study could ease traffic flow
Reprinted from the Coeur d'Alene Press
Then the light turns red and you're stopped -- only to sit and wait for what seems like an eternity.
But it's not.
In reality, cycle time is 140 seconds, because that is the maximum "wait time" for each of the state's traffic lights on U.S. 95.
While many motorists blame the timing of traffic lights for long waits and congestion, cycle times are not the enemy – traffic volume is, say state officials.
"Ideally, we try to keep traffic moving because it keeps fuel consumption and pollution down," said Michael Porcelli, District 1 traffic engineer for the Idaho Transportation Department. "But U.S. 95 has simply reached its capacity to handle large traffic volumes on a steady basis."
From Neider to Hayden Avenue, the time it takes to travel from signal to signal coincides with the 45 mph speed limit. The ITD compensates for the distance between each intersection, and then coordinates the cycle time so traffic can flow evenly.
Theoretically, a car traveling the speed limit should be able to travel through the entire "green band" of the corridor, but with about 37,000 cars traveling on U.S. 95 every day, a straight shot is nearly impossible.
However, a possible solution is on the way.
Porcelli said the ITD is conducting a study that could result in a 20-second cycle time change for north/south and east/west traffic lights on U.S. 95.
When traffic is minimal, cycle time would be reduced by 20 seconds, reducing the amount of stops. When traffic is heavier, cycle time would increase by 20 seconds, allowing more cars to get through a green light.
The adjustments would only alter cycle times for daytime traffic. The signals have already been adjusted for nighttime traffic.
Late at night in the spring, summer and fall, signals are set on timers (fully-actuated) rather than synchronized (pre-set), which means they can be "tripped" to change when a vehicle approaches an intersection. Once tripped, either by an underground sensor or a video detection camera, the light changes within about 20 seconds.
During the winter months, north/south lights stay yellow and side-street lights flash red so that snowplows can clean roadways more effectively.
While minor changes on U.S. 95 are done easily, changes to cycle times for the Northwest Boulevard/Interstate 90 interchange will not be that easy, Porcelli said.
Because the signals are so close together, they're already coordinated as best as they can be. Similarly to U.S. 95, it's the traffic volume that hampers traffic flow, and not the timing of the lights.
In addition, heavy congestion can exist on, and through, the interchange because it is only one of a handful of intersections where emergency vehicles "pre-empt" signal lights to change.
With a small device mounted on the top of its roof, an ambulance, fire truck or paramedic can interrupt signal coordination when necessary. Then, it takes a couple of cycles to get the correct coordination back.
In contrast to state-owned signals, Coeur d'Alene's city engineer Gordon Dobler said city signals are all fully actuated, and that they automatically adjust to actual traffic demand, within the minimum and maximum green times.
"Traffic lights are still not the fix-all that some people might think because some of the highest-accident intersections are signalized," said Dobler, who considers roundabouts, such as the one on Fourth and Kathleen, as a better alternative to traffic signals.
Roundabouts handle traffic more efficiently, are easier to maintain and cost $50,000-60,000, as opposed to $200,000 for a signal.
With an average "wait" time of 60 seconds at all city signals, Dobler does not foresee synchronizing the lights unless traffic volume at an intersection becomes disproportionately heavier at a certain time of the day.
When you synchronize, you lose the ability to respond to actual traffic demand at intersections, said Dobler, who, along with city officials, is evaluating the need for signals at Atlas and Kathleen, Atlas and Hanley, and Fifteenth and Margaret. The final decision will be based on increased traffic volume and development in those areas.
Porcelli said the ITD is also planning to re-coordinate the traffic signals at the Highway 41 and I-90 interchange this winter.
Photo: Mike Porcelli, a district traffic engineer for the Idaho Transportation Department, stands near a control box for traffic lights at U.S. 95 and Prairie Avenue. (Jerome A. Pollos, Coeur d'Alene Press)