals, when plans, modeling, volumes and counts all show and support progression, there are always periods where real life is going to differ from paper," Jacklett said. "That may be for unknown or unseen conditions down the street. There are so many things going on."
Pedestrians in uncontrolled cross walks or emergency vehicles approaching intersections can alter traffic progression because of the priority they are given.
Some lights in Carson City feature uncontrolled left turns that must yield to oncoming traffic. When an unforeseen event causes traffic back up on approach, there can be longer delays for vehicles waiting in those left turn lanes.
"Uncontrolled left turns are referred to as permissive operations," Jacklett said. "You are permitted to make a left turn when it's safe to do so."
He said permissive signals are preferred over controlled left turn arrows because they promote the least restrictive flow of traffic along high volume streets.
"We're not going to apply a protected turn control, because it's more restrictive, will cause more delay and actually requires more time and reduces the level of service at the intersection," he said. "It has more negative consequences than positive, unless there are safety and crash history concerns."
To change a traffic signal is no simple task, Jacklett said. Carson City Public Works engineers are required to follow specific criteria determined by the Federal Highway Administration's Manual on Uniform Traffic Control Devices.
"Modifications are subject to a warrant process," he said. "There are several standards that take into account opposing through volumes, the number of opposing and through turn lanes, cycle length, speed of opposing traffic, sight distance, and crash history."
Jacklett said Carson City's network of traffic signals operates with safety as a top priority, followed closely by efficiency of progression that give preference to the highest volumes of vehicles.
"There's definitely time and effort being spent to ensure we can provide the best system that we can," he said. "It's all about serving the majority of users, and there are definitely trade-offs with all traffic control devices."
