It means the road may be slippery, proceed with caution.
You can pretty much find a slippery when wet sign in most places wheres there is a lot of pedestrian traffic moving around. You might see them in bars, supermarkets, schools, and in most department stores.
A sign post is a post supporting a sign that offers information or directions. An example of a sign post is a traffic or road sign such as a Stop sign.
This spot is prone to being slippery with out warning. BE CAREFULL!
If you mean a diamond on the lane, that means that that lane is for BICYCLISTS ONLY, if you mean as a shape of a sign such as a "slippery when wet" sign, that usually tells you important things. I looked it up in my "Virginia Driver's Manual" and it says a diamond shaped sign is a warning sign.
Slippery conditions ahead.
A sign with a straight line and a crooked line generally means that motorists need to merge.
traffic signal.
warningA merging traffic sign an example of a warning.warning signyellow diamond
A stop sign
warningA merging traffic sign an example of a warning.warning signyellow diamond
traffic signal, sign etc
A warning sign