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RESIST IMPULSE TO SAY `PHOOEY' TO THE SAFE AND EFFICIENT `SPUI'

Published: Sunday, Jan. 20 1991 12:00 a.m. MST

Cars confidently cruising along the newest stretch of I-215 east suddenly slow to a confused crawl after exiting at 6200 South. Heads frantically turn in both directions as motorists wonder which way to go, where to stop and how to avoid getting hit by some other lost driver.

Welcome to what experts are calling the interchange of the future: The safe and efficient Single Point Urban Interchange - or "spui," in engineering jargon.And despite the confusion the new design may cause, Utahns can count on more of them along the Wasatch Front in the future. The Utah Department of Transportation is considering spuis at congested crossroads along I-15 as part of the freeway's massive reconstruction taking place in the next 10-15 years.

"You get complaints about anything new. People don't want to change. But this (spui) is far superior from a safety and capacity standpoint," said UDOT safety engineer Mac Christensen.

Those factors must mean an awful lot to state highway officials because a spui can cost 75-100 percent more than the more common diamond design interchange.

The spui isn't entirely new to Utahns. The first was built in 1980 on the 2100 South highway at 3200 West. Another one is at California Avenue on the west half of I-215.

Interchanges have evolved over the past 50 years to accommodate traffic growth. The first revolutionary change from the ground-level four-way intersection was the cloverleaf.

And while many cloverleafs still stand in Utah and in other parts of the country, the design isn't efficient, said UDOT research engineer Les Abbey. He said the cloverleaf takes up too much land and it requires motorists to go far out of their way to change directions while either getting on or off the freeway.

An improvement from the cloverleaf is the diamond interchange, which can be found at most of the interchanges along I-15. It takes up less land and has more direct routes on and off the freeway.

But Abbey explained that the diamond design hasn't performed well in areas where lots of traffic is getting on and off the freeway. The diamond only allows one direction of traffic either entering or leaving the freeway to turn left while the other three directions wait.

Another drawback to the diamond, Abbey said, is its multiple stops - as many as five traffic signals in one direction if frontage road signals are included.

All this can wreak havoc at rush hour. Abbey said left turns are time-consuming maneuvers and when that is coupled with multiple stops at the interchange, gridlock can occur. Traffic backs up in all directions as motorists become stranded in the middle of the intersection, not allowing anyone to move.

Engineers contend the spui solves this problem because it reduces four separate left-turn movements to three. For example, at 6200 South, the design allows traffic exiting the freeway to turn left simultaneously - one movement - while traffic in both directions on 6200 South turns left to enter the freeway at separate intervals - two movements.

This design can result in some interesting reactions from motorists, particularly those exiting the freeway. As motorists veer left off the off-ramp it can appear as though they are on the wrong side of road because oncoming traffic, which is also turning left, is on the right hand instead of the left.

Abbey has seen some drivers take a sharp left and head into oncoming traffic waiting at the stop light, or roam onto the opposite off-ramp.

UDOT admits signing and lighting at spuis in Salt Lake County could improve to make it easier for motorists to navigate through the foreign interchange. But so far no serious accidents have occurred.

"Despite its confusion, it's very forgiving," he said.

Abbey cites studies in Arizona showing a spui in Tempe that reduced the accident rate at a particular interchange from 1.73 to .82 per million vehicles entering the intersection.

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