It could gather this information from its own sensors, as well as from sensors in nearby wearable or mobile devices carried by others.
Honestly, if we're having a watch scream at anyone, I'd rather it be the drivers who are (a) unaware of the law about pedestrian crossings, (b) looking at their phone while driving, (c) driving like an asshole, (d) driving drunk.
In a "perfect" world where everything and everyone is connected, sure. But consider what kind of world that would be.
For modern technology to be the most useful it may be something that would be just as effective if the traffic still consisted of horses & buggys or Model T's.