The interstate running through downtown Birmingham is getting new bridges thanks to a $475 million project from the Alabama Department of Transportation.

That project helped drive the decision to change the name of the interstate from I-20/59 to I-59/20 after 40 years. It’s a switch that puzzles some Birmingham area drivers nearly four years after the decision was made.

ALDOT East Central Region Engineer DeJarvis Leonard said the decision to use 59/20 as the official name for the highway, rather than 20/59, was made in early 2015, during the early planning stages of the project to rebuild the elevated highway through Birmingham’s central business district.

“Early on when we were developing our website and talking about media around the project, we looked at the nomenclature of 59/20 and 20/59, and after some discussions with officials in Montgomery, it was determined that since 59 was the first route that was put in service, we elected to use 59/20,” Leonard said.

I-59 serves as the major interstate route from Chattanooga to New Orleans, and shares the right of way with I-20 for a 153-mile stretch from east Birmingham to Meridian, Miss., where the two roads diverge again. Highway website Interstate-Guide.com says the 153-mile overlap is one of the longest stretches in the U.S. interstate system. On the stretch of I-59/20 in Mississippi, the I-59 mile markers and exit numbers are used over I-20. In Alabama, the markers are the same because they start at the western border going east.

The stretch running through downtown Birmingham that is being replaced was finished in 1973. That roadway will close to all traffic later this month, as contractors demolish and replace the bridges elevating the highway over city streets.

An exact closure date has not been announced, but is expected to be in “mid- to late-January.” The interstate closure is planned to last 14 months, though contractors can receive up to $15 million in incentives for early completion.

ALDOT is posting detour information, project updates and history on its 59/20 project web site and using the ALGOtraffic web site and mobile app to update traffic conditions. Drivers are urged to plan alternate routes during the closure.