Construction has begun on the main phase of the long-awaited $444 million overhaul of the interchange between the 57 and 60 freeways in the eastern San Gabriel Valley – one of the most dangerous interchanges in the nation.

The interchange is unusual for freeways because the two highways merge for 2 miles through much of the cities of Diamond Bar and Industry before resuming their separation. This results in sudden lane drops and forces many vehicles to traverse several lanes of traffic to complete the transition from one freeway to another, which has resulted in many accidents.

Roughly 350,000 vehicles per day travel through this complex interchange, including substantial truck traffic on the way to and from the Ports of Los Angeles and Long Beach and to and from warehouses in the Inland Empire. According to the American Transportation Research Institute, it is the seventh worst truck bottleneck in the nation and the worst in California.

Plans to overhaul this interchange to remove many of the lane drops and add bypass bridges have been in the works for 20 years, but the major chunk of the funding – roughly $240 million in state transportation funds from Senate Bill 1 (passed in 2017) – didn’t materialize until 2018. The other main funding source, Los Angeles County’s Measure M half-cent sales tax for transportation projects, passed in November 2016; this project has received about $175 million from that measure.

The targeted completion date for the project is mid-2028.