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Utah Department of Transportation
The 10600 South interchange on I-15 in Sandy opened ahead of schedule on Saturday, Sept. 23, 2017, after being reconstructed.

SANDY — The 10600 South exit, one of the south valley's most heavily traveled interchanges, opened ahead of schedule Saturday after being reconstructed.

A new underpass, however, remains under construction and is expected to open next year, Utah Department of Transportation officials said.

The I-15 interchange closed in both directions on Sept. 7 and was scheduled to close for 16 days while crews worked 24 hours a day to finish the $23 million project.

Traffic on the interchange — which carries an estimated 42,000 vehicles per day — opened two days ahead of schedule. Westbound lanes opened around 3 p.m., while eastbound lanes were expected to open later in the day.

During the closure, crews installed a three-sided underpass that will connect the northbound I-15 ramp with Monroe Street, just west of the Shops at South Town. Crews then rebuilt the roadway over the underpass.

When the new 183-foot underpass opens next year, it will be the first of its kind in Utah, UDOT officials said. It will measure 40 feet wide and 22 feet high, twice as wide as New York City's Lincoln Tunnel and tall enough for a double-decker bus to pass through with plenty of room to spare.

The new underpass will remain closed until next year as workers install pavement and build the connection to the existing northbound off-ramp. It will also connect to a smaller underpass that is already in place on the north side of 10600 South.

Once completed, the underpass is expected to carry about 4,800 vehicles daily while providing easier access to downtown Sandy and reduce traffic jams along 10600 South, UDOT officials said.