After nearly two decades of work on Interstate 5 in Tacoma — and the traffic snarls that come with the constant construction — the next big project has started. But this may very well be the last one of the busy bunch.

Guy F. Atkinson Construction has the $324 million contract to design and build a wider I-5 between the Tacoma Dome and Puyallup River to make room for new HOV lanes. This, though, requires a new bridge over the Puyallup River to carry the southbound lanes of the interstate and the old bridges will get demolished.

“The final piece of the puzzle is now our focus,” says John Wynands, Washington State Dept. of Transportation regional administrator, in a statement. “This monumental effort represents an over $1.6 billion investment in the highway infrastructure to not only increase capacity with new HOV facilities, but also to rebuild I-5 through Tacoma.”

The work in the area started in 2001 and has remained a near-constant presence in the area since then. The new HOV component represents the final piece of the planned puzzle. But the end may not be in sight for drivers looking for a construction-free commute, as the project — including the new bridge — isn’t planned to wrap up until the end of 2021.

Once the project hits a conclusion, the number of southbound lanes will increase from four to five, including an HOV lane for carpools and transit. Final configuration of northbound I-5 will add an additional lane at Pacific Avenue, an extra lane between the I-705 on-ramp and the Portland Avenue exit and one HOV lane.

Once done, it will mark the official opening of HOV lanes in both direction of I-5 through Tacoma, allowing carpools a connection that starts on State Route 16 in Gig Harbor, east of Tacoma, all the way to I-5 in Tacoma north through Seattle into Everett.

During the roughly three-year construction window of the current project, narrowed and shifted lanes will become the norm on I-5, along with nighttime lane and ramp closures. Along with the major new bridge, crews will demolish and rebuild the L Street overpass that cross I-5, demolish the old I-5 bridge crossing the Puyallup River, modify on-ramps and replace the concrete road surface.

In all, the Tacoma area has seen about 15 projects — on both State Route 16 and I-5 — connected with the HOV program.

Follow Tim Newcomb on Twitter at @tdnewcomb