Interstate 25 will be closed in both directions for long periods this weekend, potentially detouring travelers headed to the high country Saturday and Sunday morning.

On Friday, one lane of northbound and southbound I-25 between Interstate 70 and Speer Boulevard will be closed starting at 7 p.m. with a second lane closing at 9 p.m, according to traffic planners.

Fifteenth Street over I-25 will also be closed from 9 p.m. to 9 a.m. on Friday and Saturday, said Mindy Crane, Colorado Department of Transportation spokeswoman.

The highway will be closed in both directions between Speer and I-70 from 11 p.m. Friday to 9 a.m. Saturday, and 11 p.m. Saturday to 9 a.m. Sunday, Crane said.

On Sunday, one lane of northbound and southbound I-25 between I-70 and Speer will be closed starting at 7 p.m. with a second lane closing at 9 p.m. All lanes of I-25 will close at 11 p.m. and reopen by 5:30 a.m. on Monday, Crane said. Fifteenth Street over I-25 will also be closed from 9 p.m. to 5:30 a.m., she said.

The closures are necessary for the demolition of the southern half of the existing 15th Street bridge over I-25, traffic planners said.

Major delays are expected and alternate routes are strongly advised.

“We have already demolished and replaced the northern half of the bridge and will now begin the demolition and reconstruction of the southern portion,” said Kevin Ryburn, CDOT project engineer in a news release. “Following this weekend’s work, one lane eastbound and westbound 15th Street will remain open over I-25 as we construct the new bridge.”

Suggested routes are:

• Southbound I-25: Exit onto westbound I-70 and take I-70 to Federal Boulevard. Travel south on Federal Boulevard to Speer Boulevard and then south on Speer Boulevard to I-25.

• Northbound I-25: Exit onto northbound Speer Boulevard and take Speer Boulevard to Federal Boulevard. Then travel north on Federal Boulevard to I-70. Take I-70 east to I-25.

• Fifteenth Street: Motorists on 15th Street will be detoured to Little Raven Street to 20th Street and then to Central Street.

In addition, 38th Avenue between Fox and Inca streets will be closed from 3 a.m. until noon Saturday in order to place commuter rail bridge girders over the roadway, according to RTD.

Ryan Parker: 303-954-2409, rparker@denverpost.com or twitter.com/ryanparkerdp