Is mistake of law a defense if you are given a speeding ticket for speeding in an area with no visible limit?

[janyd]

Ignorantia legis neminem excusat is usually a pretty easy way to defeat the defense of mistake of law. However, in a situation where the speed limit on a county road was 55 but then changed to 40 and an individual was given a ticket for going 17 miles per hour over the posted speed limit. Yet in this case the only speed limit sign was completely overgrown by low hanging vines from trees and not visible at all. Would mistake of law then be a usable defense? Is there another defense that could be used?

[osryd]

Technically, "mistake-of-law" is never a viable defense but if what you're saying is true, the offender may have a viable argument if he gets a sympathetic judge ... However, the truth of the matter is that traffic judges have their own little kingdoms (their rulings are virtually never appealed) and if they don't use common sense in making decisions, a violator in this scenario cannot win.