The MUTCD, or Manual of Uniform Traffic Control Devices, is the book that sets the standard for pavement markings, signs, speed bumps and basically anything else that determines how traffic flows in public rights of way. Traffic in this case is broadly defined- pedestrians, equestrians, cyclists, bus and rail transit, and automobiles all have specific regulations in the MUTCD. Bikes even get their own chapter- Part 9. The Manual is published by the Federal Highway Administration, and most states simply adopt it as their own standard. Some adopt it with state "Amendments,' which usually specify unique markings that are used in that state, such as specific state highway signs. Seven states, including California, have their own MUTCD; though this is required by law to be "substantially in conformance" with the federal MUTCD, it is sometimes the little differences that'll trip you up.
Why does the MUTCD matter? Well, any markings on pavement or road signs have to be in compliance with it. The standards dictate everything from where the signs should be placed, to how high off the ground they need to be, to what colour and text are on them and how shiny they are. (This is called "minimum retroreflectivity," but I think "shininess" sounds better.) If you want to make a stretch of road different in any way, you have to check the MUTCD first.
Now, as I said, California has their own MUTCD, which is different from the federal one. Sometimes this works in our favour- we were the first state who could use sharrows without special federal permission, because we included them in the 2004 California MUTCD. Right now, though, this is working against us. The new 2009 Federal MUTCD has some very cool bike-related stuff in it, but these changes have yet to migrate into the California MUTCD. Also, while we were the first to allow sharrows generally in our roads, the California MUTCD limits their placement and use in a way that the federal does not.
Specifically, here in Riverside, I want to do something about the atrocious Arlington Avenue, the Central/Magnolia intersection, and now the badly-redesigned University/I-215 eastbound segment. I'd love to see sharrows installed on these streets, along with a sign that warns drivers that bicycles can use a full car lane. Under the federal MUTCD, sharrows would be just fine for all of these situations, and a "Bicycles allowed full use of lane" is included (Sign R4-11). However, the California MUTCD does NOT include a "Full use of lane" sign, just the terribly ineffective "Share the road" sign. (Most motorists think "Share the road" means "Bikes get a tiny bit of it at the right, cars get the rest- sharing!") As for sharrows, the California MUTCD only permits them on streets with on-street parallel parking, restricting them from being used on University, Arlington or Central.
There is, however, good news! The 2011 California MUTCD is currently in draft form, and incorporates the more progressive federal bicycle markings. While I doubt very much that these are in danger of not being present in the final Manual (due out mid-2011), you should still comment in support of the new bike stuff. Also, if you find it useful, I suggested that a sentence in their sharrows section be slightly modified, so as to permit sharrows being used on roads that are unsafe for bikes yet signed with a speed limit above 35. Currently, there is "Guidance" that suggests otherwise, and while I think this is non-binding, traffic engineers will probably follow it anyway.