Interstate 12’s new westbound lanes between 4-H Club Road and O’Neal Lane are scheduled to open Tuesday, but motorists face the ominous closure for months of two of the three westbound entrance lanes at South Range Avenue.

Two of the entrance lanes carrying traffic toward Baton Rouge from Range will be closed about 1,000 feet past the ramp meters, officials said.

“It probably will be the worst experience we have seen in a long, long time as far as congestion and backup,” Denham Springs Mayor Jimmy Durbin said of the approaching shutdown of two of the entrance ramp’s three lanes.

It will require commuters from Denham Springs “to evaluate a different way to get to work in Baton Rouge,” the mayor said.

Durbin said 85,000 to 100,000 vehicles travel I-12 at South Range Avenue each day.

When entrance traffic backs up significantly, the ramp meters will shut off automatically to help funnel traffic onto I-12, but traffic snarls are still expected, said Bill Grass, a spokesman for the Geaux Wider Program, which is handling the state’s widening of Interstates 10 and 12 in East Baton Rouge and Livingston parishes.

“It probably will be a little bit rough for a couple of months,” Grass said.

The plan calls for shutting down all three westbound entrance lanes to I-12 at South Range Avenue on Wednesday evening, Grass said.

When the westbound South Range Avenue ramp reopens around 5 a.m. Thursday, only one of the three lanes will be operational 1,000 feet past the ramp meters, he said.

The lanes probably will remain closed until near the end of the year, Grass said.

The ramp work will follow Tuesday’s rerouting of I-12’s main traffic lanes. The re-rerouting will start around 8 p.m. if weather permits, state highway officials report.

The first change westbound motorists will experience is a slight shift to the left just west of Juban Road. That shift will allow crews to work on new lanes there, Grass said.

Just past 4-H Club Road, motorists will be routed onto newly built westbound lanes. That will eliminate the current shift to the left that westbound motorists now have to make after crossing 4-H Club at I-12, Grass said.

Tuesday night and early Wednesday morning, the Juban Road and South Range Avenue entrance ramps and exit ramps will close intermittently while work is being done, officials said.

In following days, eastbound traffic also will be shifted to the newly built Amite River bridge. This will allow the contractor to demolish the old bridge that has carried eastbound traffic and widen to three lanes the new bridge for eastbound traffic, Grass said.

The purpose of the I-12 work is to widen the interstate to six lanes from Juban Road to O’Neal Lane. That work should be completed by mid-2012 at a cost of about $146 million, Grass said.