Like the rest of America, California is still entirely pre-metric: all road distances are measured in miles; speeds in miles per hour (MPH); and gas (petrol) is sold and consumed by the US gallon. There are no concessions at all to metric users — virtually no one will understand you if you try to use kilometres, litres, kilograms, degrees Celsius, etc. Twenty-four hour clock times (e.g. 23.45 instead of 11:45 PM) are also rarely understood here.
In urban areas, distances are often informally measured in “blocks”; for example, you might be told that the nearest post office is “three blocks down Main Street”. This is usually fairly self-explanatory — city and suburban blocks are normally defined in relation to the larger cross-streets. The resulting block size varies, but in city centers a block is usually somewhere around several hundred metres long; suburban blocks can be somewhat larger. In urban areas, most street addresses are prefixed with the block number; e.g. “2125 Main Street” will normally be in the 21st block of Main Street (and the nearest cross street may well be “21st Street”). The block in this example may be referred to as the “2100” (“twenty-one-hundred”) block. A useful side effect of this numbering scheme is that 2125 Main Street is usually eleven blocks from 1025 Main Street (by the way, nearly every American city has a Main Street somewhere, just like every Australian city has a Church or Station Street, every British city a High Street or Broadway. Another frequent road name here is “Frontage Road”, which is apparently synonymous with “access road”, and is usually found running alongside a highway. There seems to be one in every city, town, and village…).
Many cities have numbered street names (i.e. those peculiarly American street names like “14th Street”); some even have lettered street names, e.g. “B Street”. This sometimes leads to odd-looking signs like the one on an exit ramp from the Interstate 880 freeway in the Bay Area which used to simply read “A Street Downtown”, or the “Eye Street” sign between H and J Streets in Bakersfield. Commonly, many cities also have schemes where avenues run in one direction, streets in the perpendicular direction (but this is sometimes unreliable).
Spoken street names are almost always given without the “Street”, “Road”, or “Avenue” part, e.g. 17th Avenue is “17th”, Oxford Street is simply “Oxford” (e.g. “It’s on Oxford about a block north of Hearst”, or “I think it’s 2125 Main”). City locations are often given by the nearest intersection, e.g. “It’s at Market and Montgomery”, meaning “it” is somewhere in the blocks defined by the intersection of Market and Montgomery Streets (not always actually on the intersection, though). “It’s on University near McGee” has a similar sort of meaning. Unlike (for example) Britain, California does not have a tendency to clump together a bunch of streets, roads, avenues, mews, etc. all with the same name — there’s unlikely to be a Muswell Hill Broadway near Muswell Hill Avenue right next to Muswell Hill Road and Muswell Hill Mews as there is in London (yes, I used to live in Muswell Hill) — so it’s almost always safe to leave off the “Street”, “Avenue”, “Road” or whatever.
Some addresses will look something like “1025 S. Figueroa” (or “1025 Sth. Figueroa”); here the “S.” stands for south, meaning the address is in the 1000 block of the part of Figueroa Street (or Road) that is south of the city center or a particular intersection. It’s not always obvious what the “south” (or whatever) is relative to, but it’s likely to be a particular local landmark or large street. Similarly with addresses like “4400 E. 14th, Oakland” (East of Broadway in Oakland) or “1400 North Elm”.