The $2.16 billion I-405 Improvement mega-project is one of the largest design-build infrastructure freeway projects in the United States, along one of the busiest and most congested freeway segments in the United States. The project improves 16 miles of Interstate Route 405 in Orange County, from State Route 73 to Interstate Route 605, crossing six corridor cities. The project aims to reduce travel times for the 509,000 travelers projected to traverse this segment per day by 2040. The goal of the project is to reduce congestion, enhance operations, increase mobility, improve trip reliability, maximize throughput, and optimize operations, while minimizing environmental impacts and right-of-way acquisitions.

In general, the project accomplishes these goals by adding two lanes in each direction, providing one new general-purpose lane in each direction and allowing for the conversion of the existing carpool lane facility to a total four-lane tolled express lane facility. The freeway widening alleviates geometric constraints, such as bottlenecks and nonstandard lane and shoulder widths, and requires interchange reconstruction providing further traffic operational and geometric improvements. To accommodate the work, the project constructs four new bridges, replaces18 overcrossing bridges (including one pedestrian overcrossing), widens or modifies six existing bridges, and constructs miles of retaining and soundwalls along the corridor and local roadways. Almost all of the replaced overcrossing bridges span the freeway at a 45-degree skew, which further lengthens and complicates the bridge design.  One of the new bridges supports a new connector ramp between the SR-73 tollway and the new I-405 express lane facility. 

With the reconstruction of the local road interchanges and overcrossing bridges, the bridge and the roadway approaches were improved to ultimate master plan widths, which can accommodate additional vehicular lanes, pedestrian, bicycle, and transit facilities, in addition to geometric improvements. Curb ramps and driveways impacted by the project were reconstructed to meet accessibility standards and guidance. Safety features such as concrete barriers, guard railings, crash cushions, terminal systems, and lighting was added throughout the corridor per standards. 

Stormwater conveyance systems were constructed along the new shoulders of the freeway, ramps, and bridges to provide collection of rainwater and discharge it to the existing county and city channels which lead to the Pacific Ocean, per current standards and project requirements. Stormwater treatment devices, such as detention and infiltration basins, were designed and installed to treat the runoff from the new pavement surfaces to achieve California’s requirement to meet the Federal Clean Water Act.