Current chain-control requirements on California mountain highways.

Data: Caltrans chain-control status for fixed checkpoints on mountain
routes (I-80 Donner, US-50 Echo Summit, SR-88, SR-89, and others).
Levels: R-1 = chains OR snow tires required; R-2 = chains required
except 4WD/AWD with snow tires on all four; R-3 = chains on ALL vehicles
(rare, usually precedes closure). Refresh: 5-minute cache.

Filters: route (e.g. "80", "US-50", "SR-88"); center "lat,lon" with
radius_km for all checkpoints around a place (e.g. around Truckee),
whatever highway they are on.

Off-season (roughly May-October) there are usually no controls anywhere;
the response says so explicitly rather than returning an empty list.
Chain requirements can change hour to hour in storms - tell the user the
data_as_of time and to carry chains anyway when snow is possible.