High Quality Content by WIKIPEDIA articles! Vehicle activated sign (VAS) is a generic term for a type of road traffic sign which displays a message conditional upon the presence, or speed, of a road vehicle. These devices are used for speed limit enforcement at some locations in the UK.These signs activate if an approaching vehicle is detected to be exceeding a preset speed threshold. The speed limit and/or a warning message will illuminate on the sign to remind the driver/rider to slow down.These signs are installed on the approaches to hazards such as bends and junctions. If triggered they may display a warning message and/or an image of the appropriate road sign (sharp bend, junction, etc).