Vehicle-activated sign
Image:Vehicle activated sign (VAS) [speed limit enforcement.jpg|thumb|right|Vehicle activated speed enforcing sign]
Vehicle activated sign 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. The devices are deployed on roadsides like regular traffic signs in various countries. They are intended to help drivers by alerting them and drawing attention to danger or other undesirable situation.
Sign types
Speed enforcing
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.Another variation of VAS displays the speed of any passing vehicle. If the vehicle is moving faster than the speed limit the number will flash, or be displayed in an alternate colour. Other signs also flash alternately between a number indicating the speed and an emoticon. A Smiley face is displayed if the vehicle is moving below the speed limit, while a sad face is displayed when the limit is being exceeded. This kind of VAS is widely deployed in the Netherlands and Belgium.
Hazard warning
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.Sydney, Australia has implemented a unique low-clearance hazard warning using a curtain of water directly in the path of the vehicle. "When a truck gets too close... a curtain of water starts flowing and a stop sign is projected onto it. The result is a warning that's so in-your-face that not only do the trucks stop, but so do all of the cars around it."