The big divide is under way . . . on Chicago’s Lakefront Trail.

Segments of the 18-mile multi-use path are being ripped up and repaved to separate cyclists from everyone else — and a $60 million project to build a “flyover” bridge spanning the Chicago River and the congested downtown area near Navy Pier remains in progress.

The goal is to decrease the chance of dangerous collisions on one of the nation’s busiest trails. It’s used by 100,000 people per day on summer weekends.

All the work, however, won’t be done until the end of 2018 — so runners, bikers and walkers will have to deal with some closures and detours until then.

“Even though we’re all going to go through a bit of pain to get it built, we hope it will be a great amenity for years to come,” said Heather Gleason, head of project planning for the Chicago Park District.

“It will be worth the wait,” said Kyle Whitehead, government relations director for the Active Transportation Alliance, which has lobbied for years for lakefront upgrades.

A $12 million gift from billionaire Ken Griffin — Illinois’ richest man and an avid cyclist — will cover the entire cost of the trail-separation project north and south of the Navy Pier flyover.

The asphalt bike trail will be 12-feet wide. It will be located to the west of the pedestrian trail, which will consist of a 14-foot wide asphalt path flanked on each side by 3 feet of soft surface for joggers.

A five block section from 31st to 35th Streets that was completed last year is an example of what the trail will look like.

Though one detour on the path went into effect last week through Lincoln Park, it’s possible others will be put in place in the future. But park district officials want to keep the path open as much as possible.

“If we can keep the trail open when its under construction and do so safely, that’s absolutely our goal,” Gleason said.

Here’s a look at construction plans for the trail, starting at its northern most point at Ardmore Avenue in the Edgewater neighborhood to its southern terminus at 71st Street in the South Shore neighborhood:

Ardmore to Montrose

Plans are under design. Construction is slated to begin and end in 2017.

Montrose to Fullerton

Plans are under design. Construction is slated to begin and end in 2018.

Fullerton to North Avenue

A detour that routes trail users inland — west of Lakeshore Drive — was put in place when work began on this section of the trail on May 22. The detour actually starts two blocks north of Fullerton at Diversey in order to save southbound traffic the headache of facing a dead end at Fullerton. Work should be finished by late August or early September. This is one of the busiest lengths of the entire path. Beach access between North and Fullerton will remain open.

North to Oak

Construction from North to Oak is under design and slated to begin and end in 2018.

Oak to Ohio

Work on this segment began May 22 and is scheduled for completion by late August or early September. A shared lane will remain open along this stretch of trail during construction — and contractors will try to limit the use of heavy equipment during busy morning and evening traffic.

Navy Pier Flyover (Ohio to Roosevelt)

The trail will connect to the Navy Pier Flyover at Ohio. Once finished, users will take advantage of the new bridge across the Chicago River where it meets Lake Michigan. Construction began in 2014 and is expected to last through 2018, with the $60 million project under management of the Chicago Department of Transportation. Until it’s done, people will have to take the same bottleneck route across the river that’s been a source of frustration for years.

Roosevelt to 31st

Construction is in design phase and expected to begin and end in 2018.

31st to 35th

Work to separate path was completed last year. Trail is open.

35th to 41st

Under construction now, to be completed in the next month. Trail users will be able to use this stretch of the trail during construction.

41st to 54th

Plans are under design. Construction is slated to begin and end in 2017.

54th to 71st

Plans are under design. Construction is slated to begin and end in 2018.