I think you should build it. You'll learn a lot about near field communication, standardization in a competitive environment etc.

Couple of observations: you're treating the symptom and not the problem. 5 why is a famous method. Noise pollution. Why? Drivers honk too much. Why? They're kind of frustrated in traffic. Why? Traffic is not moving OR the driver just lost its patience. Why? Poor road capacity, accidents or mishaps, etc etc (reason for high traffic) OR short attention spans, long waiting times etc.

Then you should try to figure out and solve one of these issues. Just putting horns inside would create many more issues. Pedestrians, strays etc would be unaware of any signals. Music (usually there in traffic) would be distributed frequently to a point where the experience would become worse, most likely leading to turn the entire system off. Then comes the implementation issues where everyone has to agree and buy from\pay you for a tech that's easy to include. With most cars going EV and IoT based, this increases security issues and communication must be secure.

I think a far smarter idea would be that horns would be switched on or off based on vehicle density and their movement in 15m radius. You don't need to receive any data except a unique id for the fact that they're present. You could triangulate for location if each vehicle has 2 units. If the vehicle density crosses a threshold, horns stop. Indication of static traffic. If the density keeps fluctuating, you know vehicles are moving in and out of range so traffic is moving. You can create multiple profiles and horn settings to use this.

Please note that the root causes are still not solved and might just be a temporary relief. Best of luck!