August 1st, 2011
IT’S COMPLICATEDOptimize Mobility, by Catherine Higgins.
This post is first in a two-part series about signal timing. Read the second part here.
If incoming calls and email to UDOT are any indication, signal timing seems to generate a lot of public concern.
“It takes forever for the light to change,” laments Kay, a road user who recently contacted UDOT. Kitty Wright, Customer Service Specialist at UDOT, gets several calls or email a day about signals. Many customers are concerned about long wait times. She directs calls and email to the UDOT Traffic Operation Center where experts address questions one by one.
Matthew Luker, a UDOT Signal Systems Engineer, provides answers. “We appreciate these calls because we can’t be everywhere all the time, and they help us to know of problems–and correct them sooner.” Kay, who spotted some unusual traffic delay, alerted traffic engineers to a malfunction that was addressed within a day.
While UDOT signal operators are continually watching traffic and adjusting signals to work more efficiently, drivers shouldn’t expect to always breeze through successive intersections – signal systems are just not that simple. Traffic engineers have a lot of factors to consider when programming signals so traffic can move efficiently and safely from signal to signal.
First, here are a few key definitions to understand:
Cycle – one complete rotation through all signal colors; from green back to green.
Phase – one segment of the cycle allowing a unique movement. For example, a green light with a protected left turn arrow with the same movement on the facing leg of the intersection is an example of one phase of a cycle.
Signal Detection – Modern signals include devices that have the ability to “see” traffic and change when needed. Signals that can change depending on traffic needs are called “actuated signals.”
Progression – the movement of traffic through a corridor from one signal to the next.
Here are some reasons why signal timing is complicated:
Multiple jurisdictions – Not all signals wear the UDOT label. Like roads, some intersections are owned and maintained by cities or counties. UDOT works closely with local municipalities to time signals so there is congruity among UDOT and municipal intersections in close proximity. Most recently, UDOT has worked closely with Salt Lake City to evaluate and re-time signals in the downtown area. Drivers should see less delay on city streets as a result of recent adjustments.
Cycle length – For traffic to progress efficiently from one signal to the next, the cycle length of all signals on a corridor must be the same. Signal operators try to make the cycle length just right – not too short, not too long. A too-short cycle can create delay when the too few cars can pass through. Long cycles reduce traffic speed and can make progression sluggish.
Phases within each cycle can vary to adjust to directional traffic flow. For example, signals can be programmed to give more green light time for commuters, but that adjustment robs other phases of time. Motorists traveling in the opposite direction may have long wait times at red lights.
Proximity to other signals – The ideal spacing of signals depends on cycle length. For commonly used cycle lengths in Utah, signal spacing should be about every half-mile. Signals in metropolitan areas often need to be closer.
The signal space-time continuum can be breached when signals are too close – especially if the traffic volume is high – because vehicles can’t clear the signal and motorists can end up waiting through multiple cycles.
Pedestrians – Signals must allow adequate time for pedestrians to cross the street safely. In downtown Salt Lake City, streets are wide – about twice as wide as other cities – and the time it takes pedestrians to cross determines cycle length. Consequently, cycle length downtown is long to allow for pedestrians to cross.
Multiple phases – Because of heavy traffic in modern cities, signals need phases that accommodate traffic movements, especially left turns. Without protected left-turn phases, no one would be able to turn left on some busy city streets. But adding more phases, like protected left turns, “makes the cycle length longer,” explains Luker. “Without moving the signals farther apart, you don’t get good progression in both directions or the speed goes down to a point where it is too slow for most drivers.”
Check back Thursday to see more reasons why signal timing is complicated.