A system to alert drivers approaching a traffic intersection controlled by a traffic signal light. The system may alert about an impending change in the state of the signal from green to red, to allow them to plan ahead and stop the vehicle safely. The system can be used to slow down and stop an autonomous vehicle (driven by a robot) safely at an intersection. It can also be used to implement “active” or “intelligent” stop signs that warn the driver if the vehicle does not slow down when it has crossed the safe stopping distance at an intersection. It may be used as a virtual hazard warning system to warn motorists of temporary hazards or bottlenecks on the roadway; or as an aid for visually impaired drivers (drivers with color blindness, partially or fully blind drivers).