An engineering firm will spend five-plus years exploring how to rebuild the Briggsmore Avenue interchange on Highway 99.

The Modesto City Council voted unanimously Tuesday for a $6.38 million contract with the Sacramento office of AECOM.

Briggsmore is the busiest and most complicated of the state highway junctions within the city. The firm will assess possible designs, construction costs and timing, funding sources and environmental impacts.

The vote came four weeks after similar action for the Standiford Avenue interchange. It gets plenty of traffic, too, but is simpler than Briggsmore.

The $6.45 million Standiford contract went to WMH Corp. of San Jose. It will stretch over 39 months, or until July 2026.

The Briggsmore contract is for 69 months, or until January 2029, reflecting the complexity of the site. The council approved it without comment.

The Briggsmore area has many big-box stores and other businesses, as well as the Modesto Junior College West Campus.

Three feeder routes — Sisk Road and Briggsmore and Orangeburg avenues — all approach 99 on tight curves. And the interchange rises over freight tracks that also could carry the Altamont Corridor Express by late 2026.

Built in 1970s

The structure was built in the 1970s to serve a state highway with only two lanes in each direction. It has been upgraded “incrementally” as traffic increased and 99 got third lanes, a city staff report said.

The state plans to eventually have four highway lanes each way. Some stretches already have this, but only for vehicles entering and exiting the highway.

The report said the interchange redo will mean “a net increase in safety and efficiency for movement of goods and services throughout the vicinity of Briggsmore Avenue and (Highway 99).”

Local funds cover contract

The city will cover the contract cost with fees on developers and the Measure L sales tax increase. The process will include chances for public comment as possible designs are put forth.

The construction cost was estimated at $118 million in a 2020 report to the Stanislaus Council of Governments. AECOM will refine it as plans get more precise.

The city hopes to use a mix of local, state and federal money for the interchange. The same goes for Standiford, estimated a $108 million by StanCOG.

After the engineering contracts run their course, a few more years would be needed for detailed design, right-of-way purchase and construction.