soyez prudents

be careful/prudent? Again an imperative and a road sign

serrez a gauche

something like keep left? Seen on the motorway approaching peage.

chausse deforme

deformed road ;-)

bouchon

As in bouchon de quatre km dirrecion sud. Heard it on the radio and avoided it on the road, now that is a listening test. Kind of feel like bouchon must have other meanings too and it does; e.g. Tout ce qui obstrue un orifice, un conduit : Un bouchon de cérumen. Also reminds me of bouche as in mouth, related?

So, how do you say it? bouchon on forvo

allumez vos feux

Noticed this on road signs outside tunnels. Love it because it makes me think 'light your fires', though presumably French speakers don't think that ;-)
Better translated as 'turn your lights on', perhaps?
Pron of feux: http://www.forvo.com/word/feux#fr
No pron of Allumer on forvo but follow the song link at the end of this post for a nice use.
Allumez is also I guess an example of imperative as with other road signs e.g. 'serrez a gauche'
A little research into uses of allumer suggests many other interesting ways it can be used including some more like the fire lighting associations I have from English, e.g. allume moi, see http://www.youtube.com/watch?v=VFgbMjKobI4