I've been a bicycle/transit/pedestrian advocate for 10 years, lead the successful fight to repeal bicycle prohibitions on expressways, but it still ceases to amaze me how traffic engineers have no regard for the law, if they think they can get away with it. (In fact, this is also true of many politicians who violate the Clean Air Act by always approving more highways which generate more air pollution). The Vehicle Code only allows prohibiting bicycles and pedestrians from freeways. I have been through many occasions where traffic engineers, after receiving dozen(s) of my phone calls, run out of excuses and acknowledge that the signs prohibiting bicycles and/or pedestrians must come down, according to law. They then take another several months or a year to get around taking the sign(s) down, in the meantime willfully violating the law. Even after going through a long fight at the city council level, being opposed by the traffic engineers, and winning when the city council repealed their discriminatory ordinance, in most cases it has taken another huge fight just to have the take the signs taken down.
There is another, much easier, way to accomplish removal of these illegal signs, I found. Get a community volunteer, including possibly yourself, to take down the sign(s). The important thing is to ditch the signs behind shrubbery, or, better yet, remove the sign from the area altogether. Otherwise, road maintenance crews would just put it back up. If it's unseen, it can go unnoticed.
You remove the signs very easily:
Wooden posts: most are easily pulled up from the ground. Use gloves to avoid splinters. First, loosen by vibrating the sign back and forth with your arms and using your body weight. Sometimes, it takes a bear hug for the initial pull, so wear old clothes. Alternatively, most posts are old and can also be broken at the base (or just pushed over if the ground is wet!).
Lamp posts: most are simply bolted on. Bring a large crescent wrench. You need a friend or a stepladder so you can reach the bolts. The easiest is for your friend to form a step with his hands (fingers interlaced) and to step onto this. A few may have a steel band instead of bolts. In this case, use a hacksaw with a carbide blade (available at hardware shores).
When to remove signs: If you remove signs during the day, some passing motorists think you are a vandal, and honk their horn. Either dress as a worker (leaving your bike a short distance away), or do it when traffic is light, like at night. Chances of getting caught by police are quite small (but take all precautions to avoid a hassle). If you do get caught, just remember that many people have gone to jail for violating "whites only" transportation laws. If you think about it, this is also a civil rights issue, and is just as illogical and unfair. The right to travel cannot be abridged just because one does not own, or prefers not use, automobiles.
Another approach is to use politicians and/or police. The California Vehicle Code (#21467) actually empowers police and "local authorities" to remove all illegal signs. While the sympathy of police departments to bicycles is questionable at best, you can use this as a leverage. Last year, I informed the director of the Santa Clara County Transportation Agency, that if the signs aren't removed within 48 hours from Central Expressway, I would get Mayor Eddie Souza of Santa Clara, who supports bicycles and pedestrians, to direct the City Police Department to enforce the city ordinance, and I would send out a press release. An axiom of bureaucracies is not to make themselves look bad (a corollary of maximizing money for their bureaucracy). The signs were gone in 48 hours. Where did I get 48 hours from? Community volunteers on four occasions had taken down the signs along the road, but just leaving them knocked over (a mistake, they should have been hidden or removed). They were all placed back up within 48 hours. Concurrently, whenever I called the traffic engineers, they stated that they just didn't have the time to take the signs down!
You might try writing a letter to the Police Chief of the jurisdiction involved, and mention that Vehicle Code #21467* authorizes and empowers them to take the signs down, and that they are sworn to enforce the law.
Signs that were removed by volunteers (by the "literal" method) from Hwy. 237 (between Coyote River and 880) half a year ago are still gone. [Update: new yellow 'caution' signs showing a bicyclist were recently placed here on 237.] Signs removed 18 months ago that prohibited bicyclists to go between Lawrence and Central (even with both roads allowing bicycles!) are still gone. [Update: still gone, and sidewalks were constructed in 1997.]
One way or another, the traffic engineers cannot keep illegal signs up unless nobody takes any action to take them down using any of the above methods. For illegal discriminatory signs, it takes a community volunteer to enforce the law.
* California Vehicle Code #21467 states "Every prohibited sign, signal, device or light is a public nuisance, and the Department of Transportation, members of the California Highway Patrol, and local authorities are hereby authorized and empowered without notice to remove the same, or cause the same to be removed."
Also see: list of bicycle articles.