STOP 'N' GO: Traffic control

Few people give much thought to traffic signals, except for the occasional second thought about whether to stop or go when the light turns yellow. Traffic engineers, however, give signals a lot of thought.

"We have families that drive up and down these streets. We want to make them as safe as possible," said O.C. White Jr., traffic engineer for the city of Las Vegas.

As the city continues to grow, so does the number of signals. The city has installed an average of 30 new signals per year during the past five years.

Traffic engineers spend about a year studying each potential site and, if approved, another six to 12 months building and installing the signals.

Federal guidelines must be met, and include calculating the number and direction of vehicles and pedestrians that pass through the intersection; physical conditions, such as hills and curves; and the number and type of accidents that have been reported at the site.

The study begins with mechanical counters that record the time of day and the speed of vehicles. Then, if there is enough evidence to show a signal is needed, crews manually count vehicles passing through or turning left or right. This helps determine what type of signal should be installed.

Meantime, a crew member checks the pedestrian and vehicle routes to nearby schools, and obtains police records to study the types of accidents that have occurred at the site.

"The city is growing so quickly we try to monitor traffic on our own, but it does help us if we know the neighborhood's concerns. We like all the help we can get," White said.

A history of accidents, however, does not necessarily mean a traffic signal is needed.

"You can have accidents where no one is hurt, but would be preventable with a signal. And there are ones where there can be an injury or fatality, but a signal wouldn't prevent it anyway," he said.

Traffic fatalities rarely play a role in signal placement, but when they do it is often because of budget constraints. If, for example, there are two intersections in need of signals, and there is only enough funding available for one, then fatalities are taken into consideration to help determine where the money will be spent.

It costs an average of $250,000 to install a new four-way traffic signal, including its design, construction and inspection. By comparison, a stop sign costs about $200. Funding comes from various sources, including gas tax dollars, developer fees, and special assessments.

A few signals are built and installed by city crews, while the remainder are handled by contractors. Because the number of signals installed each year varies, and because several signals sometimes go online at the same time, White said it is more efficient to hire contractors. The downside is that it takes several months longer to get the signals online because contracts are awarded through a bid process.

Synchronization of the signals falls under the jurisdiction of the Las Vegas Area Computer Traffic System. In operation since the early 1980s, the system controls signals in the cities of Las Vegas, North Las Vegas and Henderson, as well as in adjoining areas of Clark County.

Niel Rohleder, traffic system manager, refers to the synchronization process as "a balancing act" that involves four factors: the length of the cycle; offset (when the green light comes on in relation to the entire cycle); splits (how long the light remains green); and phase rotation (if a left-turn arrow comes on before or after opposing traffic passes through an intersection).

"There's a series of analyses and tests that must be done before we get out to the street," he said, noting that computer simulations are helpful. "The computer model sometimes sees things we don't ... it can see the big picture."

The spacing between traffic signals also is key, according to Rohleder.

"A body moving through a space at 45 miles per hour travels 66 feet per second. We must take that into account when we set up (splits) and cycle lengths," he said, noting that once the timing has been set, it is tested at the site.

"We want to make sure the model gave us valid data. We evaluate it to see if it did improve performance from a real life perspective."

One of the city's goals is to reduce the cycle time of signals. Rohleder said there were several in the valley with 180-second cycles when he came to work for the agency about a year ago.

Three minutes, he said, "is a long time," however a new system has helped "bring down those cycles by as much as 40 seconds in some locations. In drivers' time, 40 seconds is a huge amount of difference."

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