It's the first of six projects to improve the freeway from the Orange County line to I-605.
The widening project will replace the existing two-lane steel overpass with a 10-lane concrete structure. It will also expand I-5 by adding one general purpose lane and one HOV lane in each direction from Alondra Boulevard to Shoemaker Avenue in the cities of Santa Fe Springs and Norwalk, covering about 1.2 miles.
The overarching goal of the project is to ease road congestion.
"Commuters on this stretch of the Golden State Freeway are going to have a lot more time to do the things they want to do. The HOV lanes are going to make it so they won't have to sit in traffic as long," said Michael Miles with Caltrans. "For every mile of HOV lane that we put in, we figure we save about a minute a mile."
However, some motorists who use the stretch of freeway every day say they don't think the project will be effective.
"You need more than one lane really. You need about three lanes on that freeway to really help out. You know, it's just bad. There's too many people now," said motorist John Southward of Lakewood.
The project carries a price tag of $380 million, which includes federal, state and local funding. Caltrans says 173,000 commuters use this stretch of freeway on a daily basis.
CHP officers reminded motorists to drive slower near construction areas or near construction equipment or signs.
Construction is expected to begin in the fall and wrap up by early 2015.