On two successive weekends this month, Utah will take the unprecedented step of shutting down traffic completely in one direction and then the other on Interstate 80 through Parleys Canyon.

The 24-hour closures will block westbound flow between Park City and Salt Lake City starting at 4 p.m. Aug. 9 and eastbound traffic beginning at 7 p.m. Aug. 16.

During that time, Utah Department of Transportation contractors will slide pre-made bridges into place - two each weekend - at East Canyon and Lambs Canyon. Barriers and parked troopers will turn back motorists at Exit 130, a ranch ramp near the western mouth of Parleys, and at Exit 140 at Parleys Summit.

It's the fastest-ever bridge-replacement project in Utah, if not the nation, said UDOT regional director Randy Park. The department calculated that doing the job the old way and rerouting some traffic into oncoming lanes while building the bridges in place would have added up to more than 220,000 hours of motorist delays.

"It's on I-80, which is the crossroads of the West," Park said. "We do realize this is an enormous impact."

UDOT will post 175 electronic and regular signs alerting motorists as faraway as the Wyoming state line. Truckers already have been advised to avoid I-80 through Salt Lake City because of lane closures in the city, though many have ignored the call to detour to Ogden and I-84.

Building the bridges beside the road and then slipping them into place "reduces months into days," project manager Mark Parry said.

I-80's peak-hour weekend traffic - generally between 3 and 4 p.m. - averages 1,800 vehicles an hour, Parry said.

It means that if you're returning from the Park City Luxury Home Tour on Aug. 9 or driving up to Gladys Knight's Deer Valley performance with the Utah Symphony on Aug. 16, you'd best not be late.

And if you live in one county, work in the other and can't avoid the weekend drive - or if you're simply determined to escape the valley heat - you're just going to have to get acquainted with I-84 through Morgan County or U.S. 189 though Provo Canyon for one leg of the trip.

UDOT coordinated with the Utah Symphony to keep the eastbound lanes open Aug. 16 until the 7 p.m. start of the Gladys Knight concert. Those who arrive on time will not be inconvenienced, because the westbound lanes will be open and waiting for their return.

"We urge people not to be late, because we will close the canyon right at 7," Park said.

The roadblocks could become a major bummer for the Aug. 9 luxury-home show, an annual benefit for Park City's Peace House domestic-violence shelter. Organizers didn't know about the closure until it was too late to reschedule, especially because some participants fly in from around the country, Peace House executive director Jane Patten said. It usually draws about 1,000 people, most of whom drive up from the Wasatch Front.

"We hope that people are resourceful and realize this is just one thing they have to take into account in their planning," she said. "We really hope people will spend the weekend in Park City."

The show ends at 6 p.m., and the road back to Salt Lake City will have closed at 4 p.m. Patten said people can leave the tour whenever they like, though, and those participants wishing to stay will get a $30 discount at the Yarrow Hotel.

The closures were unwelcome news to White Pine Touring, a Park City shop that during summer rents bikes and guides tours. At least half the customers are clients at destination resorts, assistant manager Brent McElhaney said, so how much time they're willing to spend with a bike might depend on how quickly transportation services can get them through the detours.

I-84, branching off near Ogden, seems a long way to go, he said. "That's not convenient."

All Resorts Express is one business that's thinking ahead, changing its bus and limo routes from Salt Lake City International Airport up to I-84, and its smaller vehicle up Big Cottonwood Canyon, over Guardsman Pass to Deer Valley.

"It's actually a very pretty road, and it's an added benefit to our guests," All Resorts transportation director Barry Gross said. "We anticipate heavy traffic through I-84, so [Big Cottonwood] may be the best way anyway."

Gross said he has been pleased with UDOT's I-80 work and minimal delays so far, though he worries about the effects to home-show participants, some of whom use his services.

Up to 400 golfers who usually find their way to Mountain Dell Canyon Golf Course also will have to plan ahead and use Emigration Canyon to get there. Salt Lake City golf director David Terry said the 36-hole course will be open. Officials are still devising a transportation plan, he said, but generally will encourage golfers to use Emigration.

bloomis@sltrib.com

Another closure, on Highway 6

The state will shut down U.S. Highway 6 overnight Tuesday for bridge repairs at milepost 200.

* The closure, from 10 p.m. Tuesday to 6 a.m. Wednesday, will block traffic at the bridge two miles west of the Tucker rest area to just west of Soldier Summit.

* State highway officials encourage southbound motorists and truckers to detour on Interstate 15 to Scipio and connect to I-70 via U.S. 50. Northbound traffic can reverse that detour, heading west on I-70 to U.S. 50 at Salina, then to Scipio and I-15.

* The department will post electronic signs to alert motorists.