From about November through March, dense Valley fog and mountain snow shape when trucks should roll and which routes stay open. Here's where it matters, how to plan the season and what to check on the policy.
Quick answer
Caltrans puts tule fog season in the Central Valley at roughly November 1 to March 31 and says snow closes the I-5 Grapevine three to five times in an average winter. Most winter losses tend to be collision and liability questions, so make sure each truck carries the physical damage it needs and drivers have a written fog and chain plan. Each winter claim is weighed against the policy wording and that truck's coverages.
Tule fog forms overnight over the cold, damp Valley floor and can cut visibility to almost nothing by morning, right when crews and delivery trucks head out. A Caltrans report on its SR-99 fog warning system describes a fog season running roughly from November 1 to March 31 and recounts a November 2007 pileup near Fresno that involved 108 vehicles, 18 of them big rigs.
Mountain passes add snow and ice. Caltrans says full snow closures of the I-5 Grapevine average three to five a winter and usually last four to six hours, and chain controls go up on the Sierra and northern passes whenever conditions call for them. A truck that can't get over a pass can mean a missed delivery; both problems are easier to plan around than to drive through.
Official sources
Winter losses tend to be crashes. In fog the pattern is often a chain reaction: One driver slows, the next can't see in time, and vehicles pile into each other along a long stretch of road. Damage to your truck in that kind of crash is typically a collision question, and injuries or damage to others generally fall under liability. A unit carried without collision usually has no coverage of its own for that damage, and in a multi-vehicle crash, recovering from another driver's insurer can take months while fault is sorted out.
Snow and ice bring jackknifes, slide-offs and low-speed scrapes at chain-up areas. Caltrans sets chain controls by level, and what's required of heavier trucks can differ from what's required of passenger cars, so drivers need to know the rules before they reach the checkpoint. Caltrans' QuickMap (opens in a new tab) shows active chain controls, closures and highway cameras.
Closures cost time even when nothing is damaged. When the Grapevine closes, Caltrans' Operation Snowflake plan sends traffic toward detours such as SR-58 and SR-166 when those are open, which can add hours and put drivers on two-lane roads they may not know. Build those detours, and the extra time they take, into winter dispatch.
Official sources
Fog belongs to the Valley floor; chains belong to the passes. These metros see one or both in most winters.
Fresno–Visalia. SR-99 south of Fresno is where Caltrans built its fog detection and warning system after repeated multi-vehicle crashes in fog, the 2007 pileup among them. Farm, packing and distribution trucks leave before sunrise through the heart of fog season. See truck and fleet insurance around Fresno and Visalia.
Stockton–Modesto. The Delta and the flat ground along I-5 and SR-99 can hold fog well into the morning, and the area's distribution centers run early shifts that put trucks on the road in it. Staggered start times are an easy lever here. Our page on Stockton and Modesto truck insurance covers the area.
Sacramento Region. The National Weather Service office in Sacramento publishes fog safety guidance for Valley drivers, including the advice to watch for CHP pace cars. Fleets here also send trucks up I-80 over Donner Summit and US-50 toward Echo Summit, where chain controls are common in winter. See commercial auto insurance in the Sacramento region.
Bakersfield / Kern. Kern gets both hazards: Valley fog on SR-99 and I-5, and the Grapevine to the south, where snow closures are counted in hours. SR-58 over Tehachapi Pass is the other winter route to watch. More on Kern County fleet insurance.
High Desert. The Cajon Pass on I-15 and the routes over the San Gabriel and San Bernardino mountains can see snow and ice, and strong winds across open desert can shove high-profile trucks and empty trailers. Check conditions before sending trucks over the pass. See High Desert truck fleet insurance.
North State. I-5 over the Siskiyous and past Mount Shasta, and SR-299 and SR-44 into the mountains, often carry chain controls in winter, while the northern Valley floor around Red Bluff and Redding gets its share of fog. Long routes mean drivers need chains, food and a plan for waiting out a closure. Our North State commercial truck insurance page covers the area.
Official sources
Fleets that get through winter cleanly tend to settle these questions before the first fog morning, not during it. One page covers most of it.
Distributors running early delivery routes along SR-99 sit squarely in the fog's window; wholesale distributor truck insurance covers how those route fleets are written.
Before November, read the schedule with winter in mind.
See physical damage coverage for how collision and comprehensive are written, and commercial truck insurance in California for the heavier units that cross the passes. For the routes themselves, read about California's high-claim truck corridors; because storm season overlaps fog season, our guide on flood and atmospheric river risks for fleets covers the yard side of winter. Then get a quote before fog season.
The fleet's written winter rule should, not each driver. Name the visibility at which trucks hold in Central Valley tule fog, and state plainly that waiting is never disciplined, so nobody feels pushed to keep moving.
A driver who has never put chains on that truck learns how at a crowded chain-up area in the dark. One practice fit in the yard before November can save time and help avoid low-speed scrapes.
In a multi-vehicle fog crash, sorting fault among several insurers can take months. Collision on your own truck can let the repair start without waiting for that, and rental reimbursement helps keep routes running meanwhile.
Because most winter losses are fog and ice crashes, an older truck kept on Valley routes without collision is likely the unit that leaves the business paying its own repair. Decide on those units before November.
Official resources
Requirements change without notice. Confirm what applies to your fleet with the agency before relying on it.
How wildfire season affects parked trucks, evacuations and power shutoffs, which metros face it most and what to confirm on a fleet policy before red flag days.
Read the guideWhere vehicle, trailer and catalytic converter thefts concentrate in California, which work trucks get targeted, how yards are secured and where auto coverage usually stops.
Read the guideWhat can change for a contractor's auto coverage during a wildfire rebuild: adding trucks mid-term, rentals and borrowed units, debris hauling and access to burn zones.
Read the guideThe Grapevine, Cajon Pass, SR-99 and the busiest Los Angeles, Inland Empire, East Bay and Sacramento freeways: what makes them costly for fleets and what to check.
Read the guideHow atmospheric-river floods affect trucks parked in low yards, how flood damage is usually handled under comprehensive and what fleets in flood-prone metros should check.
Read the guideHow commercial auto usually treats earthquake damage to parked trucks, what yards and parking structures add, which metros sit near major faults and what to check.
Read the guideTiming depends on the carriers and your loss history.
Have ready: vehicle list with VINs, driver list, current declarations page, loss runs.
Next Level Experience. Insurance Done Right.
Fleet quotes for California businesses running up to 50 trucks and vans.