The scene at a downtown intersection is typical - the irritated driver who missed the green light yet again, shaking his fist and muttering unpleasantries.

It's likely the driver is launching his diatribe against Salt Lake City's Traffic Systems Analyst Tom Stetich, who is responsible for maintaining the city's 117 traffic lights.But Stetich would probably be unashamed and unimpressed by the driver's angry words - he's heard it all before, as he fields daily phone calls.

A typical complaint: "They say, `I've lived here 20 years, and I used to be able to drive straight up Fifth South and now you guys have screwed it up.' "

But Stetich will point out that he and Transportation Engineer Timothy Harpst haven't "screwed it up." And not only does he have the numbers to prove it, he can show you during a drive about town.

Since the city installed a $1.4 million traffic control system in 1984, driving through the city's core has become significantly more efficient, Stetich said, pointing to 1985 figures.

Average travel speed has improved 21.2 percent, the probability of stopping has declined 35.9 percent, and the average length of time spent waiting at a stop light has dropped 12.1 percent, according to a study conducted for the city.

"And we're making improvements in every one of those categories," Harpst added.

Not only does the system save time, Harpst said, it also saves money. Less time spent twiddling thumbs at a red light means less time a car engine idles, burning less gas.

Harpst estimates the system pays for itself once every 10 weeks in money saved by drivers paying less for gas and maintenance. "It puts some change back in your pocket."

The city's traffic control computer, housed at the transportation division, controls 117 traffic signals within an area bordered by North Temple, 27th South, Second West and 13th East.

Sitting at the computer and monitoring a map of the city, Stetich can control traffic flow by manipulating the length of time it takes for lights to change from red to green to yellow.

His objective is to synchronize traffic lights so that drivers can capture the "green window" - that elusive phenomenon that occurs when drivers cruise through green light after green light.

The basic principle, Stetich said, is to create a progression of "double alternates." He demonstrates the principle while driving in the late afternoon through downtown Salt Lake City.

Stetich, stopped at a traffic light, points out that when the light turns green, so does the next light down the street. That light is timed such that it stays green long enough to permit a driver to pass through.

As Stetich progresses down the street, the lights, timed by his computer, continue to turn green as the "double alternate" moves down the street, which should ensure a smooth flow.

Indeed, the system seems to work as Stetich drives through downtown Salt Lake observing the speed limit, avoiding traffic snarls and enjoying a relatively stop-free journey.

"But if you go too slow or too fast, you're going to be penalized . . . it's a pretty safe bet that if you go the speed limit, you'll stick with the greens," he said.

The system has its shortcomings, Stetich admits. Some less critical streets "take it in the shorts" so others that provide access to busy street routes like I-15 can flow more smoothly.

And unpredictable variables like car accidents, slow drivers and crowded streets simply throw off the system, irritating drivers and making Stetich's job difficult.

Also, Salt Lake City's wide streets can make the "green window" elusive. Pedestrians must be allowed extra time to cross the wide boulevards - something Brigham Young, who championed the wide avenues during the city's infancy, probably didn't think of. That can shorten green time, Ste-tich said.

However, aggravation from being held up at too many traffic lights can be self-induced, Harpst said, suggesting a number of driving tips that will keep drivers in the "green window."

"What you should try to do when you are on a trip is to take a direct route," Harpst said. Turning from one street to another will take a driver out of the green widow, he said.

Speeding won't help because going too fast also moves a driver out of the green window, along with increasing his blood pressure unnecessarily.