Vehicle-activated sign

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.

If the vehicle is moving faster than the speed limit the number will flash, or be displayed in an alternate colour.

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.