The San Diego Association of Governments and Caltrans will begin a series of overnight closures of southbound Interstate 5 tonight to complete construction work between the I-5/Interstate-805 interchange and La Jolla Colony Drive.

Southbound I-5 will be closed from the interchange to La Jolla Colony Drive from 9:30 p.m. to 5 a.m. each night, Sunday through Thursday, through at least June 28, according to SANDAG, although the nightly closures may continue into July. Motorists will be detoured along southbound I-805 and state Route 52 during the closure window.

The closures are necessary to continue construction of two Metropolitan Transit System trolley overpasses, which will span I-5 at Voigt Drive and south of Nobel Drive. Crews will remove wooden structures that support the bridges during construction from the Voigt Drive overpass and add them to the one near Nobel Drive.

The closures are expected to affect freeway traffic to and from the San Diego County Fair, which is being held at the Del Mar Fairgrounds through July 4. Residents are advised to plan accordingly as the closures may extend into July.

After work on southbound I-5 is complete, a full closure of northbound I-5 will also be required, according to SANDAG. Dates and work windows for closures of northbound I-5 have yet to be released.

The closures are part of the $2.17 billion Mid-Coast Trolley Blue Line Extension, which includes a planned 11-mile extension of trolley service by MTS from Santa Fe Depot in downtown San Diego to University City. The extension will add trolley stops in Mission Bay Park, UC San Diego and Westfield UTC. SANDAG is receiving $1.04 billion in funding from the Federal Transit Administration to complete the project.

The extension and related projects are intended to reduce traffic congestion as the county's population increases. Construction on the extension began in 2016 and is scheduled to be completed in 2021.